Marlene Dietrich, in her strong German accent, said this in the old movie ‘Grand Hotel’, but plants say it too, in a silent deadly way. Nature has a way of giving certain plants an advantage over the rest. They contain an unfriendly substance that prevents other plants from growing underneath. They want to be alone.
The word allelopathy comes from two Latin words, allelon meaning ‘of each other’ and pathos which means ‘to suffer’. Allelopathy is the chemical inhibition of one plant to another. The chemical can be in the roots, any part of the plant or even in the soil where it affects the growth development of other competitors. Sometimes it means the plants are aggressive enough to crowd out the competition.
Plants already compete for sun, water and soil nutrients and, like animals, even for territory it seems. Their allelopathic qualities are actually controlling or limiting their surrounding environment causing other plants to decline if they are unlucky enough to seed or be planted too close.
Most commonly, California walnuts trees, Juglans californica and Juglans hindsii , and any walnut tree really, have allelopathic effects on plants growing beneath them. The substance produced by walnuts is called juglone. (Ain’t Latin great!)
Other trees with allelopathic traits include Bearberry, Oaks, Sycamore, Manzanita California Bay laurel, Cottonwood, Forsythia, Tree-of-heaven, Black locust and Eucalyptus. In my garden, oaks and manzanita, Arctostaphylos viscida, are the ones needing their space, so I don’t plant anything under their canopies.
California chaparral plants, like Brittlebush, Encelia farinose, Purple sage, Salvia leucophylla, California sagebrush, Artemisia californica, and Chamise, Adenostoma fasciculatum have been known to be allelopathic. However, botanists now know that bare zones under chapparal plants are caused by animals seeking cover and, basicly “eating at home”
In the garden, when you discover a tree or shrub is resisting any plants growing underneath, you can begin to work with nature. Raking under a tree or spreading mulch to add neatness can help blend these areas into your scheme. You no longer have to fight it, but plant outside the drip lines of these difficult characters.