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?