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You ever wonder why almost nothing will grow under a black walnut tree? Or why old timer gardeners plant marigolds in their vegetable gardens? I’ve heard tales of why this is so, but wondered what else fits into the categories here. Let me tell you research wasn’t easy on this subject, and finding first hand experience was really hard. Whenever I bring up the subject of companion planting to others, they usually think I’m referring to color combinations in the garden. I get responses like "Oh I like Coneflowers with Liatris." Actually companion planting has nothing to do with aesthetics. I even managed to stump almost all of the experts in the chat room.
Companion planting is the technique of combining two plants for a particular purpose (usually pest control). But for all the reasons to combine plants together there are also reasons NOT to combine plants too. There are several ‘flavors’ of companion planting; allelopathy, pest control (repelling with smells and lures), and complimentary crops.

First is the reason nothing grows under the black walnut tree. This tree, and plants like it are Allelopathic. Allelopathy occurs when plants release inhibitory chemicals into the soil or air, to make sure nothing is going to compete with a plant for its share of rooting space, moisture and nutrients. There are many other plants that are allelopathic that we don’t even notice: legumes, grains, brassicas, and marigolds can kill seedlings or limit seed germination. Red clover releases 9 different compounds that prevent new red clover seedlings from sprouting. Wormwood (Artemesia) can interfere with plant growth, especially when interplanted with other herbs. Fennel can inhibit the growth of veggies and herbs. Even wildflowers (or weeds, depending on your outlook) can have allelopathic properties: Milkweed can inhibit the growth of sorghum seedlings, while Peppergrass, evening primroses, and crabgrass can stop germination of other seeds. Sunflowers are widely allelopathic and also inhibit nitrogen fixation.
Marigolds are probably the most widely grown companion plants. Many gardeners swear by their power to repel all kinds of pests. Scientists have only been able to show that marigolds can affect root know nematodes and root lesion nematodes, microscopic soil-dwelling pests. They can be widely allelopathic while growing and decomposing. The roots and, to a lesser extent, the shoots produce compounds called thiophenes. If you turn them under after the growing season, nothing will grow where they grew, and as they decay, they’ll kill anything you plant. But by spring, the soil is safe for planting again.
Allelopathy isn’t always a problem; you can use it to your advantage for keeping weeds and other vegetation under control. Annual Rye is useful for this in the veggie garden, as it decomposes; it suppresses the growth of redroot pigweed, ragweed, and green foxtail weeds.
When you have a plant that just doesn’t ‘do good’ and you’ve met the right growing conditions, etc., look at its neighbors. They could be the culprits.

Many insects use their sense of smell to find their way to favored food. On way to use companion planting to protect your plants is to mask their odors with other powerful smells. Onions and garlic, for instance, release powerful aromas in the air. You can plant onions with strawberries and tomatoes, mint discourages cabbage loopers, and basil repels tomato hornworms.
Some plants have an almost irresistible appeal for certain pests, and you can use this to your favor. Nasturtiums are an excellent attractant plant because they are a favorite of aphids. Attractant plants can protect you crops in two ways. First, they act as decoys to lure pests away from your desirable plants. Second, they make it easier to control the pests since the insects are concentrated on a few plants. Once the pests are trapped, pull out the attractant plants and destroy them along with the pests. Keep in mind, though, that not all insects are garden enemies. You can encourage beneficial creatures by planting their favorite flowering plants. Also remember, that what a vegetable gardener considers a beneficial, a butterfly gardener will consider a pest. Most gardeners consider parasitic wasps and assassin bugs beneficial, but we know that they kill caterpillars. It all depends on what (and why) you are growing. Dill attracts spiders, lacewings, and parasitic wasps – and black swallowtail caterpillars.

Some plants make ideal garden companions simply because they don’t compete, even when planted closely together. Plants like deep-rooted squash and shallow rooted onions. Crops that are heavy feeders combine well with light feeders. Taller plants can provide welcome light shade for ground huggers. Legumes have a mutually beneficial relationship with nitrogen fixing Rhizobium bacteria. The bacteria colonize legume roots, absorbing up to 20 percent of sugars the plants produce. The bacteria use this energy to capture atmospheric nitrogen (nitrogen gas) and convert it into nitrogen compounds that plants use. Part of the nitrogen trapped by the Rhizobium bacteria is released into the soil as the niodule-bearing roots die off and decompose. This nitrogen is available during the season to boost the growth of any companion plants growing nearby. The big bonus comes when you turn the foliage and roots of the legumes into the soil. When they decay, they can release enough nitrogen to feed the next crop you grow.

I took most of the information here from two books: Roses Love Garlic, by Louise Riotte and Rodale's Successful Organic Gardening Companion Planting , by Susan McClure and Sally Roth. Roses Love Garlic looks mostly at the ‘old gardener’s’ wisdom. It tells what to plant or not plant with other plants, but doesn’t exactly tell you why. Rhodale’s is more scientific (as scientific as companion planting can be anyhow). I enjoyed reading Roses Love Garlic (it’s a good read), but I gained most information from the Rhodale book. There is also another good book, called Companion Planting , by Richard Bird.

I took information I found in these books and attempted to make a spreadsheet from them. Hopefully, I did my job in condensing 3 books into one document.   I'm new at importing Excel spreadsheets to the web, so it's a little crude, but hopefully, it will help.  Click here to see it.updated_md_wht_18888.gif (1807 bytes)

What the heck is a host plant?  Feb 2000
What is a butterfly garden?  Jan 2000

Next month's article:  Unlocking the mysteries of a seed


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