December 31, 2016
“I want to be alone.” That’s what the California bay laurel I cut down seemed to be saying to me, and to anything I planted near its stump. Nothing thrived even though the same plants were doing great just a few feet away. I was puzzled. But then I read that the California bay laurel exhibits allelopathic traits and it all became clear.
Allelopathy comes from the Greek word allelon (mutual) and pathy (suffering). When it comes to plants, it means that chemicals released by one plant have a negative effect on another plant. These chemicals are released by the plant itself or may be emitted during decomposition. Allelopathy is a survival mechanism that allows certain plants to better compete with nearby plants by affecting their nutrient uptake, root development, or seed sprouting. Allelopathic plants don’t just not play well with others; they kill others.
Ancient Greeks noticed how chickpea plants exhausted the soil and killed weeds. Ancient Romans observed walnut trees injured anything planted in its vicinity. A modern-day, comprehensive list of allelopathic plants is difficult to come by. Marin gardens with acacia, maple, manzanita, pine, fir, walnut, eucalyptus, ginkgo, hackberry, sycamore and locust trees might notice that they want to be alone.
Allelopathic traits are not limited to trees however. Ferns, rhododendrons, elderberry, artemisia, even some grasses (fescue and Kentucky bluegrass in particular) like to fly solo.
Even one of summer’s most beloved flowers is a culprit. Have you ever noticed that nothing grows well under a bird feeder that contains sunflower seeds? It turns out all parts of a sunflower contain allelopathic toxins that inhibit seed germination and growth. These chemicals are so effective that they are being studied for their use in weed control.
Please know that there is no reason to panic. Allelopathy may sound like horticultural hell but it can actually refer to harmful or beneficial effects of one plant on another. In one study, wild mustard was interplanted with broccoli, and crop yields increased by as much as 50 percent compared with the broccoli that was planted alone. That’s because the wild mustard and the broccoli are both in the brassica family, and together suppressed weed competition. Studies like this show prospects for allelopathy being utilized for increasing crop productivity and decreasing our reliance on synthetic pesticides and herbicides.
If allelopathy sounds like a convenient rationale for why plants aren’t performing in your garden, I have two words for you — nice try. There are many reasons why plants underperform, and allelopathic traits are not super high on the list. One plant might be hogging water or blocking sunlight from a neighboring plant, but that doesn’t mean it’s allelopathic. Though it’s challenging to separate the effects of conventional competition from chemical interference, those crafty botanical scientists have become increasingly adept at isolating specific plant chemicals that are toxic to nearby plants. Also, some plants are immune to allelopathic chemicals — that is, they can put up with the bullying. For instance, tomatoes and potatoes won’t grow near a walnut tree, but onions and beets do fine.
So what does a plant suffering from allelopathic toxicity look like? It’s really difficult to pinpoint, as different plants use different chemicals with different effects. However reduced seed germination and seedling growth is a common sign, as are wilting yellowing leaves that do not respond to water and eventually die.
If you have allelopathic plants in your garden, dig in organic material such as compost into the soil and add a layer of mulch on top. Allelopathic toxins leach out of healthy, well-draining soil faster than in heavy clay soil where they build up. Grow vegetables in raised beds lined with weed cloth to keep the roots of allelopathic plants out. Try different plantings that may not be sensitive to allelopathic toxins. Or, if all else fails, simply grant an allelopathic plants its wish and let it be alone.