Allelopathic Plants: Lone Killers

These Plants Compete Chemically with Other Plants

May 25, 2008

Some plants just can't get along with other plants. In fact, they might just be out to kill their neighbors.

Allelopathic plants don’t want any other plants near them. In fact, like Greta Garbo, they just want to be alone.

But what plants are allelopathic and how do they affect other plants? Are their chemical attacks specific to one genus, or are they indiscriminate? What are the chemicals they use?

There are many allelopathic plants. Some discriminate, and have a stronger effect on specific plants. Others are indiscriminate. They use a variety of chemicals. Some use phenols. Others use coumarins. Plants also use juglone, cyanogenic glycosides, and terpenoides.

These chemicals affect plants in different ways. Some act on the plant’s respiration, blocking it, and inhibiting the plant’s transfer of energy. Some slow plant growth, or stop cell division, thereby interfering with seed germination. Some allelopathic chemicals interfere with a plant’s ability to take in water and nutrients. In a few instances, the chemical can kill a plant, or prevent it from becoming established, or reduce plant growth.

Allelopathic plants spread their chemicals mainly through the soil (although some may also be absorbed directly from the air). How much chemicals accumulate there depends on how well the soil drains, how much aeration there is in the soil, temperature and a number of other factors.

Chemicals accumulate in clay soils, because clay soils do not drain well. In sandy soils, however, they leach out quickly. So if you’re planting near allelopathic plants, don’t put plants sensitive to the chemicals in heavy clay soils.

Here are some examples of plants that are allelopathic and some of the plants they affect:

Salt Cedars

Sugar maple

Hackberry

Eucalyptus

Black Walnut

Juniper

Sycamore

Oaks

Sassafras

Balsam poplar

Laurel

Manzanita

Bearberry

Elderberry

Forsythia intermedia

Despite the negative effects of allelopathic plants, there are some positive aspects too.

Agriculturalists and scientists are now looking at the chemicals in allelopathic plants for their possibilities as natural pesticides and natural weed killers.

Costs of pruning trees are considerable, especially those trees that are growing up around power lines. Studies are being conducted on the possibilities of using allelopathic chemicals, which inhibit growth to slow or contain the growth of trees along those lines.

Planting cover crops of allelopathic vegetation can lower weed infestation and lower or eliminate use of herbicides.

At this time, there are no ways to reduce the effect of allelopathic vegetation on the plants they affect. The best solution is to put susceptible plants far enough away from allelopathic plants so they won’t be affected.

The copyright of the article Allelopathic Plants: Lone Killers in Desert/Water-wise Gardens is owned by Robert Dailey. Permission to republish Allelopathic Plants: Lone Killers in print or online must be granted by the author in writing.