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The Scientific Basis for Companion Planting

By on May 26, 2013

Happy garden companions -- Tricia and Caesar! Learn about happy garden companion plants in our new video.

The expression “companion planting” sounds like garden lore you should have learned at your grandmother’s knee, right?

If you missed out, you can now get the 21st century update on the topic, at the knee of Washington State University Associate Professor Linda Chalker-Scott.

The Science of Companion Planting

Professor Chalker-Scott points out that there is a lot of “lore” and not as much logic in many of the traditionally suggested planting combinations.

Instead of using time-honored charts, she suggests looking specifically at the interactions between the plants. She breaks the kinds of interactions down into three categories—chemical, physical, and biological.

In our latest video, Tricia explains these three reasons for companion planting, and shows you some popular plant combinations.

Chemical Companion Planting

Legumes change the soil chemically when they take nitrogen out of the air and fix it in the soil. For extra nitrogen plant peas, beans, clover, and alfalfa.

Marigolds produce thiophenes, that deter nematodes in the soil.

Physical Companion Planting

People need elbow room and plants need root room. Lettuce, carrots, and onions grow roots that go down to different depths—and these vegetables can grow comfortably side by side.

Biological Companion Planting

This is probably what you think of when you hear the term “companion planting”—the above-ground interactions between different plants.

Plant flowers that act as “trap crops” to attract pests and keep them busy, well away from your vegetables—nasturtiums are a common trap crop to keep the pests away from kale.

Aromatic herbs (think basil, rosemary, lavender, sage) and alliums (onions and garlic) will repel many insects. Mix these in with your more susceptible plants, such as tomatoes. Planting different kinds of herbs and vegetables together is called “intercropping”. Basil emits a strong aroma that will keep some insects from noticing a neighboring tomato plant.

Some plant combos are destructive. For instance, onions will stunt the growth of peas and beans, from the legume family.

Lure the good bugs to your garden with flowers called umbels. These flat-topped flowers are easy dining for many good bugs that have small mouths. Dill is a classic umbel plant; you can plant a mixture of Good Bug attractors with our high or low-growing Good Bug Blend seed mixes. Or open a 24-hour-buffet for beneficial insects with our Good Bug Food painted on wooden garden labels.

Pests and diseases will come back to the same old place if you keep planting members of the same family in the same garden spot. So rotate your crops. Refresh your recollection about vegetable families by taking a quick peek at our article and short video about crop rotation.

For more information we recommend two excellent books.

vegetable gardeners bible A favorite reference on all things vegetable is the 2009 update of The Vegetable Gardener’s Bible ($24.99). Noted for being easy to use, the Bible lives up to its reputation with two pages of simple-to-understand companion planting charts (pages 50-51).




carrots love tomatoes Want all companion planting, all the time? You’ll enjoy Carrots Love Tomatoes ($14.99) with 200+ pages of details about which vegetables, herbs and flowers grow best together.

Companion planting is one more way to make your organic garden a healthy spot to grow vegetables.

  Comments (1)


Thank you for this informative article.

Posted by Laura on Apr. 13, 2015 at 6:54:54 PM

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