THE BACKYARD SERIES W 346-F BACKYARD VEGETABLES

THE TENNESSEE VEGETABLE

SEASON EXTENSION METHODS

Natalie Bumgarner, Assistant Professor and UT Extension Residential and Consumer Horticulture Specialist Department of Sciences

Vegetable production is increasingly of conditions — thus extending the popular for Tennessee residents. THE WHY AND growing season. Growing vegetables at home WHAT OF SEASON This practice of allows provides financial and nutritional EXTENSION gardeners to have some control over benefits through the bounty of the environment around their crops a fresh harvest, and the activity (roots and/or shoots) to enhance One of the basic facts of gardening enhances personal health and productivity or maintain survival until is that crops will grow, develop and well-being. However, a basic conditions are more appropriate for produce when temperatures are growth. Certainly there are limits, but understanding of soils, site selection appropriate. Gardeners carefully plant adding an extra few days to weeks and crop maintenance is required cool- and warm-season crops to grow to the growing season can be quite before backyard growers can take during the time of year that targets useful in vegetable . Methods full advantage of the benefits of their ideal temperature range. Managing are divided into two main groups: home food production. To meet crops according to surrounding management practices that can extend conditions can work very well and fit these needs, this series of fact growing periods and structures or the needs of many gardeners. However, sheets has been prepared by UT materials that can be used to alter gardeners have the option of altering Extension to inform home gardeners temperatures and extend seasons. and propel them to success in the environment around their (called the microclimate) to enable residential vegetable production. crops to be grown in a wider range

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Season Extension Methods 1 Figure 1. A young tomato plant can be grown in a to receive optimum conditions for growth prior to transplant.

CULTURAL RAISED BEDS TRANSPLANTS PRACTICES THAT Soil temperatures can impact speed of One of the most common methods germination, root growth, and water to extend seasons is CAN EXTEND and nutrient uptake — all of which older plants into the garden rather SEASONS affect plant growth. Raised beds have than direct seeding (Figure 1). This a couple of benefits in altering soil simple practice enables plants to temperatures to extend seasons. In be grown when outdoor conditions SITE SELECTION spring, water in the soil can slow down would not allow or would slow growth. the process of soil warming because Additionally, conditions can be tailored Vineyards often are located around it takes more energy to raise water for each type of plant, and early lakes to allow temperatures in the area temperature than air. Raised beds that stresses that could limit growth can be to be moderated by the water — a allow water in soil to drain more quickly avoided. Pest pressures may also be good example of selecting a site to take can allow warmer air to enter the soil reduced by an early planting for advantage of natural microclimates. spaces and warm up the soil faster. some crops. Smaller scale examples include sites Additionally, raised beds have more close to buildings that retain daytime surface area that can be exposed to the Some of the most commonly heat, sites where plants have the warming effects of the sun. However, transplanted crops are tomatoes and benefit of a windbreak to protect from these factors also can lead to more peppers because these longer season cold and drying winds, or sites near rapid cooling in raised beds. Another crops can be harvested earlier when a slight slope that enables cold air to benefit of raised beds is that they are transplants are used. Additionally, drain away from plants. Gardeners can well suited to accommodate other planting can be delayed until create these season-extending sites season extension practices. They easily conditions are more optimal for plant around the residence, so pay close can be fit with plastic mulches, row growth. These warm-season crops are attention to discover where you might covers, low tunnels or even soil heating frost sensitive, and their growth can be use microclimates to benefit the home cables to warm the air and soil around limited by cool soil temperatures even vegetable garden. plants and make earlier and later when air temperatures remain gardening possible. above freezing.

