Trees and turf release natural herbicides,"allelochemicals", that inhibit the growth of their neighbors and keep other plants from growing too close. Some allelopathic chemicalsretard growth or inhibit germination by disrupting cell division. Some interfere with respiration and other energy-transfer processes. Many affect plant nutrition by inhibiting water and nutrient uptake. In some instances, allelopathy prevents the establishment of a plant or kills established plants, but most often it simply reduces plant growth. Allelochemicals may be absorbed directly from the air, but most must pass into the soil before being absorbed. In the soil, the chemicals may be deactivated by adsorption onto clays or organic matter, or they may be decomposed by microorganisms. The level of toxins in the soil is affected by soil types, drainage, aeration, temperature, and microbial activity. Clay soils drain poorly, and toxins do not leach readily, so toxin-sensitive plants may be at higher risk when planted in heavy soils. The only practical controls to prevent the interaction of incompatible plants.are physical separation and planning ahead since there are no chemical controls available to stop the potential toxicity of one plant toward another.
| Allelopathic Species | Type of Chemical | Affected Species |
| Trees: | ||
| Sugar Maple | Phenolics | Yellow Birch, White Spruce |
| Hackberry | Coumarins | Herbs, grasses |
| Eucalyptus | Phenolics | Shrubs, herbs, grasses |
| Black Walnut | Juglone (Quinone) | Pines (Austrian, Scots, red, white), Apple, Birch, Black Alder, Hackberry, Basswood, Azalea, et al. |
| Juniper | Phenolics | Grasses |
| Sycamore (Planetree) | Coumarins | Yellow Birch, herbs, grasses |
| Black Cherry | Cyanogenic glycosides | Red Maple, Red Pine |
| Oaks | Coumarins, | Herbs, grasses |
| Other phenolics | ||
| Sassafras | Terpenoids | Elm, Silver Maple, Boxelder |
| Balsam Poplar | Green Alder | |
| Southern Red Oak | Sweetgum | |
| Shrubs: | ||
| Laurel -- Kalmia angustifolia | Phenolics | Black Spruce |
| Manzanita | Coumarins, | Herbs, grasses |
| Other phenolics | ||
| Bearberry | Phenolics | Pine, Spruce |
| Sumac | Phenolics, terpenoids | Douglas fir |
| Rhododendron | Phenolics | Douglas fir |
| Elderberry | Phenolics | Douglas fir |
| Forsythia intermedia | Kentucky Bluegrass | |
| Other: | ||
| Goldenrod, Aster | Phenolics, terpenoids | Sugar Maple, Bl. Cherry, Tulip Poplar, Red Pine |
| New York Fern | Phenolics | Black Cherry |
| Bracken Fern | Phenolics | Douglas fir |
| Shorthusk Grass | Phenolics | Black Cherry |
| Clubmoss | Phenolics | Black Cherry |
| Reindeer Lichen | Phenolics | Jack Pine, White Spruce |
| Tall Fescue | Phenolics | Sweetgum, Black Walnut, White Ash |
| Red Fescue, Kentucky Bluegrass | Azalea, Barberry, Forsythia, Flowering Dogwood, Yew | |
| Colonial Bentgrass | Azalea, Barberry, Yew, Forsythia | |
| Perennial Rye | Apple, Forsythia, Flowering Dogwood | |
| Foxtail, Smooth Brome | Populus spp. |
But there is hope for some potential benefits for allelopathic plants
- acting as natural weed killers or pesticides, substituting for chemicals, and promote sustainable agriculture.
- suppressing tree growth might someday reduce the cost of pruning or herbicide applications in conflicts between trees and power lines.
- using allelopathic cover crops (e.g., rye) for weed suppression can decrease reliance upon herbicides
| Table 1. COMPANION PLANTING CHART FOR HOME & MARKET GARDENING (compiled from traditional literature on companion planting) | ||
| CROP | COMPANIONS | INCOMPATIBLE |
| Asparagus | Tomato, Parsley, Basil | |
| Beans | Most Vegetables & Herbs | |
| Beans, Bush | Irish Potato, Cucumber, Corn, Strawberry, Celery, Summer Savory | Onion |
| Beans, Pole | Corn, Summer Savory, Radish | Onion, Beets, Kohlrabi, Sunflower |
| Cabbage Family | Aromatic Herbs, Celery, Beets, Onion Family, Chamomile, Spinach, Chard | Dill, Strawberries, Pole Beans, Tomato |
| Carrots | English Pea, Lettuce, Rosemary, Onion Family, Sage, Tomato | Dill |
| Celery | Onion & Cabbage Families, Tomato, Bush Beans, Nasturtium | |
| Corn | Irish Potato, Beans, English Pea, Pumpkin, Cucumber, Squash | Tomato |
| Cucumber | Beans, Corn, English Pea, Sunflowers, Radish | Irish Potato, Aromatic Herbs |
| Eggplant | Beans, Marigold | |
| Lettuce | Carrot, Radish, Strawberry, Cucumber | |
| Onion Family | Beets, Carrot, Lettuce, Cabbage Family, Summer Savory | Beans, English Peas |
| Parsley | Tomato, Asparagus | |
| Pea, English | Carrots, Radish, Turnip, Cucumber, Corn, Beans | Onion Family, Gladiolus, Irish Potato |
| Potato, Irish | Beans, Corn, Cabbage Family, Marigolds, Horseradish | Pumpkin, Squash, Tomato, Cucumber, Sunflower |
| Pumpkins | Corn, Marigold | Irish Potato |
| Radish | English Pea, Nasturtium, Lettuce, Cucumber | Hyssop |
| Spinach | Strawberry, Faba Bean | |
| Squash | Nasturtium, Corn, Marigold | Irish Potato |
| Tomato | Onion Family, Nasturtium, Marigold, Asparagus, Carrot, Parsley, Cucumber | Irish Potato, Fennel, Cabbage Family |
| Turnip | English Pea | Irish Potato |
Mechanisms thought to create beneficial plant associations include:
Trap Cropping-Sometimes, a neighboring crop may be selected because it is more attractive to pests and serves to distract them from the main crop. An example of this is the use of collards to draw the diamond back moth away from cabbage.
