Allelopathic Plants

Customers frequently bring us samples of declining Rhododendrons, Azaleas, Kalmia (Mountain Laurel) and Pieris (Andromeda). We usually inquire about the growing conditions the individual plant(s) are exposed to on their properties. Rhododendrons, Azaleas*, Kalmia (Mountain Laurel), and Pieris are all members of the plant family Ericaceae (Heath) family. These plants all require moist acid soil and usually prefer a light, dappled shade. Once we ascertain that the plant in question is receiving these cultural requirements, it is time for the next question. This question is, 'What other plants are growing nearby?' In response we frequently hear, 'There is a large maple, beech, or walnut tree nearby, providing the preferred shade protection.' Although they provide the needed shade, none of these trees, or any of their relatives for that matter, is a good companion plant for Heath family members. The reason for this is that all of them are said to be allelopathic.
Allelopathic plants are those which secrete harmful chemicals into the environment. They do this as a means of enhancing their own survival chances. Plants that have to compete with other plants for soil moisture, nutrients, and light are at a disadvantage. By secreting chemical toxins to inhibit the growth of other plants, the allelopathic plant increases its chances of survival. This natural phenomenon of allelopathy in certain plants was noted by the great Roman naturalist Pliny the Elder, almost 2000 years ago. He noted the deleterious effect of Walnut trees on other nearby plants.
Many plant families exhibit allelopathic abilities. Among those families that concern the home landscaper/gardener are the Maple Family (Aceraceae), the Beech Family (Fagaceae), and the Walnut Family (Juglandaceae). It should be noted that the Walnut family includes the hickories. The non-native, invasive weed tree Ailanthus altissima (Tree of Heaven, Ailanthus) is also extremely allelopathic. The following is a list of plants that are tolerant of allelopathic conditions. Many allelopathic plants will not just poison the soil through their roots, but, will also secrete the toxins into the air through their leaves. This process is known as volitazation. Some Allelopathic plants will also leach toxins from their leaves during heavy rainfall.

Latin Name Common Name
Juniperus   virginiana  Eastern Red Cedar
Picea   abies  Norway Spruce
Pinus   virginiana  Virginia Pine
Thuja   occidentalis  American Arborvitae
Tsuga canadensis  Canadian Hemlock, Canadian Hemlock
Acer   negundo  Boxelder, Ashleaf Maple
Acer   palmatum  Japanese Maple
Acer   saccharum  Sugar Maple
Amelanchier  Serviceberry
Betula  Birch, many species known to tolerate
Catalpa   bignonioides  Southern Catalpa, Indian bean
Catalpa   speciosa  Northern Catalpa, Indian Bean
Cercis canadensis  Eastern Redbud, Judas Tree
Chionanthus   virginicus  Fringetree, Old Man's Beard
Fagus   grandifolia  American Beech
Fagus   sylvatica  European Beech
Fraxinus   americanus  American Ash
Gleditsia   triacanthos  Honeylocust
Halesia carolina  Carolina Silverbell
Liquidambar   styraciflua  Sweetgum
Liriodendron   tulipifera Tuliptree,   Yellow-Poplar, Tulip-Poplar
Platanus   occidentalis  Sycamore
Quercus   alba  White Oak
Quercus   rubra  Red Oak
Ulmus   americana  American Elm Shrubs
Berberis   spp.  Barberry species
Daphne   spp.  Daphne species
Hamamelis   spp.  Witchhazel
Hydrangea   spp.  Hydrangea species
Hypericum  St John's Wort
Rhus spp.  Sumac species
Syringa   spp.  Lilac species
Parthenocissus   tricuspidata  Parthenocissus, Boston Ivy
Wisteria   spp.  Wisteria species
Achillea   spp.  Yarrow
Arisaema   triphylla  Jack-In-The-Pulpit
Hemerocallis  Daylily
Heuchera  Coral Bells
Hosta spp.  Hosta
Phlox ssp.  Phlox species
Rudbeckia   hirta  Black-Eyed-Susan
Sedum Stonecrop
Verbascum  Mullein
Veronica  Speedwell
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