Horticulture 101

(Marianne Ophardt)

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Hi, this is Marianne Ophardt and today we‟re going to be talking about Horticulture 101. Horticulture is a topic that most gardeners might not think would be very interesting, but it lays the basis for a lot of things we do in gardening. So let‟s get started.

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We‟re all taxonomists. We like to name and classify things. Early humans classified plants by their use, such as if they were used for medicines, seasoning or dye.

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We also like common names. Common names are easier to pronounce and to remember. But common names can be confusing. For example, Chamaecyparis lawsoniana, also known as Port Orford cedar or Lawson‟s cypress, is neither a true cedar or a cypress. Juniperus viginiana, also known as red cedar in the east, is not a cedar. Thuja plicata, also known as red cedar in the west, is not a cedar. When we use common names properly in writing, it‟s not capitalized unless it‟s a proper noun, such as something like Forsythia, which comes from the name Forsyth.

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We need to thank Carl von Linne. Carl von Linne was a Swedish doctor who developed the hierarchy system of scientific binomial names for plants and animals in 1753. He even changed his own name to Carolus Linnaeus to give himself a scientific binomial name.

He did groupings that were based on reproductive structures. His hierarchal system created order out of chaos. If you had someone in one country talking about a plant using a common name, they might not be talking about the same plant because in another country it might have a different common name. Thus the binomial naming system for plants and animals really has made things a lot easier in talking about plants to gardeners around the world.

Plant names are reviewed by International Commission for the Nomenclature of Cultivated Plants. Often scientists find some relationships that are different than they first thought and names of plants change because they find some plants are more closely related to others than they thought.

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Gardeners are usually only interested in perhaps the family, genus, and species of a plant. But there‟s more when we talk about the taxonomy hierarchy. For example, the tomato, a common garden plant. It‟s in the plant kingdom. It‟s in the division of flower-bearing plants. It also is in a

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special class and order before we get all the way down to family. So we‟re mostly going to talk about plants from the family, genus, and species.

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A family is a group of plants having a number of similar characteristics, especially in their reproductive structures. A family can be composed of one or more genera.

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For example, the Rose family, the Rosaceae family. One member is Rosa. That‟s the genus. Rosa – rose. Rubus, another genus, is raspberry. Prunus includes cherry, plum, apricot, peach, and almond. The Malus genus includes apple and crabapple. Cotoneaster includes cotoneaster. Fragaria – strawberry. Chaenomeles – flowering quince.

But we‟re not done yet, there‟s more.

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Crataegus – Hawthorn Potentilla – Potentilla Amelanchier – Serviceberry (or some people know it as shadbush) Sorbus – Mountain Ash Spiraea – Spiraea Pyracantha – Pyracantha and Photinia – Photinia

These are all members of the rose family but different genera. If you look at the flowers on these you‟ll find that there are similarities in the flowers and in the fruiting structures.

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Next is the genus. The genus is a group of plants more like each other than any other with similar structure, appearance, and chromosome makeup. Inter-grafting sometimes is possible between the different genera. Occasional hybrids from breeding may occur, too, but only occasionally.

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Next is species. Species is the specific division of a genus. The plants are morphologically similar within the genus and interbreeding is possible to form hybrids.

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Next, even more closely related is a variety. We often as gardeners refer to a variety but we are Page 2 of 15 using it wrongly. Variety is a fairly consistent naturally occurring variation of a species and when we‟re writing the plant name it‟s preceded by “var.” indicating variety.

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When we write out plant names – if we want to do it in the proper form – the genus is always capitalized. And if you‟re writing it or typing it, it‟s in italics or underlined. The species is usually lowercase (with the exception of proper nouns) and it‟s in italics or underlined. Variety is in lowercase and it‟s also in italics or underlined but preceded by “var.” indicating variety.

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I would imagine you‟d like some examples of that. Here we have an example of the mustard family:

Brassica oleracea var. botrytis (Cauliflower) the genus is Brassica, the oleracia is the species, and the variety is botrytis.

