ID-241: After Your Ash Has Died: Making an Informed Decision On

University of Kentucky College of Agriculture, Food and Environment ID-241 Cooperative Extension Service

After Your Ash Has Died Making an Informed Decision on what to Replant

William M. Fountain, Horticulture, Ellen V. Crocker, Forestry, Lee Townsend, Entomology, Nicole Ward Gauthier, Plant Pathology

s a homeowner, even under the opy cover in parts of the east. The death and red maple (Acer rubrum) became best of circumstances it can be of these trees had dire consequences popular as urban trees. challengingA to know how to provide the for the ecology of the woodlands and Green ash (Fraxinus pennsylva- best care for your trees. The arrival of wildlife that depended on their nuts nica), black ash (F. nigra), white ash (F. the emerald ash borer to Kentucky has for food. However, there was minimal americana), and a few other species presented extra challenges. As the em- impact to the urban landscape since of ash replaced the American elm. The erald ash borer rapidly kills young and this species was rarely found in cities. form of these trees was not as attractive old, healthy and struggling ash trees, A disease of elms was found in Eu- but they were easy to propagate and homeowners must decide whether rope and later brought to North Ameri- grew rapidly in nurseries. In addition to save their trees with insecticides. ca later in the same decade. Throughout they tolerated the harsh urban condi- Untreated trees will die and should be the 20th century, millions of urban elms tions that prevented many other tree removed if there is a chance that their would die across North America and species from growing in our industrial- failure could result in harm to people Europe. This disease became known as ized cities. or property. The question then arises: Dutch elm disease (DED), not because Mark Twain is often credited with What should be planted to replace it? it came from the Netherlands but for the statement, “History doesn’t repeat Unfortunately the emerald ash borer the person who discovered it. It was itself but it often rhymes.” The emerald is only the latest in a series of invasive actually three different but closely re- ash borer (Agrilus planipennis) was pests that have recently decimated our lated diseases (Ophiostoma ulmi, O. unintentionally introduced into greater trees. Here, we provide basic informa- himalulmi, and O. novoulmi). Like Detroit in the late 1990s. By 2016, hun- tion on the death of our ash trees and chestnut blight, DED was also intro- dreds of millions of ash trees native to what types of species are less likely to duced from Asia. This time the affected North America and Europe were dead be impacted by invasive insects and trees were the dominant species in the or dying and many millions more were diseases in the future. urban landscape. threatened. Like chestnut blight and The American elm Ulmus( ameri- Dutch elm disease, this insect borer The Loss of Two Giants cana) and related elms were planted was an unintentional introduction The year was about 1904. A fungus extensively in cities during and after the from Asia. Three different problems that became known as chestnut blight Industrial Revolution. Elms were easy were attacking three unrelated groups (Cryphonectria parasitica) was unin- to grow and had high branching that of plants. Our ability to see the connec- tentionally introduced into the US on allowed traffic to pass below a canopy tions and understand the problems can nursery stock imported from Japan. that spread, providing cooling shade in allow us to avoid making the same mis- Within the coming decades it caused the days before air conditioning. These take as we replace our dying ash trees. the death of 4 billion American chest- trees were tolerant of urban pollution nuts (Castanea dentata) growing and compacted soils. They could live Continental Drift and throughout the Appalachian wood- in wet or dry sites and in soils that Species Diversification lands and eastern North American for- were acid or alkaline. In effect, they We need to look back about 200 mil- ests. American chestnuts were massive; were the perfect tree. This majestic lion years ago to the breakup of large clearly the largest species in eastern tree lined entire streets, exceeding 50 super continents. Laurasia formed North America. They routinely reach- percent of the total urban tree canopy North America, Europe, and Asia; ing heights of 100 feet with trunks 14 in some cities. With the demise of the Gondwana separated into India, Africa, feet in diameter. Their branches made elms, people began looking for the South America, Australia, and Ant- up an estimated 25 percent of the can- best replacement. This is when the ash arctica. Plant and animal life on both

