Sunday, May 25, 2014

Companion Planting

Now that the danger of frost appears to be past, planting of many seedlings commences. A quick review of companion planting suggestions may prove helpful in planning layout of the plants.  

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. 

Table 1. Some allelopathic plants, the chemicals they produce, and the plants they affect.
Allelopathic SpeciesType of ChemicalAffected Species
Sugar MaplePhenolicsYellow Birch, White Spruce
HackberryCoumarinsHerbs, grasses
EucalyptusPhenolicsShrubs, herbs, grasses
Black WalnutJuglone (Quinone)Pines (Austrian, Scots, red, white), Apple, Birch, Black Alder, Hackberry, Basswood, Azalea, et al.
Sycamore (Planetree)CoumarinsYellow Birch, herbs, grasses
Black CherryCyanogenic glycosidesRed Maple, Red Pine
OaksCoumarins,Herbs, grasses
Other phenolics
SassafrasTerpenoidsElm, Silver Maple, Boxelder
Balsam PoplarGreen Alder
Southern Red OakSweetgum
Laurel -- Kalmia angustifoliaPhenolicsBlack Spruce
ManzanitaCoumarins,Herbs, grasses
Other phenolics
BearberryPhenolicsPine, Spruce
SumacPhenolics, terpenoidsDouglas fir
RhododendronPhenolicsDouglas fir
ElderberryPhenolicsDouglas fir
Forsythia intermediaKentucky Bluegrass
Goldenrod, AsterPhenolics, terpenoidsSugar Maple, Bl. Cherry, Tulip Poplar, Red Pine
New York FernPhenolicsBlack Cherry
Bracken FernPhenolicsDouglas fir
Shorthusk GrassPhenolicsBlack Cherry
ClubmossPhenolicsBlack Cherry
Reindeer LichenPhenolicsJack Pine, White Spruce
Tall FescuePhenolicsSweetgum, Black Walnut, White Ash
Red Fescue, Kentucky BluegrassAzalea, Barberry, Forsythia, Flowering Dogwood, Yew
Colonial BentgrassAzalea, Barberry, Yew, Forsythia
Perennial RyeApple, Forsythia, Flowering Dogwood
Foxtail, Smooth BromePopulus 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)
AsparagusTomato, Parsley, Basil
BeansMost Vegetables & Herbs
Beans, BushIrish Potato, Cucumber, Corn, Strawberry, Celery, Summer SavoryOnion
Beans, PoleCorn, Summer Savory, RadishOnion, Beets, Kohlrabi, Sunflower
Cabbage FamilyAromatic Herbs, Celery, Beets, Onion Family, Chamomile, Spinach, ChardDill, Strawberries, Pole Beans, Tomato
CarrotsEnglish Pea, Lettuce, Rosemary, Onion Family, Sage, TomatoDill
CeleryOnion & Cabbage Families, Tomato, Bush Beans, Nasturtium
CornIrish Potato, Beans, English Pea, Pumpkin, Cucumber, SquashTomato
CucumberBeans, Corn, English Pea, Sunflowers, RadishIrish Potato, Aromatic Herbs
EggplantBeans, Marigold
LettuceCarrot, Radish, Strawberry, Cucumber
Onion FamilyBeets, Carrot, Lettuce, Cabbage Family, Summer SavoryBeans, English Peas
ParsleyTomato, Asparagus
Pea, EnglishCarrots, Radish, Turnip, Cucumber, Corn, BeansOnion Family, Gladiolus, Irish Potato
Potato, IrishBeans, Corn, Cabbage Family, Marigolds, HorseradishPumpkin, Squash, Tomato, Cucumber, Sunflower
PumpkinsCorn, MarigoldIrish Potato
RadishEnglish Pea, Nasturtium, Lettuce, CucumberHyssop
SpinachStrawberry, Faba Bean
SquashNasturtium, Corn, MarigoldIrish Potato
TomatoOnion Family, Nasturtium, Marigold, Asparagus, Carrot, Parsley, CucumberIrish Potato, Fennel, Cabbage Family
TurnipEnglish PeaIrish 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.

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