Next we have Brassica oleracea var. capitata (which is Cabbage) So the genus again is Brassica. The species is oleracia and the variety is capitata and that means “head” in Latin.

Brassica oleracea var. italica (is Broccoli) Brassica oleracea var. gemmifera (is Brussel Sprouts)

You can see that all of these are in the same genus and species but each is a different variety. What we often refer to as variety such as a variety of cauliflower or cabbage is actually the cultivar.

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A cultivar or cultivated variety is maintained by cultivation. It may be the result of chance sexual reproduction where it was selected because of good or interesting traits that it had. It may be the result of a mutation or bud sport that appealed to someone as interesting or different. Or it may be the result of selective breeding by plant breeders.

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When we see a cultivar or cultivated variety name in print, the name may be in Latin or English, capitalized, and not underlined. It‟s put in single quote marks or preceded by cv. – meaning cultivated variety. One example would be „Big Boy‟ or „Better Boy‟ tomatoes – both cultivars or cultivated varieties.

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Gardeners need to know that names keep changing. Scientists continue to study evolutionary relationships. New species are discovered. Studies reveal closer or different relationships than first thought. For example, the Brassicaceae family used to be the Cruciferae family. The Asteraceae family used to be the Compositae family. Scientists have found that these are more Page 3 of 15 closely related than they thought and so they renamed the families.

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A hybrid is the result of sexual reproduction or a cross between parents that differ in one or more genes. The term hybrid can be used to describe a cross between two different species such as Clematis x jackmanii, which is a hybrid that resulted from a cross between two different species: Clematis languinosa and Clematis viticella.

The term hybrid can also be used to refer to a cross between two different varieties. An F1 hybrid is the result of a cross between two inbred lines.

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Inbred lines are created by plant breeders by self-pollinating closely related plants that would normally cross-pollinate. The plant breeders will usually do this for ten generations to create an inbred line. Inbreeding in plants results in death and sterility of some offspring and weaknesses in the line, such as reduced growth and yield. But the purpose is to get an inbred line so that you can get an F1 hybrid.

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An F1 hybrid is the result of a cross between two inbred lines. The result is offspring that are very uniform. These offspring may or may not have a higher yield and earlier maturity. Hybrid corn was first introduced in the 1930s. It was an F1 hybrid and the corn was higher yield and earlier maturing and very uniform, which definitely was an advantage for commercial agriculture and the farmers who were growing the hybrid corn. Since then, more and more vegetable and flower hybrids have been bred for the commercial and home garden markets.

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Break Time

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Now let‟s move on to the different parts of the plant. First we‟ll look at the roots. What‟s the function of roots? One of their primary functions is the absorption of water and nutrients for the plant but they also anchor plants to the soil. They provide storage of carbohydrates or energy reserves for the plant and they‟re a site of synthesis for new compounds and for new cells.

When we look at the structure of the root, at the very tip is the root cap. This is a protective layer that protects the meristem zone beneath it. The meristem is where new cells are being formed by cells dividing. After the cells are produced, then they elongate and become larger and then start to differentiate or mature.

You‟ll notice in the diagram the root hairs. The root hairs are some differentiated cells that are elongated and have a larger surface area. This provides for the root by allowing more water absorption and nutrient uptake. As the cells mature, they become different types of cells that have different functions. One of those is vascular tissue, which is basically the plumbing of the Page 4 of 15 plant and we‟ll talk more about that soon.

We have to remember that roots require oxygen and carbohydrates for growth. They‟re a living part of the plant. If we deny them oxygen or food they won‟t function properly. It‟s important for gardeners to remember that because often when people overwater, the roots can‟t get oxygen and then the roots can‟t function properly.

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Next let‟s talk about the trunk or stem of a woody plant. The function of the trunk is to connect the roots to the leaves. It‟s a conduit for water and nutrients and of course it provides support or structure to a tree. If we look at a cross-section of a trunk we‟re going to find bark, bark cambium, phloem, cambium, and xylem.