Cooperative Extension Service | Agriculture and Natural Resources | Family and Consumer Sciences | 4-H Youth Development | Community and Economic Development was already very advanced. However, Table 1. Common Kentucky landscape plants • A disease or insect pest is more likely as the new continents drifted apart, native to North America, Europe, and Asia with to attack a plant species in the same life-threatening disease/insect problems. different selection pressures sent evolu- Number of species genus than one belonging to a differ- tion along very different survival paths. in genus (Genera in ent genus. Likewise, a plant from a Consequently the chestnuts of North family/ total different genus in the same family is America and Europe were not exposed Genus (common name) species in family)1 more likely to be attacked than a plant to the same diseases and plant feeders Abies (fir) 48 (11/255) from a different family. Plants with as the Asian species. The same is true Acer (maple) 154 (137/1,751) close relatives on different continents for elms, ash and many other species. Amelanchier (serviceberry) 28 (104/4,828) are called disjunct species. Accidental or intentional transcon- Betula (birch) 119 (6/234) Carpinus (hornbeam) 42 (6/234) • The larger the number of species in tinental movement of organisms can Carya (pecan & hickory)a 27 (12/89) a genus, the larger the number of have devastating effects on native spe- Castanea (chestnut) 9 (9/1,101) diseases and insects that are likely to cies that lack effective survival traits. Cornus (dogwood) 45 (12/124) attack these plants. (See Table 1) We generally remain unaware of this Crataegus (hawthorn) 380 (104/4,828) • Genera with few or no relatives that worldwide problem unless we travel Fagus (beech) 11 (9/1,101) are native to other continents are to these other areas and study their Fraxinus (ash) 63 (25/688) less likely to have life-threatening environmental challenges. Juglans (walnut) 21 (12/89) problems. (See Table 2.) Juniperus (juniper) 75(32/166) Plums and cherries (Prunus), oaks Offsetting the Negative Larix (larch) 14 (11/255) (Quercus), and maples (Acer) are Impact of World Trade Liriodendron (tulip poplar)a 2 (6/250) Magnolia (magnolia)a 242 (6/250) examples of genera with hundreds One strategy for preventing the next Malus (apple/crabapple) 62 (104/4,828) of species across all three Laurasian big insect or disease catastrophe is to Picea (spruce) 40 (11/255) continents. These and similar groups avoid excess use of a species. Ideally, any Pinus (pine) 130 (11/255) of plants are widely used in landscapes one genus of plants should not exceed 5 Platanus (sycamore) 9 (1/9) in spite of having serious disease and percent or 10 percent of our landscape. Populus (poplar) 98 (54/1,269) insect problems. If, or more likely when, a new pest is Prunus (plum, cherry) 254 (104/4,828) Not all of these are large trees or aes- introduced into North America we do Pyrus (pear)b 69 (104/4,828) thetic replacements for American elm not want to see a rhyming of history as Quercus (oak) 597 (9/1,101) and ash. Some have traits, including the occurred with the chestnut, elm and Salix (willow) 552 (54/1,269) potential to be invasive, that may make Tilia (linden) 46 (245/4,465) ash. Developing a diverse landscape Tsuga (hemlock)a 10 (11/255) them unsuitable for use in specific land- can prevent the staggering losses that Ulmus (elm) 40 (8/64) scapes. Evaluate and match the cultural occurred with the American chestnut, 1 Nomenclature from: The Plant List: A Working List conditions of the site with the cultural elm, and now our ash trees and the of All Plant Species (http://www.theplantlist.org/) requirements of the plant. resulting loss in percent canopy cover. accessed 18 July 2016. There is no such thing as a perfect This practice also slows the spread of a Native to North America and Asia, but not Europe plant. Species listed in Table 3 are ac- problems that inevitably develop. It b Native to Europe and Asia, but not North America ceptable for use in urban landscapes gives green industry professionals time but should not exceed 5-10 percent of to develop pest management strategies. existing species in the region. Species in In addition it produces a more interest- Table 4 are already greatly over-planted ing landscape throughout the year. Basic Principles of Pathogenicity and should not be used except in ex- Second, we should avoid the overuse Some basic principals of pathology tenuating circumstances. of species that are found on all three and entomology strongly influence former Laurasian continents: North the potential for a disease or insect to The Value of Preservation America, Asia, and Europe. This in- attack a host plant. Insects such as the You may have mature trees in your cludes some of the most common trees Japanese beetle that feed on hundreds landscape that are on the “do-not-plant in our landscapes (Table 1). of different types of plants are rare. list” (tables 1 and 4). This publication These lessons are: does not encourage the removal of • The larger the number of species in healthy trees that are already serving a genus, the larger the number of an important role in making our homes diseases and insects that are likely and cities desirable places to live, work, to attack these plants. (See Table 2.) and play. When these trees are either • Plants without close relatives are no longer safe or do not serve their referred to as monotypic plants. They intended purpose, replace them with tend to have fewer disease and insect a species that is less prone to a cata- pests. (See Table 2.) strophic disease or insect.

