Companion Plant Associations
Science, Tradition & Experience
The Art and Science of Plant Associations
Generations of gardeners have practised the art of 'companion planting'. We say 'art' because much of the 'companioning' was, and is, based on gardeners' personal observations and a dependence— more or less— on information that some scientific sources label as 'folklore', not necessarily rigorously-researched scientific fact.
Be that as it may, experienced gardeners do what works for them, so we encourage you to do your own research, consult traditional as well as scientific sources & then experiment. Gather your own data in the beneficial plant associations area. We try to.
What are Good Plant Associations?
Mutually beneficial groupings of vegetables, herbs & flowers are based on above-ground and below-ground beneficial exchanges among plants.
Above ground, some plants can provide necessary shade for others. Some can crowd out weeds. Some ward off or attract insect pests, or even attract beneficial predatory insects.
Below ground, some plants produce chemical compounds that not only protect themselves, but other plant species around them as well. And some chemical exchanges between two different plants' root systems can mutually enhance the health & productivity of both.
Peppers, Sweet Basil & Marigolds among the Tomatoes
It's Busy Underground
There's a great deal of meaningful activity happening beneath the soil's surface.
It's a jungle down there. There's the obvious competition for water, nutrients and space among weeds and vegetables. But there's even more going on: with their own survival & reproductive interests in mind, many plants produce substances that discourage and even kill other plants, insect pests, microbes and disease-causing bacteria and fungi.
This capacity to inhibit other plants' growth or even kill them is called allelopathy. One of the most well-known examples is the allelopathic effect of the compound called juglone, produced by the roots and leaves of Black Walnut tree (Juglans nigra) on plants that endeavor to grow under its canopy.
But it's also cooperative down under the soil's surface, too. Many plants form beneficial nutritional relations with other plant, fungal & microbial species, growing all the better in close proximity to them. Plants' associations with mycorrhysal fungi are a case in point. The tiny filaments of the fungi assist plants to better absorb water and nutrients from the soil; in return, the fungi absorb sugars produced by the plant.
Chances are that if your soil is healthy and unpolluted by synthetic fertilizers and pesticides, mutually beneficial relations among your plants, microbes and fungi are already well-established. You just need to support them with compost, mulch, and the right amounts of moisture.
Root Exudates & Plant Defenses
Plant roots and the soil around them are very alive chemically, and the compounds roots secrete into the soil around them are called exudates.
According to H.P. Bais and colleagues (2004), plant roots can sense specific pathogens and exude the compounds necessary to ward them off. Needless to say, healthy plants have this chemical capacity in spades; stressed plants do not. All the more reason to plant all your veggies in the light, soil and water conditions that suit them best.
Root exudates play a profoundly important role in the creation and sustaining of plants' mutually supportive chemical exchanges with beneficial microbes and fungi. This is why it pays to plant the right plant in the right place and to add microbial support to your garden regularly: compost, green manures, mulch & the right amount of water.