First on the outside is bark. The bark is a protective layer to protect the tissues underneath. It‟s produced by the bark cambium, just beneath the bark. Beneath that is the phloem, then the cambium, and then the xylem. The xylem is made up of sapwood – which are living xylem cells – and heartwood – which are non-living xylem cells. More about that in a minute.

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Let‟s talk a little more about vascular tissue. The xylem is the vascular tissue in plants responsible for transporting water and mineral nutrients from the roots. The only direction is upward, no plant energy is used. While they once thought it was capillary forces that pulled the water up through the xylem tissue, they found that that wasn‟t really enough force to get the water all the way to the top of a tall tree. Now they feel it‟s the pull of the water evaporating through the leaves and coming in through the roots and they feel that‟s how it moves upward. But no plant energy is used.

The phloem is the vascular tissue in plants responsible for conducting food around the plant. This food is in the form of carbohydrates. Transport materials are moved up and down to where the energy is needed, such as for growth in the roots or leaf growth or shoot growth. Energy is used for this transport. So carbohydrates are used up for the vascular tissue to function.

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When we look at non-woody plants, they have a similar structure. On the outside is a protective layer called the epidermis. If we look inside at a cross-section, you‟re going to find that there‟re vascular bundles. In those vascular bundles, the phloem is toward the outside – toward the epidermis – and the xylem cells are toward the inside – toward the center of the stem. They‟re produced by cambium that runs through the vascular bundles in the stem. Undifferentiated cells within the stem that are non-woody are pith.

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Next we‟re going to look at leaves. Leaves have a very important function: photosynthesis. But before we talk about photosynthesis, let‟s first look at the structure of the leaf. On the outside of the leaf, both top and bottom, is the cuticle. This is a waxy layer that protects the leaf. It protects it from fungal spores, insects, and protects water from evaporating from the leaf surfaces. Page 5 of 15

Below that cuticle is the epidermal layer again, that is a protective layer of cells. Just beneath that on the top of the leaf are the palisade cells. Those are elongated cells that you see in the diagram. The small dots within the elongated cells are chloroplasts and chloroplasts are where photosynthesis occurs. And you can notice that there‟s a lot of chloroplasts within those palisade cells. And that they rotate around the leaf, making the most of the sunlight because that‟s helping create carbohydrates through the process of photosynthesis.

There are chloroplasts in the lower part of the leaf, but they‟re not as dense because the sunlight is more directly coming from above rather than from underneath on the leaf.

Below the palisade cells, you‟re also going to notice some other cells called mesophyll cells and they have air spaces between them and that allows for gas exchange for the gasses that are coming in through the openings at the bottom of the leaf called stomates. And so the stomates basically are like pores in the leaf that are allowing gas exchange in, water vapor out, and so those stomates are opening or closing depending on the needs of the plant.

You‟ll also notice that there is vascular tissue – xylem and phloem – that you can find in the leaf, such as the leaf veins when you look at a leaf. Those are part of the vascular tissue.

The purpose of leaves is to produce food for the plant. It‟s always important to remember that a plant produces its own food. When we talk about fertilizer we shouldn‟t be calling it “plant food” because the real energy from the plant comes from the sun through the process of photosynthesis.

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So what is photosynthesis? Photosynthesis takes place in the chloroplasts using the green pigment, chlorophyll. Photosynthesis requires sunlight. It‟s a chemical process using light energy to convert carbon dioxide and water into carbohydrates and oxygen. The carbohydrates are that energy or food for the plant. So this process is very important for the plant surviving.

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Now the opposite of photosynthesis is respiration. That‟s because it‟s the oxidation of food in living cells that brings about a release of energy and occurs continuously in all living cells because all living cells need energy to continue. Energy is used for growth and cell functions. Now this chemical process of respiration converts carbohydrates – the food – and oxygen into carbon dioxide, water, and energy.