Symbiotic Nitrogen Fixation-Legumes (such as peas, beans, and clover) have the ability to fix atmospheric nitrogen for their own use and for the benefit of neighboring plants via symbiotic relationship with Rhizobium bacteria. Forage legumes are commonly seeded with grasses to reduce the need for nitrogen fertilizer. Likewise, beans are sometimes interplanted with corn.
Biochemical Pest Suppression-Some plants exude chemicals from roots or aerial parts that suppress or repel pests and protect neighboring plants. The African marigold releases thiopen, a nematode repellent, making it a good companion for a number of garden crops. A positive use of plant allelopathy is the use of mow-killed grain rye as a mulch. The allelochemicals that leach from rye residue prevent weed germination but do not harm transplanted tomatoes, broccoli, or many other vegetables.
Physical Spatial Interactions-Tall-growing, sun-loving plants may share space with lower-growing, shade-tolerant species, resulting in higher total yields from the land. Spatial interaction can also yield pest control benefits. The diverse canopy resulting when corn is companion-planted with squash or pumpkins is believed to disorient the adult squash vine borer and protect the vining crop from this damaging pest. In turn, the presence of the prickly vines is said to discourage raccoons from ravaging the sweet corn.
Nurse Cropping-Tall or dense-canopied plants may protect more vulnerable species through shading or by providing a windbreak. Nurse crops such as oats have long been used to help establish alfalfa and other forages by supplanting the more competitive weeds that would otherwise grow in their place. In many instances, nurse cropping is simply another form of physical-spatial interaction.
Beneficial Habitats or refugia—are derived when companion plants provide a desirable environment for beneficial insects and other arthropod, especially those predatory and parasitic species which help to keep pest populations in check. Predators include ladybird beetles, lacewings, hover flies, mantids, robber flies, and non-insects such as spiders and predatory mites. Parasites include a wide range of fly and wasp species including tachinid flies, and Trichogramma and ichneumonid wasps. Agroecologists believe that by developing systems to include habitats that draw and sustain beneficial insects, the twin objectives of reducing both pest damage and pesticide use can be attained.
Security Through Diversity-A more general mixing of various crops and varieties provides a degree of security to the grower. If pests or adverse conditions reduce or destroy a single crop or cultivar, others remain to produce some level of yield. The mixing of cultivars of a single crop, can reduce aphid infestation, as demonstrated with broccoli in University of California research.
Trap Cropping-Sometimes, a neighboring crop may be selected because it is more attractive to pests and serves to distract them from the main crop. An example of this is the use of collards to draw the diamond back moth away from cabbage.
Symbiotic Nitrogen Fixation-Legumes (such as peas, beans, and clover) have the ability to fix atmospheric nitrogen for their own use and for the benefit of neighboring plants via symbiotic relationship with Rhizobium bacteria. Forage legumes are commonly seeded with grasses to reduce the need for nitrogen fertilizer. Likewise, beans are sometimes interplanted with corn.
Biochemical Pest Suppression-Some plants exude chemicals from roots or aerial parts that suppress or repel pests and protect neighboring plants. The African marigold releases thiopen, a nematode repellent, making it a good companion for a number of garden crops. A positive use of plant allelopathy is the use of mow-killed grain rye as a mulch. The allelochemicals that leach from rye residue prevent weed germination but do not harm transplanted tomatoes, broccoli, or many other vegetables.
Physical Spatial Interactions-Tall-growing, sun-loving plants may share space with lower-growing, shade-tolerant species, resulting in higher total yields from the land. Spatial interaction can also yield pest control benefits. The diverse canopy resulting when corn is companion-planted with squash or pumpkins is believed to disorient the adult squash vine borer and protect the vining crop from this damaging pest. In turn, the presence of the prickly vines is said to discourage raccoons from ravaging the sweet corn.
Nurse Cropping-Tall or dense-canopied plants may protect more vulnerable species through shading or by providing a windbreak. Nurse crops such as oats have long been used to help establish alfalfa and other forages by supplanting the more competitive weeds that would otherwise grow in their place. In many instances, nurse cropping is simply another form of physical-spatial interaction.
Beneficial Habitats or refugia—are derived when companion plants provide a desirable environment for beneficial insects and other arthropod, especially those predatory and parasitic species which help to keep pest populations in check. Predators include ladybird beetles, lacewings, hover flies, mantids, robber flies, and non-insects such as spiders and predatory mites. Parasites include a wide range of fly and wasp species including tachinid flies, and Trichogramma and ichneumonid wasps. Agroecologists believe that by developing systems to include habitats that draw and sustain beneficial insects, the twin objectives of reducing both pest damage and pesticide use can be attained.
Security Through Diversity-A more general mixing of various crops and varieties provides a degree of security to the grower. If pests or adverse conditions reduce or destroy a single crop or cultivar, others remain to produce some level of yield. The mixing of cultivars of a single crop, can reduce aphid infestation, as demonstrated with broccoli in University of California research.
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