Bad Neighbors

Master Gardener Cheri Melton has been writing about companion planting-grouping plants that seem to be particularly content in each other's company- but what about the flip side? What about plants that don't get along together and shouldn't be planted near each other?

Plants that need lots of lebensraum and are antisocial to other plants, often even their own off spring, exhibit what botanists call allelopathy. They produce natural herbicides called phytotoxins and surround themselves with a chemical barrier that kills other plants by attacking their root systems or seedlings. Phytotoxins are most commonly produced in roots and leaves and usually reside in the soil as either water soluble solids or volatile gasses. Many examples of allelopathy occur in arid climates such as our own where, because of the great competition for water and nutrients, plants find it advantageous to kill off their rivals.

Salvia leucophylla (a sage) is a classic example of an allelopathic plant growing in the wild. Where these salvias invade grasslands in California, there is a zone 1 to 2 meters in diameter around each clump that is devoid of herbaceous plants. Natural herbicides produced by these salvias are highly volatile chemicals that have been found to interfere with the germination of seeds of herbaceous plants and are found both in the air around and in the soil adjacent to the plants.

In addition to the general allelopathy described above, some plants exhibit a selective allelopathy that makes them bad neighbors for only certain other plants. These are the plants that shouldn't be grouped together in a garden. Many gardeners know, for example, that Black Walnut (Juglans nigra) is incompatible with a large variety of garden plants - including, pine, birch, azaleas, rhododendrons, domestic grapes, tomatoes, and potatoes - but is compatible with others such as, hickory, oak, poison ivy (of course!), salvia, and wild grapes. Radishes provide another example of allelopathy. The subject of allelopathy is not without controversy, however. For example, in 1994 Marcus and Burz published an article in The American Biology Teacher (Vol. 56, No. 3, pp 180-81) titled, "A simple demonstration of allelopathy." In that article they describe an experiment that showed that radishes are allelopathic to lettuce. In 1996 Santaniello and Koning, publishing in the same journal (Vol 58, No. 2, pp 102-03), describe an unsuccessful attempt to duplicate the results achieved by Marcus and Burtz. The lesson to be learned from these articles is to approach the subject with skepticism. Establishing that an allelopathic relationship exists between two plants can be tricky. Never-the-less, gardeners should be aware that such relationships do exist and be on the lookout for them. Jerry Baker, in his book. Talk to Your Plants, describes some of the common vegetables he believes are allelopathic to each other. Here is his list. You can be the judge as to whether he's right or wrong about them.

* Asparagus- onion, leeks, garlic, beans

* Cabbages- strawberries, tomatoes, pole beans

* Carrots-dill

* Chives-peas

* Cucumbers-herbs

* Peas-onion, garlic

* Pole beans- onions, bush beans, sunflowers, kohlrabi, beets

* Potato- pumpkin, sunflowers, tomatoes, cucumbers, raspberries

So next time your potatoes croak ask yourself, "Who did it? Was it the pumpkins? The sun flowers? Cucumbers? The tomatoes? Or the raspberries?" Of course there's always the possibility that it was the gardener!

Gary Gruenhagen
April, 1996