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Now let‟s talk about plant shoots. When we look closely at plant shoots, if we could see at the cellular level, we would see at the very tip a meristematic zone. The meristematic zone is where new cells are being produced. Cells are dividing and new cells are occurring. Then, as we go back from the very tip of the shoot, is the zone of elongation. That‟s where the cells are elongating and getting larger. And then, even further back, we‟re going to find the zone of maturation. That‟s where the cells differentiate into different tissues such as xylem tissue or leaf tissue.

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When we look at a twig, we‟re looking at a very different structure. First of all, at the very tip, is the terminal bud. That‟s where the shoot growth will occur in the next year. Right now, we‟re looking at the twig of a deciduous tree, one that loses its leaves over the winter. So when that terminal bud starts to grow that will become the first leaves on the stem and the shoot will grow from that point. Below the terminal bud you can see the lateral buds and then you can see other buds down the stem. Each of the locations where the buds are is called a node. The space in between each of the nodes is called the internode.

You can also notice by one of those buds are the leaf scars. Those are where the leaves from the previous season were attached to the twig. The shape of that leaf scar is very characteristic along with the number of the vascular bundle scars and those are the small dots you see. The number and the shape of those vascular bundle scars can be very characteristic as well as the whole shape of the leaf scar of the species of tree.

During the winter you don‟t have the leaves to look at, so there‟s different characteristics for plant identification that you look at. And the leaf scar and vascular bundles are one of those characteristics as well as the shape and nature and color of the terminal bud.

The side buds are the axillary buds. They‟re not a very large one but you‟ll see them on the stem. And another thing you might notice here are little things that look like polka dots or spots on the twig. And when you look at a real live twig you‟ll see that they‟re a little bit corky spots and those are called lenticels and those are for gas exchange in and out of the stem.

The important thing for you to notice here is this is the scar from where the terminal bud was the year before. And this was the growth between here and the terminal bud that occurred last season. It can help you tell whether a plant is doing well or not doing well. That amount of growth might be very limited and telling you that there's something wrong with the plant, but also plants as they mature have lesser and lesser terminal growth. But the distance between the scar of the terminal bud and the scar of the terminal bud from this year is one year‟s growth and many times you can also tell how old a twig is by looking back and seeing the different terminal bud scars that are located on that.

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Now let‟s look at the flower. Flowers are parts of the plant that gardeners very much like because a lot of times they‟re very pretty, but some flowers are fairly insignificant. Right now we need to talk about the function of the flower and flowers are for reproduction. There‟s a female portion of flowers and there‟s a male portion of flowers. The female portion is called the pistil and it consists of the stigma, the style, and the ovary. Within the ovary is where the ovule or egg is located. That‟s the genetic material that will combine during the pollination and fertilization process and create a new plant. So the female portion of the flower is called the pistil.

The male portion of the plant is called the stamen and it consists of the anther and the filament. On the anther is where pollen grains are produced and pollen grains contain sperm that will then join with the ovule in the ovary. So the stamen is the male part of the plant.

One of the other things to note that isn‟t indicated on the diagram is the receptacle. The Page 7 of 15 receptacle is where the ovary sits. Now, the ovary can sit on top of the receptacle, it can be all the way encased by the receptacle, or it can be part way in, part way out. The thing to know about that is that when we eat fruit, sometimes we are eating just plain ovary tissue, other times we are eating ovary and receptacle tissue. And so that‟s an important thing to know about fruit because we‟re not always eating just the ovary but sometimes we‟re eating part of the receptacle.

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When the pollen grain lands on the stigma it germinates a tube which goes down into the style and then reaches the ovary. When it gets to the ovary you‟ll see there‟s two sperm. One of those sperm joins with the ovule and creates an embryo that will become the new plant within the seed. The other sperm develops into tissue that will support the new embryo and it‟s called endosperm tissue.

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Other parts of the flower to keep in mind are the petals and when you think about it, what are the functions of petals? Petals don‟t do much except they‟re pretty and they attract insects to flowers that need cross pollination or pollination by the insects. They move pollen from anthers to the stigma. The sepals aren‟t on every flower, but when they‟re there, they‟re there to protect the buds. One good example of sepals are the green sepals that protect rose buds. And then we‟ve already talked about the receptacle. That‟s where the ovary sits.