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Season Extension Methods 2 Cool-season crops such as cabbage systems to alter environmental and broccoli are also good candidates STRUCTURES AND conditions. In passive systems, growers for spring transplanting. Transplanting CROP MANAGEMENT cannot increase the temperature if these crops in spring reduces time to there is no sunlight to provide heating, harvest and allows gardeners to ensure METHODS THAT CAN although they can temporarily capture crops do not mature during hotter EXTEND SEASONS heat that has been stored in the soil weather, which can deteriorate crop to provide warmth during cloudy quality. Additionally, transplanting periods. Also, they only have natural in the late summer can allow cool- WHAT IS REALLY HAPPENING? air movement to provide cooling. In season crops to be protected from the active systems, passive heating is used, The basis for most season extension warmest and driest times of mid- but it is combined with active heating methods involves absorbing or trapping summer to reduce stress and allow for and cooling sources to maintain more radiation from the sun to warm up higher quality fall-maturing crops. precise control, which always comes the environment around crops. For at a price. This discussion will focus instance, dark mulches absorb light (or Conditions after transplanting will on passive systems because they solar radiation) and conduct this heat influence how much time to harvest are the most flexible, cost efficient to the soil below. Clear plastic covers was gained by transplanting. The and applicable for home gardeners. transmit and then trap light, causing age of the transplant cannot always will be covered in a the air temperature under the be directly subtracted from days to separate publication. harvest because low temperatures and covering to increase. lack of moisture or nutrients in soil can Agricultural plastics revolutionized stress transplants after planting and season extension and provided a reduce the benefits of the practice. variety of tools that work across a range of scales (Table 1). The biggest decisions for the gardener are how much money to invest in the purchase of materials and how much time to invest in installing and managing

Table 1. The range of control over growing environments provided by different types of growing

Control No control Some control High control

Cost Lowest cost of production Highest cost of production

Method Open field* Mulch* Low tunnel or row * High tunnel* Greenhouse** cover*

Details Crops are at the Plastic that A thin, temporary A permanent A permanent A permanent mercy of the tightly covers sheet of plastic or structure often plastic-covered structure covered surrounding soil or natural agricultural fabric made of glass or structure that with glass or conditions. materials placed covers the crop rigid plastic to trap can be vented plastic. Features on soil surface to trap sunlight. solar radiation. to manage heating and to reduce weed Can be supported Can be closed temperatures. cooling to manage growth, moderate (typically 2 feet to to trap heat or Typically large temperatures. temperatures and 3 feet) or floating vented to reduce enough to walk conserve moisture. over plants. heating. in and unheated.

Plastic mulch Warm-season Benefit depends Warm-season Enables year- Specific NA benefits in increases soil crops can often on whether crops can be round growing vegetable temperatures, be planted 1 to heating is used. planted from 1 to 4 and harvesting. gardens germination, 2 weeks earlier. Transplants can weeks earlier and and early root Cool-season crops be started several harvested later. growth. Organic could be planted weeks before Cool-season crops mulches can earlier or benefit planting. Cool- often can be kept reduce summer from better early season crops can alive all winter. soil temps. Both or late season also be grown can conserve growth. early and late in soil moisture. the season.

* Passive — only solar energy for heating and air movement for cooling ** Active — other energy sources used to heat and cool as well as passive

