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Exudates & Companion Planting

Organic gardeners have engaged in companion planting for a long time. The squash, pole bean & corn grouping, called the Three Sisters, is a technique that dates back to the First Nations peoples in North America.

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​Squash shades the roots of the other two plants, helps prevent moisture loss in the soil via evaporation and shades out weeds. The corn gives support for the climbing beans, and the beans provide nitrogen compounds the corn & squash need via nitrogen-fixing bacteria on their roots.

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Bean plant roots secrete exudates in the form of sugars that the bacteria consume; in return, the bacteria take up nitrogen from the air in the soil and convert it to nitrogen compounds that the bean plant and its two sisters can use.

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Science can safely say that the "...effects of root exudates on soil processes and microbial populations can lead to some positive effects on neighboring plants." (Bais et. al., 2006) And more research is going on in order to understand in more detail how plants relate chemically under the soil's surface.

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The Magnificent Marigold

​Scientific research—and gardeners' practical experience— has given us some plant associations to try out. For example, African marigold roots exude the chemical alpha-terthienyl, which inhibits the hatching of nematode eggs (Siddiqui and Alam, 1988). These microscopic worms can really savage tomato plant roots, and even kill the plant.

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Experts advise us to plant this variety of marigold where we're going to plant tomatoes about 2 or 3 months before we put in our tomato seedlings. Before planting the tomatoes, chop up the marigold plants and dig them in to your tomato bed.

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You can also plant African marigolds and basil among your tomatoes to discourage above-ground insect pests.

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Companion Planting Suggestions for Us Organic Gardeners

While research continues to try and isolate and study the complex chemical processes happening in, around and in-between the roots of plant species, we organic gardeners can do the following:​

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1. Avoid mono-cropping. Plant flowers and herbs (except Fennel which inhibits the growth of most veggies) among your vegetables to attract beneficial insects and discourage pests. Dill can be anywhere, but not with carrots.​

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2. Use plant groupings intelligently. Do the Three Sisters planting of corn, squash & beans. Use taller plants to provide shade for smaller ones that prefer more shade. Shallow-rooted lettuce can be protected from the sun by the broad leaves of cucumber or squash. Group plants that don't compete for nutrients & that discourage their companion's insect pests. Potatoes & bush beans are good companions for these reasons.

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3. Make use of cover crops and green manures like fall rye to inhibit weeds and add organic matter to your soil.

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4. Experiment with the info presented on various companion plant sites on the Internet such as Westcoast Seeds:

​​​​​​​​​​​​http://www.westcoastseeds.com/topicdetail/topic/companion-planting/

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References

1. R. Krueger, K. E. Dover, R. McSorley, K. -H. Wang.  Marigolds (Tagetes spp.) for Nematode Management. The Entomology & Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published: August 2007.

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2. Harsh P. Bais, Tiffany L. Weir, Laura G. Perry, Simon Gilroy, and Jorge M. Vivanco. The Role of Root Exudates in Rhizosphere Interactions with Plants and Other Organisms. Annu. Rev. Plant Biol. 2006. 57:233–66. 2006.

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3. Harsh Pal Bais, Sang-Wook Park, Tiffany L. Weir, Ragan M. Callaway and Jorge M. Vivanco. How plants communicate using the underground information superhighway. TRENDS in Plant Science Vol.9 No.1 January 2004.

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