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Talking about flowering plants, some plants have separate male and female flowers on the same plant. This is called monecious, meaning “one house.” But they‟re both on one plant. An example of this is squash. In squash plants you have both a male flower and a female flower. You can tell which one is the female flower because it has something that looks like a tiny squash at its base. That‟s the ovary.

So remember, monecious plants have both male and female plants on the same plant but dioecious plants have male and female organs or flowers on separate plants. An example of this is kiwi. Kiwi has a male plant and a female plant. Hollys have male and female plants. Ginkgo has a male and a female plant. You might want to know if you‟re growing holly for the berries that not only do you need the female plant for producing the berries, but you‟re also going to need a male plant for the pollen so that the berries get produced.

Ginkgo plants it‟s interesting because you want to have a tree that‟s a male tree. The female trees of course produce the fruit and the fruit of the ginkgo tree is known to be quite stinky and many times people want to make sure they have a male one but you can‟t tell until they flower and many ginkgo trees don‟t flower until they‟re very mature. After that you have an established tree in your landscape that produces stinky fruit. So it can be quite a problem. The way they know whether it‟s a male or female is hopefully they‟ve taken cuttings from the male tree that produced only male flowers and that way they knew that when they took cuttings and got those to root is they were able to produce only male trees. However, if a nurseryman took seeds from a plant then it could be either a male or a female tree and no one will know until it starts to flower and fruit. And they are very stinky, so it‟s important to know the difference. Page 8 of 15

A plant is known as hermaphroditic if both the male and the female organs are in the same flower. And that‟s many of the flowers that we know in our garden.

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Another flowering plant term is gynoecious. These are plants that produce only female flowers. An example of this are certain cultivars of pickling cucumbers. On these cultivars, only female flowers are produced. One might wonder then, “How do the fruit develop?” Well, what happens is the seed company will also put in several seeds from a different cultivar that does produce male flowers and this will provide the pollination and fertilization for the development of the fruit.

Parthenocarpic are fruit that develop without fertilization or development of an embryo or seed. Examples of these are grapefruit, some pears and some tomatoes. Some of the very early tomatoes are this way.

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Some other terms that gardeners should know are things that have to do with stems. Rhizomes are enlarged underground stems with adventitious roots. An example of a rhizome is iris and also some of the grasses like Kentucky bluegrass grow with rhizomes.

A tuber is an enlarged, fleshy, underground stem bearing axillary buds called "eyes." These usually grow horizontally and near the surface. Some examples of those are the potatoes and dahlias which we all know about the eyes or buds that they have on their surface.

A stolon is an above-ground trailing stem or "runner" that may form roots at nodes or at the tip. An example of this are strawberry runners. These are actually stolons.

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A bulb is a storage organ with a short stem surrounded by fleshy leaves. And at the right time of year there‟s also a flower bud in the center. An example of bulb is an onion or a tulip. And you know at the base of a tulip is the place where the roots come out and then that tulip is surrounded with these fleshy scales or basically what are fleshy leaves. And in the center at the very base, near the roots, is the short stem.

A corm, which a lot of people refer to as a bulb, is actually a swollen, fleshy, underground stem. And an example of a corm is a crocus or a gladiolus.

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Now let‟s talk about fruit types. There‟s different types of fruit. Some are fleshy and some are dry. Some examples of fleshy ones are pomes, which are apples; drupes, and example of that is cherry; berries, examples are grape and tomato; and a pepo, an example of that is squash.

For the dry fruit you have samaras, which you‟ll find on elm trees; pods, like a pea pod; capsules, you‟ll see the capsules on impatiens plants that cast out their seeds and pop open; and then on oaks you have acorns.