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Season Extension Methods 3 also soil temperatures by trapping to flower. Often this coincides with TEMPORARY sunlight. Because of their light weight warmer temperatures when additional METHODS AND and permeability, they do not need heating is no longer desired. Thanks to structural support. However, some the improved climate under row covers in STRUCTURES crops, such as tomatoes and peppers, early and late seasons, weeds can also have tender growing points that may thrive under covers as well as crops, MULCHES need protection from abrasion by so proper weed control is essential to floating covers. maintain benefits of row covers. Mulches can influence soil temperatures in two important ways. The first Because these covers are permeable Row covers come in a variety of is through absorbing or reflecting to air and water, irrigation may not thicknesses, which vary in their ability incoming light. Dark mulches, such as be required. Also, these covers are to increase daytime temperatures and solid black or woven plastic, absorb naturally vented due to air movement retain heat at night. Thicker covers solar radiation and can transfer this through the material. These aspects retain more heat but block more energy to warm the soil below. Lighter of floating row covers make them incoming sunlight. It is common for mulches, such as white plastic or straw attractive to home gardeners. gardeners to use thinner covers in mulches, reflect more light and do not Additional benefits of floating row the warmer seasons to retain some warm soil temperatures as much as covers are their ability to protect plants heat and protect crops from insects. darker mulches. Under warm summer and soil by reducing the speed of rain Gardeners typically use thicker covers conditions, lighter mulches can be a drop impact (reducing erosion and for fall to spring cropping of cool- benefit because they may keep soil crusting) and to serve as a protective season vegetables, installing them in temperatures lower and less variable, barrier from insects, such as flea the late afternoon and removing them which can be an advantage to beetles on eggplant. in the morning. Row covers can often plants. be reused from season to season if Gardeners generally use row covers as managed carefully and kept clean. The second way that mulches alter a temporary measure in spring and fall All things considered, row covers of soil temperatures is by reducing since the covers can trap sunlight and permeable nonwoven plastic may be heat loss. Heat can be lost from soil may cause plants to overheat under the the most versatile and useful, yet cost through the evaporation of water or warm, sunny conditions of late spring effective, tool for the home gardener when air temperatures are lower than and summer. Overheating can cause seeking to extend growing seasons. soil temperatures (such as at night). leafy crops to be lower in quality and Mulches, both plastic and natural, can can reduce fruit set in some warm- LOW TUNNELS retain heat by reducing both of season crops. It is also a good practice these losses. to remove row covers when crops that Low tunnels (Figure 2) provide many require pollination by insects begin of the same benefits as row covers, When choosing a mulch, pay special attention to watering needs. Irrigation is required when using impermeable plastic mulch. If using a natural mulch instead, select wisely to avoid the potential for introduction of weed seeds or herbicides. For more information on these topics, see W 346-D “The Tennessee Vegetable Garden — Plant Management Practices.”

FLOATING ROW COVERS

Also known as direct covers, floating row covers are usually nonwoven plastic films or agricultural fabrics that can be applied directly over crops. These covers can be installed in large sections that cover many rows in the garden. The edges are usually secured with soil, wooden posts or Figure 2. Slitted plastic low tunnels supported by wire hoops. A floating row cover of other materials. They increase air and agricultural fabric can be seen covering small transplants on the right.

