Companion planting: intercropping certain vegetables enhances their yield, so long as you choose companions on the basis of science, not garden lore
Clue Tyler Dennis
In nature, living creatures of all sizes and types are brought together in a delicate harmony. We throw off the balance in our vegetable gardens – artificial environments where the gardener, not nature, selects what will be allowed to live and what will be excluded. Gardeners can take a step toward reconciling nature’s harmony through the process of companion planting.
Plants grown together interact and influence each other. These relationships can be beneficial or detrimental. Companion planting is a botanical “buddy system” that takes advantage of the benefits of growing plants together. Teaming the right vegetables and herbs can reduce the need for pesticides in your garden by repelling insects and limiting the spread of diseases. In other cases, plants make good companions because they coexist in a non-competitive fashion, share similar growing requirements, allow for more efficient use of limited garden space – even provide nourishment.
Successful plant companions usually have similar needs for moisture, soil and nutrients – factors their proximity to one another requires them to share. For this reason a pairing of tomatoes, a crop very sensitive to soil moisture, with watercress, which loves to have its roots in water, would be unlikely to serve either.
Mixing plants with differing light requirements is possible, however; tall sun-lovers can be used to cast shade on a crop more sensitive to sunlight or heat. For example, a cool-season crop like spinach benefits from sharing the same row with peppers. As the weather gets warmer, the growing peppers shade the spinach, keeping its quality from declining as rapidly. The spinach will be gone by the time the peppers grow to fill their allotted space.
Another successful combination pairs celery and leeks; the upright nature of the leek permits plenty of light to reach the bush celery plants. In addition, both are potash lovers, making an application of a potassium-rich fertilizer doubly beneficial.
To save space, intercrop an early-maturing vegetable with one that matures late. Lettuce and kohlrabi are good together because the lettuce will be harvested by the time the kohlrabi needs all the space in the row.
Two plants will coexist amiably if their roots occupy different strata of the soil, as do Swiss chard and tomatoes, or potatoes and beans. The deep-rooted vegetables plunge their roots into subsoil, drawing nutrients and moisture from soil layers inaccessible to shallow-rooted crops.
A traditional plant combination, first used by Native Americans, pairs corn with pumpkins or squash. Both crops need a rich, fertile soil; combining them maximizes the use of garden space and sunlight.
One of the most important nutrients used by plants is nitrogen. Nitrogen is abundant in the air, but in a gaseous form that cannot be used by plants. Some plants – notably those that belong to the legume family – have the ability (when aided by certain soil bacteria) to transform nitrogen from the air into a form that can be assimilated by plants. Since the nitrogen is tucked away in nodules on plant roots, the roots of beans and other legumes should be left in the soil after harvest. As the roots decompose, they release the nitrogen, nourishing any plant sharing the same space.
Companion planting can also produce crops that are less affected by pests and diseases. Some gardeners explain this phenomenon with the theory that plants grown among companions have increased vigor and less stress, so insects pass them by looking for weak or sick plants. Members of the onion family, especially garlic, have been used for centuries in intercropping schemes because of their ability to repel insects. Cole crops or carrots planted near garlic are less prone to insect attack.
Some marigolds have a substance in their roots that repels the microscopic soil pests called nematodes. In soils where nematodes are present, marigolds make good companions for tomatoes, beans, carrots and other crops that are susceptible to nematode damage.
When choosing vegetable companions, it is wise to keep closely related plants separated. Vegetables of the same family group like eggplants and peppers, or the cole crops, are more likely to share pests and diseases.
There is, indeed, good ecological basis for companion planting. But it is only one of several cultural practices used to promote better plant health. The best harvests result when companion planting is practiced alongside other good gardening techniques, including green manuring, composting, mulching and crop rotation. All these practices work together to improve the soil and strengthen plants, resulting in better health and vigor and less insect damage.
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