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The Coastal Gardener: Eucalyptus forests aren't poisonous

July 15, 2011|By Ron Vanderhoff
  • Gardening myths, like Eucalyptus poisoning the soil, are repeated so often they often are accepted as fact. In reality, you can grow a garden under mature Eucalyptus, or any other tree species.
Gardening myths, like Eucalyptus poisoning the soil,… (Ron Vanderhoff,…)

I live just outside a large Eucalyptus forest. Out the window, across my garden and over the fence I can see hundreds of old, 50- to 75-foot-tall Eucalyptus. The trees dominate the neighborhood. Underneath these trees the soil is often bare and the landscaping sparse.

I'm sure you've heard it before: "Almost nothing grows under Eucalyptus. Their leaves have a toxic compound in them that poisons the soil."

And, finally, "A Eucalyptus forest is a poison forest."

Unfortunately, gardeners are surrounded by a wealth of myths and folklore. It seems in gardening that if a statement is repeated often enough, eventually it becomes fact. One gardener repeats it to another; that gardener repeats it to a third gardener. Exponentially multiplied, statements like these eventually become facts. In reality, these statements are often nothing more than emotion and frustration.

Allelopathy is a real biological phenomenon by which an organism produces one or more chemicals that influence the growth, survival and reproduction of other organisms around it. Rumors persist that a chemical in the foliage of Eucalyptus "poisons" the soil beneath it, rendering it inhospitable to other plants. This theory is reinforced by driving through a large, old Eucalyptus grove. But is it a chemical in the leaves that is to blame for the meager gardens or something else?

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For gardeners, allelopathic effects of Eucalyptus should be their least concern. When planting under Eucalyptus, research shows us that the primary factor limiting successful gardens is competition, not chemicals. The real culprit in this battle for survival is the inability of most plants to compete with aggressive Eucalyptus for water, nutrients and sunlight.

It is difficult to establish a plant in a large grove of just about any tree species, but especially when the trees are from arid and semi-arid regions of the world. Eucalyptus, native to areas of sparse rainfall, are incredibly efficient at collecting and hoarding any moisture near their roots.

One of the unfortunate stereotypes we learned at a young age is that a tree's root system is pretty much a reflection of its growth above ground — a sort of mirror image. In reality, almost all trees possess roots that spread far wider than its branches. Perhaps most important is that almost all tree roots are in the top 12 to 24 inches of soil, no matter how large or old the tree may be. Trees just don't root very deeply; forget the pretty pictures you drew in third grade.

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