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Season Extension Methods 4 and sometimes the terms are used be installed over hoops to form low referred to as a hot bed. If the frames interchangeably. In this discussion, tunnels, which can protect sensitive have access to electric, then electric we will use low tunnels to describe a crop growing points or create a larger soil heating cables could be used to temporary hoop-supported structure. air space to retain heat. add heat. Often, gardeners use polyethylene plastics over these 2- to 3-feet-tall SHADE STRUCTURES Limitations of cold frames are their wire or plastic hoops and stretch the size, accessibility and construction plastics tight to create the appearance Most of the season extension tools expense. Additionally, hinged of a miniature greenhouse. These discussed so far have centered on cold frames typically require close commercial wire hoops (pictured in adding and retaining heat for early management to prevent damage to Figure 2) are available for purchase, and late-season garden production. crops when bright sunlight can rapidly but other materials, such as electrical However, shade structures in garden increase the temperature in the cold conduit or plastic pipes, also can be areas also can extend seasons by frame. Automatic opening and closing used to form tunnels. reducing light and temperature mechanisms are available as well if accumulations. In Tennessee climates, gardeners are willing to invest in tools Low tunnels are installed down the these shade structures may allow to reduce management time. length of a row to create a distinct air gardeners to harvest cool-season space around the crops, which can crops, such as leafy greens, for a longer HIGH TUNNELS provide more consistent temperature period of time as spring temperatures benefits. Also, the hoops protect rise or to seed or start fall cool-season High tunnels are more permanent, sensitive crops from abrasion by crops under summer conditions. To plastic-covered structures built over the covering or by cold temperature create shaded areas, place white growing areas to provide warmer damage from contact with the covering or black shade netting over metal temperatures for crops. They also during freezing conditions. Like floating or plastic hoops. These commercial keep leaves drier, therefore potentially row covers, gardeners often remove materials are described by how much reducing some foliar disease. However, these tunnels to prevent overheating sunlight they block and come in a this exclusion of rainfall means that when warmer late spring and summer range of percentages. To extend the irrigation will be necessary. While row temperatures arrive. It also is best growing season for cool-season crops, covers and low tunnels are generally practice to remove them when the a 30 percent to 50 percent shade removed after a certain period of time, crop outgrows the structure or when is common. Make sure that shade crops are usually grown in high tunnels pollinating insects need access to structures are well ventilated to prevent for the entire season. The sizes of high the crop. trapping heat under the shade netting. tunnels can vary greatly from smaller structures (12 feet wide x 24 feet in When using these small polyethylene length) to more commercial sized units tunnels, keep in mind that they can (30 feet wide x 80 feet in length). heat rapidly under bright sunlight PERMANENT Typically, a high tunnel is tall enough and should be vented to prevent crop for a person to comfortably work in damage due to overheating. Slits along METHODS AND while standing. Definitions vary, but the top of the tunnels (as seen in Figure STRUCTURES high tunnels are commonly described 2) or small perforations are common as unheated greenhouses. This as a part of the manufacturing process. definition obviously limits the amount These slits reduce heat retention COLD FRAMES of environmental control gardeners at night, but they lower the risk of can practice and is one of the main daytime high temperature damage. Cold frames are familiar to many distinctions between high tunnels and home gardeners. Often constructed greenhouses. Polyethylene tunnels generally of wooden frames with glass, shed water, which can be a benefit Plexiglas, or polycarbonate panels, High tunnels are generally constructed in protecting crops. However, they these structures have long been used of wood or steel frames and are usually require irrigation to be installed for transplant production. It is also covered with flexible or rigid plastic. under the tunnel. Another limitation common to use cold frames for early Material choices depend on cost, length of polyethylene low tunnels is that or late-season root or leafy vegetable of productive life, and the weather the plastic is more difficult to reuse production. Cold frames can retain conditions of the area. Snow and wind from season to season. It is common heat well when closed, and hinged lids loads are one of the most common for plastic mulch, drip irrigation and on the boxes provide ventilation to determining factors for high tunnel polyethylene low tunnels to be installed prevent overheating on warm, sunny design and construction. Many aspects together to achieve maximum benefit days. Some gardeners also set up a of high tunnels design are related to for early planting of warm-season composting system under their cold the crops that will be grown in them crops. Agricultural fabrics also can frames to provide heating. This is often and the seasons they will be used. Most

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Season Extension Methods 5 high tunnels have doors and sidewalls season crops in the warmer times ADDITIONAL RESOURCES that can be used for ventilation while of year. other have vents in the end walls that Season extension techniques for can provide additional air movement. GREENHOUSES market gardeners — ATTRA https:// Managing these methods of ventilation attra.ncat.org/attra-pub/summaries/ is one of the most time-consuming While greenhouses use passive solar summary.php?pub=366 aspects of high tunnel growing. heating, the addition of active heating and often active evaporative cooling Venting, shading, and using multiple distinguishes them from other season layers of plastic or row covers are all extension practices. Because of these practices that enable gardeners to heating and cooling systems, the maintain more environmental control installation and operation costs of in the high tunnel. Figure 3 illustrates greenhouses are generally much higher the use of row covers and low tunnels than the methods discussed here. inside a high tunnel. When gardeners Please refer to home and commercial combine these methods, they can greenhouse publications to learn more extend the growing season for cool- about the design, installation and uses season crops thanks to better heat of greenhouses for home vegetable retention during the night. Likewise, production. sidewall venting and/or shading could extend the growing season for cool-

Figure 3. Season extension methods can be combined to produce additive benefits. This image illustrates the use of floating row covers and hoop-supported low tunnels inside a high tunnel.

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