2 Table 2. Monotypic or limited number of species Provenance of genus, number of species in this genus2 (number of Cultural and aesthetic characteristics: (The following species are considered Species (common name) Size3 genera in family/total species in family) relatively disease and insect free in their native habitat & North America) Ailanthus altissima (tree of heaven) LT Asia 7 (19/121) This species is highly invasive by suckering and seed. The use of this species is not advised for home landscapes and open parks. If it is used it should only be used where physical confinement is possible. This species is dioecious (male plants and female plants). Only male selections should ever be considered and only where the roots are physically confined by pavement to the base of the trunk. This species has extremely rapid growth and is very urban tolerant. Asimina triloba (pawpaw) MT North America 11 (128/2,106) This is the only member of this family in North America. Can spread by suckering. Produces edible fruit but is self-incompatible (needs cross-pollination from multiple plants for good fruit set). Cercidiphyllum japonicum (katsuratree) LT Asia 2 (1/2) Interesting leaf shape, good fall color, upright habit, must be watered during droughts. Citrus trifoliata (was: Poncirus t.) (trifoliate orange) LST Asia 33 (158/1,730) Considered by many taxonomists to be monotypic in its own genus (Poncirus), others put it into Citrus. Large thorns are produced on stems. Cultivar ‘Dragon’s Claw’ makes an interest- ing container specimen. Cladrastis kentukea (yellowwood) MT North America, Asia 8 (946/24,505) Kentucky native; considered the most attractive native flowering tree. Often flowers in alternate years Cunninghamia lanceolata (Chinafir) MT Asia 2 (32/166) Evergreen conifer. Needles have sharp points. Davidia involucrata (dovetree) LST Asia 1 (12/124) Needs excellent soil, water during drought, flowers have a unique appearance but are sporadic. Eucommia ulmoides (hardy rubbertree) LT Asia 1 (1/1) Monotypic; drought tolerant, very dark green foliage, minimal or no fall color. Franklinia alatamaha (Franklin tree) LST North America 1 (15/370) Requires acidic, moist soils high in organic matter, not drought tolerant. Excellent fall color. Ginkgo biloba (ginkgo) LT Asia 1 (1/1) Monotypic; dioecious (only male selections should be used to avoid the offensive odor of fruit); excellent yellow fall color, leaves drop at same time, very tolerant of heat and drought once established. Coarse and open when young. Gymnocladus dioica (Kentucky coffeetree) LT North America (Asia) 5 (946/24,505) Dioecious (use of male selections is encouraged to avoid litter from fruit); very tolerant of heat, reflected light (parking lots), and drought once established. Kalopanax septemlobus (castor-aralia) LT Asia 2 (40/1,533) Small thorns on trunk and branches. Very tolerant of alkaline soils and heat, reflected light (parking lots), and drought once established. Koelreuteria paniculata (golden raintree) MT Asia 3 (138/1,751) Very tolerant of heat, reflected light (parking lots), and drought once established. Often listed as an invasive species (seedlings are almost always limited to immediately under the tree). Maackia amurensis (Amur maackia) MT Asia 9 (946/24,505) Attractive flowers and reddish bark; tolerant of heat and drought once established. Maclura pomifera (Osage orange) LT North America (Asia, Africa) 12 (40/1,217) Dioecious (male selections are highly encouraged to avoid the excessively large fruit). Very tolerant of alkaline soils, heat, reflected light (parking lots), and drought once established. Metasequoia glyptostroboides (dawn redwood) LT Asia 2 (32/166) Deciduous conifer; tolerant of wet sites; requires acidic soils. Nyssa sylvatica (blackgum) LT North America (Asia) 9 (12/124) Outstanding ornamental tree (dark green foliage, scarlet fall color) but rapidly becoming over-planted. Oxydendrum arboreum (sourwood) MT North America 1 (151/3,554) Outstanding ornamental tree (interesting flowers, bright red fall color) but requires acidic, well-drained soils high in organic matter. Parrotia persica (parrotia, Persian ironwood) MT Asia 1 (22/99) Interesting bark, tolerant of heat and drought once established. Phellodendron amurensis (Amur corktree) LT LT Asia 5 (158/1,730) Dioecious (fruit has an odor like varnish), reported to be slightly invasive; very tolerant of heat, drought, and reflected light (parking lots) once established. Poncirus trifoliata (hardy orange) Asia 1 (159/1,730) see: Citrus trifoliata Ptelea trifoliata (wafer-ash, common hop-tree) LST North America 5 (159/1,730) Interesting fruit, sun or shade, requires well-drained soils. Sciadopitys verticillata (umbrella pine) LST Asia 1 (1/1) Evergreen conifer; monotypic. Very slow growth. Styphnolobium japonicum (Japanese pagodatree) LT Asia 8 (946/24,505) Attractive and prolific yellow flowers and fruit; fruit can be a litter problem. Taxodium distichum (baldcypress) LT North America 2 (32/166) Deciduous conifer; tolerates well-drained to flooded sites; requires acidic soil. Tetradium daniellii (was Evodia d.) (Korean evodia) MT Asia 8 (158/1,730) Tolerant of acidic or alkaline soils, wet or dry sites. Reported to be slightly invasive on wet sites. Xanthoceras sorbifolium (yellowhorn) LST Asia 1 (138/1,751) Striking large shrub or small tree (depending on prujing); rare in commerce, less heat toler- ant; not considered by APHIS to have a high potential to become invasive. Zelkova serrata (Japanese zelkova) LT Asia 6 (8/64) V-shaped branching; tolerant of heat and drought once established, circling roots are a common problem. Ziziphus jujube (jujube) MT Europe, Asia 53 (53/839) Edible fruit; very tolerant of heat and drought once established. 2 Many of the number of species may appear to be inflated because of the splitting of species by taxonomists into multiple groups to account for seemingly minor differences. 3 LT (large tree - 50 feet or larger at maturity) MT (medium tree - 25 to 50 feet tall at maturity) LST (large shrub - 10 to 25 feet tall at maturity) [pruning is often used to determine habit]

