When two living organisms, such as plants, live together, it is called “symbiosis.” Companion planting, the study of beneficial symbiotic relationships among plants, can help predict whether plants can be either beneficial or harmful to one another when designing your garden plot.
There are many ways that plants affect each other. Here are a few:
Better Growth – Some plants simply grow better around certain plants, and not as well with others. No one yet knows the exact science of why. Example: Green beans grow well with strawberries. Bibb lettuce grows well with spinach.
Nutrition – In the plant world, there are “heavy feeders” and “heavy givers.” Heavy feeders are plants that take and keep a lot of nutrients from the soil as they grow. A lot of the plants we love to eat (tomatoes, lettuce, corn, and squash to name a few) are heavy feeders. Heavy givers, on the other hand, are plants that give back a lot of nutrients to the soil. Nitrogen-fixers or legumes such as peas, beans, alfalfa, clover, and vetch are examples of heavy givers. After growing heavy feeders in a particular area, it’s a good practice to plant heavy givers to restore the soil.
Accumulators – Many wild plants including those we consider “weeds” play a vital role in the plant community as doctors and healers of soil. Many weeds either collect trace minerals from the soil for future fertilization or remove harmful elements from the soil. Example: pigweed, lamb’s quarters, and thistles have deep roots to bring up minerals from the lower soil, which they hold in their stalks and leaves. When they die and decompose into the topsoil, these minerals become available to crops with shallower roots.
Pest Control – Some plants give off chemicals from their roots or leaves. Example: certain kinds of marigolds release a chemical from their roots that are absorbed by plants around them. When whiteflies come to suck on the leaves of those plants, they think they are eating the bad-tasting marigolds and they leave.
Physical – Many plants have a special need for sunlight or shade. One plant can help provide these conditions for another. Example: Lettuce plants like to nestle among other taller plants for shade.
Pollinator Attractors – Many plants reproduce with the help of beneficial insects and other pollinators. By putting plants that pollinators love in your garden, you will help the other plants as well! Example: Bees love hyssop, thyme, catnip, and lemon balm. Once you bring bees into the garden, they will help pollinate everything.
Within a garden, plant one set of companion plants close to one another and another set farther away from each other on the same day (carrots and leeks are great for fall for example.) Hopefully, when brought together, the plants are growing visibly healthier or bigger than the other plants which are separated.