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So be aware that there‟s different types of fruit. There‟s different reasons based on what part of the fruit is fleshy whether it‟s the inside, outside, or middle wall of the ovary and also the shape and way it‟s formed for the dry fruit. So be aware that there‟s different terms for those. You usually don‟t need it, but you might need to use it when you‟re doing plant identification, especially if you‟re using its fruit to help identify it.

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Break Time

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Now that we‟re done talking about plant structure, let‟s talk about plant growth regulators. These are chemicals that are produced by the plant and they help regulate plant growth and development.

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The first plant growth regulator ever discovered was auxin. It‟s now known as indole acetic acid or IAA. It‟s produced in sub-apical regions just beneath the buds and in expanding leaf tissue. It inhibits lateral bud growth. This is called apical dominance. So when it‟s produced in the terminal buds, it keeps the lateral buds from growing. That‟s called apical dominance.

IAA is also involved with plants bending toward the light source. And you‟ve noticed this when you have plants inside your house and they bend toward the window or where the light is greater. This is called phototropism.

IAA is also involved in downward growth of roots in response to gravity. This is called geotropism.

IAA is also involved with flower formation, fruit set and growth, and the formation of adventitious roots. IAA is a very common ingredient in most rooting compounds that gardeners use to root cuttings of woody plants.

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Another growth regulator is gibberellins or GA. They stimulate cell division and elongation, they help break seed dormancy, and they help speed germination. One example of the use in agriculture or gardening of gibberellins or GA is with seedless grapes. The gibberellins are produced in seeds so when you have seedless grapes, there‟s no chemical being produced that will make the grapes bigger. So gibberellins are applied to seedless grapes so you can have a larger grape. That‟s why those grapes that you see in the grocery store (the Thompson seedless ones) are so big compared to the ones you grow in your garden, because they‟ve been able to apply gibberellins or GA to make them larger.

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Ethylene is another plant growth regulator. But it‟s only found only in the gaseous form. Ethylene will induce ripening, it causes leaves to droop (which is called epinasty), and it can lead to the Page 10 of 15 drop of leaves (this is called abscission), and it promotes senescence (or the aging of tissues.)

Stress often causes an increase in ethylene production within plant cells. Ethylene is often found in high concentrations within cells at the end of a plant's or a leaf‟s life. Ethylene on a commercial basis is used to ripen fruit such as green bananas.

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Abscisic acid or ABA is also another plant growth regulator. It‟s a general plant-growth inhibitor, though. It induces dormancy and prevents seeds from germinating. It can cause the abscission of leaves, fruits, and flowers and its presence will cause the stomata (those pores on the bottom of the leaf) to close. Abscisic acid - ABA

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Cytokinins are plant growth regulators that stimulate cell division. They‟re used in tissue culture media and they‟re used to delay aging or death of plants (or senescence).

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On this chart you‟ll see some other plant growth regulators and what they do within a plant. So you have indolebutyric acid (or IBA), naphthalene acetic acid (NAA); both of these stimulate root growth. And naphthalene acetic acid (or NAA) is used to slow respiration and is used as a dip on holly from commercial holly production areas to help preserve the holly so it can be used for decorations during the holiday times.

There are also growth retardants that will slow growth on woody plants and on other ornamental plants. These help prevent stem elongation in selected crops.

And then we have herbicides that use plant growth regulator chemistry to make a plant basically kind of grow itself to death. Things like 2,4-D distort plant growth and will lead to plant death when used in certain concentrations.

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Plant growth regulators are involved in many different plant processes. One of those would be the cold requirement that some plants have, such as the chilling requirement that apples and pears have. A chilling requirement is the number of hours below 45 degrees Fahrenheit required to break bud dormancy in the springtime. That‟s why you‟ll see no apples or pears growing in the southern parts of the United States is because they don‟t have enough hours below 45 degrees Fahrenheit during the winter that would allow for the bud break during dormancy.

Occasionally in catalogs you‟ll find different fruit trees indicating a low chilling requirement so people a little bit further south in the US can grow some of these trees that they would like to. But generally you‟re not going to find a commercial apple or pear industry happening in Florida because of the chilling requirement.

