The word allelopathy first appeared in print in 1937, defined by Austrian botanist Hans Molisch as “The Effect of Plants on Each Other.” More than 80 years later, scientists regard allelopathy as a chemical language between plants, in which many of the words are volatile organic compounds (VOCs) released by roots, stems and leaves.
Allelopathy in action is companion planting, in which we gardeners presume to know much more than we do about how plants relate to each other. We do know that some plant partnerships work well while others don’t, and it’s always a delight to discover a winning combination in your garden.
Good companion planting depends in part on compatibility of roots and canopy, but we should also consider actual above-ground communication via airborne molecules. Because plants are stationary life forms, incapable of moving about, it is reasonable that they use gases, or “smells,” as communication tools. There is a comparison here with human speech, though gaseous communication is extremely direct. Communication between plants likely consists of very clear messaging such as “You can stay,” or “Get out of my way,” or “I’m under attack from insects.”
Exciting New Developments
Like animals, it appears that plants use their VOC-based communication to get what they need, and to warn their neighbours. A fascinating 2006 study looked at how the parasitic dodder plant locates hosts using VOCs. In a series of lab experiments (illustrated here), dodder consistently located tomato plants as hosts, while rejecting less desirable wheat, based on gaseous messages passing between the plants.
It also appears that plants use VOCs to speak up for themselves, especially in mixed plantings. In nature, plants most often grow among non-related neighbours. Diversity is associated with higher levels of VOC emissions consisting of a more varied array of gases, or increased overall communication. In one study, crimson clover more than doubled its VOC output when grown in the company of orchard grass.
One intriguing recent research finding is that plants seem to enjoy the company of other plants. In a 2009 study with lupin seedlings, plants grew more vigorously in non-homogenous groups. When the stand of seedlings was made up of genetic siblings, they languished compared to seedlings grown with non-relatives. A read of the gaseous signs validates that nature really does dig diversity.
New Roles for Carrots and Herbs
If plants’ lives are in part aroma based, can we cut to the chase and simply douse them with extracts of their preferred neighbours? Perhaps. A team of Bulgarian researchers went back to the companion planting drawing board and studied the effects of simple extracts made from parsley, carrot, dill and onion sprayed on tomato plants. The onion treatment reduced growth by 30 percent, while the carrot extract increased growth by a similar margin. In a different study, a spray made from ground carrot seeds improved the productivity of pea plants and helped control weeds.
A spritz of carrot tea appears to stimulate growth of tomato, as can sprays made from aromatic herbs including basil and Mexican mint. Growing basil with tomatoes is a popular companion planting combination that can be mimicked or intensified by giving tomato plants aromatherapy sessions with basil tea from time to time.
Whether we grow plants as selected partners or use aromatic plant-based compounds to support healthy growth, we are communicating with plants in their own language. It is the only one they know.