The Coastal Gardener: Eucalyptus forests aren’t poisonous
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?
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.
So imagine the prospects for a small annual, perennial or shrub planted under a large canopy of old, water- and nutrient-hogging trees. Add to this the greatly reduced amount of light under this canopy. Now add the constant deluge of fallen branches and leaves, especially in the case of Eucalyptus, and you can get a pretty good sense of the chance for a successful garden under these trees.
Intelligent plant choices and attentive garden maintenance are the keys for success when gardening under a grove of large old trees. First, begin with plants that are well adapted to dry shade. Next, do your major planting in the fall or early winter, not during the dry hot summer. Start with small plants. This will not only reduce soil and root disturbance to the trees, but it will make digging and planting far easier and the plants will establish better.
Maintain a 2- to 3-inch deep layer of organic mulch, such as shredded cedar or shredded redwood, to conserve moisture in the dry soil. Water the new planting well and often, until the plants are firmly rooted. Remember, most of the water you are applying is going to be stolen by the tree roots. The same is true for nutrition. The soil under the large Eucalyptus in my neighborhoods is almost completely void of any nutrition. The tiny fibrous roots of the Eucalyptus have invaded almost every square inch of soil and have long ago consumed whatever fertilizer may have been present.
Yes, you can grow a garden under a grove of Eucalyptus or any other tree. The soil isn’t poisoned; it’s just dry, filled with tree roots and shady. The right plants, proper planting and the proper maintenance can overcome these obstacles.
RON VANDERHOFF is the nursery manager at Roger’s Gardens in Corona del Mar.
Ask Ron
Question: My plumeria hasn’t bloomed in the past four years. I received it as an unlabelled cutting. It is growing fine, with nice big leaves, but I want flowers. Can you give any suggestions?Carrie, Huntington Beach
Answer: I just listened to John Tarvin and Mike Maertzweiler, of the South Coast Plumeria Society speak again on plumeria culture last Saturday. These are two of the most knowledgeable plumeria experts in California and they discussed this issue. My suspicion is that the plant is not growing in full sun. Even a half day of shade on a plumeria can often be enough to discourage flowering. In addition, be sure the plants are well fed, but use a low nitrogen, high phosphorus fertilizer during the growing season, from about May to September or October.
If after another year it still doesn’t bloom I would remove it and start with another plant. This time buy a labeled variety that has a good reputation in our climate. Even better, join the South Coast Plumeria Society. They meet at the Murdy Park Community Center in Huntington Beach six times a year. Their meetings are amazing, often with more than 200 in attendance. Check them out at https://www.southcoastplumeriasociety.com.
ASK RON your toughest gardening questions, and the expert nursery staff at Roger’s Gardens will come up with an answer. Please include your name, phone number and city, and limit queries to 30 words or fewer. E-mail stumpthegardener@rogersgardens.com, or write to Plant Talk at Roger’s Gardens, 2301 San Joaquin Hills Road, Corona del Mar, CA 92625.