Vernalization is the requirement of certain plants for a cold period before they will flower. An Page 11 of 15 example of this for gardeners is tulips. Tulips need to have a certain amount of chilling or cold period before they will flower. So if you carry your tulips in a warm place and forget to plant them over the winter they won‟t bloom because they haven‟t had their vernalization.

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Break Time

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When we talk about the term hardy, hardy refers to the plant‟s ability to withstand cold freezing temperatures during the winter. From this quote in the Sunset Western Garden book in 1988, they said "This term describes a plant's resistance to or tolerance of frost or freezing temperatures. The word does not mean tough, pest resistant, or disease resistant.” So when gardeners refer to a plant as hardy, they are simply talking about their winter hardiness of how much frost or cold temperatures they can take without injury or death.

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Hardiness in a plant involves physiological changes. The onset of these changes is triggered by shorter day length and cooler temperatures in the fall. Plants acclimate or go through the hardening process which means they‟re getting ready for winter cold temperatures. Ultimate winter hardiness is genetically determined and can vary within a species due to ecotypes. You may have a plant of the same species that evolved in the northern part of the United States and one of the same species that evolved in the southern part of the United States. The one that evolved in the northern part of the United States will be hardier than the one that evolved in the southern part of the United States. An example of that is some of the flowering dogwoods.

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There are different factors that affect plant hardiness. One of those is genetics as we just talked about. The other is the time of year. Plants are not as hardy in early fall as they are in early winter or mid-winter.

There‟re also differences in plant parts. Stems tend to be hardier than leaf buds, leaf buds are hardier than flower buds, and flower buds are hardier than the roots. The roots you need to keep in mind are the least hardy tissue in woody plants. One of the things to note is that we need to be concerned about this when we plant things in containers that are above ground. They don‟t have the insulating soil around them that they do when they‟re planted in the ground and thus those roots will be exposed to colder temperatures than they would be if they were planted in the ground. And so while a plant may be hardy in an area, if you put it in a pot it may be exposed to colder temperatures and may end up with winter injury.

Dramatic temperature fluctuations will also affect the hardiness of plant tissues. If a plant is going into winter and there‟s some very warm fall temperatures and then it very quickly gets severely cold, that dramatic temperature fluctuation will have an effect on the plant because the plant has not achieved full hardiness at that point. And so dramatic temperature fluctuations can lead to more winter injury than if those reductions in temperature were gradual.

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Other factors also affect plant hardiness. A plant that‟s weak or not very healthy when it goes into winter and doesn‟t have good food reserves because of pruning or stress that‟s happened on the plant through the growing season will not be as hardy as a healthy plant with a full crown of leaves. A plant that‟s drought-stressed also will not be as hardy as one that hasn‟t been stressed through the summer. Keep in mind that a healthy plant is going to be more hardy than one that isn‟t healthy. And also maturity of plant tissues. When people fertilize in late fall or prune and stimulate new growth, a lot of times that new vegetative growth won‟t be able to get ready for winter or acclimate and it will also be subject to winter injury if there‟s severe cold temperatures.

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Loss of hardiness occurs in the springtime. It‟s called de-acclimation. It‟s triggered by the warming temperatures and the longer day length that occurs in the springtime. It‟s usually gradual but can be rapid during extended warm temperatures. This sometimes leads to what‟s called southwest winter injury during cold parts of the winter. This happens on sunny days with lots of sunlight that‟s occurring and the bark on the southwest side of a tree will warm up and de-acclimate. But then the sun goes down and the temperatures may drop rapidly to severely cold temperatures. And this will cause ice crystals to form in those bark tissues and lead to damage. This is called southwest winter injury.

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You need to know about hardiness and climate maps. The USDA has plant hardiness zone maps and they‟re based on average annual minimum temperatures. What you might want to do is check out the USDA Plant Hardiness Zone Map to determine what zone you are located in. The USDA has been trying to revise their hardiness map for a while so you may have some difficulty finding maps that all say the same thing but it‟s a good thing to consult at the USDA site to see what hardiness zone you‟re in.