3 Table 3. Trees that are currently over-planted and should only be used with caution Species (common name) Size Provenance: Cultural notes: Acer campestre (hedge maple) MT Europe Tolerant of alkaline and dry soils once established Acer griseum (paperbark maple) LST Asia Attractive bark; produces large number of seeds but they are rarely viable (alive) Acer miyabei (miyabei maple) MT Asia Good as street tree Acer saccharum (sugar maple) LT Eastern North America Prefers good soils; excellent fall color; the subspecies A.s. nigrum is more drought tolerant Acer tataricum ginnala (Amur maple) LST Asia Reported to be invasive; excellent fall color and interesting leaf shape; tolerant of hot, dry and compacted soils once established Aralia spinosa (Hercules’ club) LST Eastern North America One of the best plants for hot, dry shade; prominent thorns on trunk Carpinus caroliniana (American hornbeam) MT Eastern North America No serious disease or insect problems but common to Europe and Asia; tolerant of wet sites; interesting bark Castanea sp. (Hybrid chestnut) LT Hybrid of Asian and North American species Resistant (but not immune) to chestnut blight; flowers are odoriferous Castanea pumila (chinquapin) LST Eastern North America Some resistance to chestnut blight; shrubby tree Fraxinus quadrangulata (blue ash) LT Eastern North America Some resistance to emerald ash borer Nyssa sylvatica (blackgum) LT Eastern North America Outstanding dark green foliage and red fall color; becoming over-planted Ulmus sp. (Hybrid elms) LT Hybrid of Asian and North American species Resistant (but not immune) to Dutch elm disease

Table 4. Species that should not be planted (already significantly over-used) Species (common name) Size Provenance: Cultural notes: Acer rubrum (red maple) LT Eastern North America The most commonly planted species in North America; not tolerant of dry sites or alkaline soils Betula nigra (river birch) MT Eastern North America Not tolerant of alkaline soils; surface roots, especially on compacted soils; tolerates wet sites Cornus florida(flowering dogwood) MT Eastern North America Prefers to be in a shaded site with excellent soils; numerous of disease and insect pests related to being planted on less than optimum sites Gleditsia triacanthos inermis (thornless LT Eastern North America Numerous disease and insect problems related to cultural stresses honeylocust) Platanus xacerifolia (London planetree) LT Hybrid of Asian and North Tolerant of urban stresses and poor sites; surface roots especially on compacted soils American species Quercus palustris (pin oak) LT Eastern North America Prefers wet sites but often planted on dry, compacted soils; not tolerant of alkaline soils; numerous disease and insect problems Quercus phellos (willow oak) LT eastern North America Prefers wet sites but often planted on dry, compacted soils; not tolerant of alkaline soils

