Not all that long ago, companion planting was looked on as a sort of a fringe ‘science,’ more related to moon worshipers than to serious gardeners. When I first heard about the practice I was in my early 20’s and it greatly appealed to me. The basic premise of companion planting is that some plants actually like to be near each other, while at the same time, there are plants that actually despair in each other’s company. The idea of plants having a soul, of liking and disliking each other, even at the species level appealed to my young sense of idealism.
Companion planting arose out of ancient and on going observations that clearly pointed to some plants doing better (or worse) when growing in the presence of certain other plants. Perhaps it is anthropomorphism to describe these affinities as likes and dislikes, but we humans tend to describe almost everything from our own perspectives. So here ya go: In this view carrots are said to like onions and peas. While potatoes are said to dislike sunflowers and walnuts. Perhaps you have noticed that some of your plants like or dislike being too close to your favorite shrubs or trees. There are actually some explanations that serious gardeners can understand.
While some of the more mysterious effects plants have on each other occur on a molecular level down in the root zone, a lot of it starts in the leaves. Plants absorb carbon dioxide through their leaves, and depending on the species, to a greater or lesser extent they can also absorb nitrogen, sulfur, and other compounds right out of the air. These compounds are used to for the plants life processes, to form vitamins, phytochemicals, essential oils, anti-oxidants, etc. Any absorbed compounds in excess of what the plant needs are sent through the plants vascular system down into the roots. Here is where the real magic begins to happen.
Plant roots secrete a very subtle, moist, jell like substance, a few millimeters into the surrounding soil. Through this substance they exert a profound effect on soil chemistry. This jell is rich in plant produced compounds, and one of its chief roles is to nourish soil micro-organisms that fuel the soil nutrient cycle. It is in this zone that the cation and ion exchanges between the roots, soil organisms, and mineral and humus particles are unfolding. The plants exchange excess carbon, oxygen, and other compounds they have absorbed through the air, or that they have stored in their tissues - for soil and humus bound nutrients that are necessary for their lives. They are also capable of secreting their own home made chemicals that effect the plants growing around them. And they definitely do this, some of them all the time, and some only if the need arises.
Lets imagine a place, like Hoopa, where day time temperatures and night time temperatures can be as much as 50 degrees different, and are often as much as 30 degrees different. This temperature difference is pretty hard on many plants. Plants vascular functions are tied to something called the evapo-transpiration rate. This is fueled by air temperature, and our large temperature differences cause an extra stress on plants. This can create a situation where plants may need more soil borne essential nutrients than is commonly expected.
Now, lets continue to imagine these plants are growing in a place, like Hoopa, where the rain fall is often over 30 inches a year. Perhaps they are growing out there in our yards right now, after a very wet year of nearly 60 inches of rain. As we discussed in a previous article, when rain fall is up, soil nutrient levels can be down. (The cure is organic matter – but it is not a quick fix.) What on earth is a poor plant to do. He can’t move to a different neighborhood, so he endeavors to change the neighborhood.
Some plants have adapted to survive rigorous conditions by either improving conditions for the entire plant community or by attacking the rest of the plant community. They do this, in part, by the chemicals they exude from their roots. These chemicals can improve the bio-availability of soil minerals, or they can inhibit germination of seed, inhibit growth of neighboring plants, or in extreme cases, they can be down right toxic to the plants growing near them. (These selfish plants want to keep all the soil nutrients for themselves.) While science can explain the generalities, it can’t explain what is going on out there in your garden. Because it is all too complex and variable.
What chemicals a plant will produce and exude and how adaptable the surrounding plants are is dependant on a myriad of things that change from time to time and place to place, even inside a single garden. So what’s a gardener to do? First and foremost, trust your own observations. If you have plants that get along just fine, while a book or magazine article claims they don’t, ignore the article. What they are saying may be true in a different soil or climate – but if it works in your garden, who cares.
If on the other hand your petunias (for instance) won’t grow for nothing, but your neighbor’s petunias thrive, look around to see if you might have shrubs or plants that could be inhibiting growth. Look for something growing near your petunias that isn’t found in your neighbors yards. And then adapt. Try planting your petunias far away from your sunflowers, corn, or walnut trees, for instance. Or grow them in raised beds or containers. If all else fails, just claim you are a moon worshiper and don’t have the petunia mojo and grow four o’clocks instead.
Some plants are so good at making growth inhibiting chemicals that they not only exude them from their roots, the exude them from their leaves, they permeate the very tissue of their stalks and branches, and upon decomposition of those leaves the chemicals permeate the soil. However, for every plant that specializes in poisoning other plants, there are a dozen that specialize in thriving anyway. By your own observations and experimentation you will discover which plants make the best companions for the specimens in your own garden, and that is knowledge you can pass down.
In the mean time, when you hear long time local gardeners talking about plants that like or don’t like each other, pay attention. That is the companion planting advice that is most likely to hold true in your gardens. You can smile in reply and say “Yes, it is all because of some magical stuff that happens way down in the dirt.”
For more information ask your book store or librarian to recommend some titles or see: Soul of the Soil, by Grace Gershuny (Chelsea Green Publishing), and Designing and Maintaining Your Edible Landscape Naturally by Robert Kourik (Metamorphic Press).
Stay tuned, next time we will be getting ready to plant our fall greens. Meanwhile, you can probably find me out in the garden, Digging the Dirt.
Copyright 2006 Harvest McCampbell, from my column "Digging the Dirt," published in The Hoopa Valley People Newspaper, Sept. 26, 2006. Posted here with permission. http://www.hoopa-nsn.gov/enterprises/newspaper.htm