If you live in the western part of the US you may also want to know what the climate zone is that your plants are rated for. And the Sunset Climate Zones consider temperature ranges, humidity patterns, and other geographical and seasonal characteristics. Check the Sunset Western Garden Book to determine your zone and they‟ll also help you determine which plants grow in what zone.

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Let‟s talk about growing season. The period between the last frost in the spring and the first frost in the fall is the growing season. Due to microclimates, this can vary even within a small area. Gardeners should know the last average date of frost in the spring and the average date of first hard frost in the fall for their area. That‟s important when growing annual crops or flowers for your garden because you want to know whether you have a plant that will flower and produce before frost comes and kills the plant.

You can find this date for Washington locations in WSU Extension Publication EB 0422 Home Gardens and it can be found on the web: Page 13 of 15 http://cru.cahe.wsu.edu/CEPublications/eb0422/eb0422.pdf

Slide 55

Some plants also need a certain amount of heat, not just the right length of growing season. Heat units, called growing degree days, are the difference between the mean temperature for the day and the base temperature. Using a base of 50 degrees Fahrenheit, the average number of degree days for the different areas in Washington include:

Richland – 3,262 Walla Walla – 3,095 Spokane – 2,366 Yakima – 2,296 Seattle – 2,147

The reason why that‟s important in Washington is that certain crops we can grow in certain areas but not others. Such as Cabernet grapes – important to the wine industry in Washington – require 2000-3000 units. You can probably grow these in Richland and Walla Walla but not very well in Spokane, Yakima, or Seattle.

Slide 56

Now we‟re going to talk about photoperiod. Plant growth regulators are involved and photoperiod can be important to flowering, bulbing, seed dormancy, winter dormancy and hardiness. And plants respond to long days versus short days. Or, actually, it‟s short nights versus long nights, but they originally thought it was long days versus short days and so plants got labeled that they were a long-day plant or a short-day plant. Not to confuse you, but they‟re actually short-night versus long-night plants.

And again, this response is involved in flowering, bulbing, seed dormancy, winter dormancy and hardiness.

Slide 57

Short-day plants (or long-night plants) include poinsettias, chrysanthemums, June-bearing strawberries, dahlias, and bulbing for short day onions. When they decide to grow a poinsettia crop, what they have to do is make sure that no light reaches the plant so that they can get them in bloom in time for the holidays. And so what they‟ll do in many greenhouse situations is at night-time, cover the plants with black cloth that will keep the light out so that they can make sure that they all have a dark night and a long night.

This also happens with getting chrysanthemums to bloom at a certain time and it‟s also involved with June-bearing strawberries, dahlias, and again, the short-day onions.

Long-day (or short-night) plants include spinach, rose-of-sharon, petunia, and bulbing for long- day onions and we‟ll talk more about them in a minute.

There‟s some plants that don‟t respond to either day length or night length and those include tomatoes, dandelions, day-neutral strawberries, roses, and African violets.

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Slide 58

So let‟s talk about onion bulbing. Photoperiod along with temperature controls onion bulbing. Short-day onions bulb in response to 12 hours or less of daylight (this occurs in the southern states). Long-day onions bulb in response to 15 or more hours of daylight (this occurs in the northern states). Because of day length some onion varieties are unsuitable for northern climates because they begin to bulb when the plants are too small. Walla Walla Sweets are long-day onions and Vidalia sweet onions are short-day onions.

Slide 59

Southern states grow short-day onions. As the northern latitude increases, the day length requirement for bulbing to occur increases. During the summer months, northern latitudes will have longer day lengths than southern latitudes. You need to prevent bulbing in order to get large plants; otherwise you‟ll have small plants and small bulbs. Long-day cultivars at northern latitudes grow longer before bulbing than short-day cultivars. Long-day cultivars grown in southern latitudes never reach critical day-length for bulbing, so they grow large plants without ever bulbing.

Slide 60

Conclusion

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