Finding New and Different have these characteristics. Consumers We trust the decisions of others more Types of Plants must buy and plant these trees based than our own judgement. This caused on what it will look like in a few years. trouble when previous generations Consumer demand ultimately drives For example, the Kentucky coffeetree planted too many elms and ash turning the availability of tree species and culti- ( ) is essentially our cities into monocultures. Planting a vars. However, this is tempered by what Gymnocladus dioica disease and insect free and tolerates diversity of species is the best strategy the wholesale nursery industry grows. the disturbed and alkaline soils that to keep our urban forests healthy and Nurseries must try to predict demand are all too common in cities. This able to provide environmental benefits 4 to 7 years into the future and develop North American native is very toler- for generations to come. efficient production systems to meet ant of reflected light and heat making When it is time to replace the dead it. Wholesale nurseries cannot afford it especially well-adapted to parking ash on your property, begin by looking to get ahead of the curve and grow a lots and along streets. Because it often at the list of suggested trees in Table 2. large numbers of new plants before does not produce branches until it Consider all of the cultural limitations there is a market for them. Education of is 8 or 10 feet tall, it looks like a stick of the proposed site and the aesthetic consumers and tree producers must go in a large pot when consumers see it traits that you want. Your local county hand-in-hand. In time, recommended in the retail nursery. It takes a skilled Cooperative Extension Service, green trees will become more common in the salesperson to convince the customer industry professional (nursery, garden marketplace. that this ugly duckling will grow into center, or landscape management pro- The market requires plants that a beautiful swan. This is true of many fessional) or landscape architect can are attractive and look like a tree at other underutilized trees and shrubs. assist you. If your local retail nursery or a relatively small size. Some species It is human nature for us to make garden center does not have a particu- that make great trees in the residential decisions based on the familiar with lar plant, ask if they can find it for you. and commercial landscape do not or what we already see in abundance. Kentucky wholesale producers can be

4 searched at: http://www.kyagr.com/ Alkaline: Soil pH is measured on a Laurasia (Laurasian): The super conti- marketing/plant/plant-guide.html. scale from 1 to 14. Values above 7.0 nent that broke apart approximately If you are unsuccessful in finding an are referred to as being alkaline or 200 million years ago to form Asia, in-state source consult a mail-order basic. Soils that are too alkaline may North America, and Europe. nursery. have mineral elements that become Monoecious: A species that has male Mail-order nurseries often carry so available that they become toxic and female flowers or these flower unusual or specialty plants. However, or become unavailable. Very alkaline parts on the same plant. Opposite the limitations on the size of materials soils can also kill beneficial micro- of dioecious. that can be shipped means they are organisms. Monotypic: A genus that has only one often smaller than those available from Compacted soil: Soils that are com- living species. a local nursery. A plant search web site pressed have reduced pore space and Provenance: the geographic location such as the one operated by the Ander- increased density. Roots grow poorly where a species evolved. sen Horticultural Library (Minnesota in compacted soil. Water does not Species: (abbreviated as sp. or spp. Landscape Arboretum) called Plant In- infiltrate into or percolate through [plural]) taxonomic group of organ- formation Online can help you. You can compacted soils. isms composed of individuals of find it at: http://plantinfo.umn.edu. Cultivar: A cultivated variety of a the same genus that can reproduce You can search for cultural information species. Cultivars are selected and among themselves and have similar about a plant you are not familiar with grown for their aesthetic qualities offspring. Species is both singular and find a retail source for it. or tolerance for diseases, insects or and plural. Taking a little extra time to make an cultural challenges. informed decision will help to ensure DED: Dutch elm disease; one of three References that future generations do not have the fungi that originated in Asia that is Dirr, Michael A. Dirr’s Encyclopedia heartache and expense associated with lethal to many elm species (Ulmus). of Trees and Shrubs. Portland OR: losing a large elm or ash. Dioecious: A species that has male Timber Press, 2011. Society grows great when old men and female flowers on separate Dirr, Michael A. Manual of Woody plant trees whose shade they know they plants. Opposite of monoecious. Landscape Plants, Their Identifi- shall never sit in. (Greek proverb) Disjunct: species that are geographi- cation , Ornamental Character- cally separated in their native range istics, Culture, Propagation, and Glossary of Terms but very closely related as a result Uses, 5th ed. Champaign, IL: Stipes Acid: Soil pH is measured on a scale of having evolved from a common Publishing, LLC. 2009. from 1 to 14. Values below 7.0 are ancestor. Jones, Ronald L. Plant Life of Ken- referred to as being acid or acidic. Family: A subdivision of an order that tucky: An Illustrated Guide to Soils that are too acidic may have contains closely related genera. the Vascular Flora. Lexington KY: mineral elements that become so Genus (plural genera): a subdivision University of Kentucky Press, 2005. available that they become toxic. of a family; a group of species with Warton, Mary E and Roger W. Barbour. Others may become unavailable. fundamental traits in common but Trees and Shrubs of Kentucky. Lex- Very acidic soils can also kill ben- differing in the lesser characteristics. ington, KY: University of Kentucky eficial soil microorganisms. Gondwana: The super continent Press, 1973. that broke apart approximately 200 The Plant List: A Working List of All million years ago to form Africa, Plant Species. Accessed 18 July 2016 South America, Australia, India, and at http://www.theplantlist.org. Antarctica.

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