Australian willows make good street trees because of their well-behaved roots and tolerance to harsh conditions. They also display delicate, pendulous foliage.

Photograph by Shari Kaplan

Proper planting can help control pests in gardens

By Tony Tomeo

I can't imagine how garden pathogens were controlled years ago when generous applications of toxic and nonspecific pesticides were so common. It should have been obvious that such insecticides would also eliminate the predatory insects that would have helped control the target pathogen. Fortunately, contemporary integrated pest management, known as IPM, exploits the natural ecology of insects and nontoxic pesticide alternatives.

Pest management begins with the selection of species in the garden and the maintenance of their health. Many species are much more susceptible to various pathogens than others. The degree of susceptibility to endemic pathogens among each species should be considered and evaluated prior to installation in the garden. For example, if scale (brown soft scale) is often apparent in the garden, holly oak should not be installed because it is so susceptible. Any pathogen is more likely to proliferate among large quantities of the same species simply because there is more host material.

Specimens of any species, including those that are more resistant to pathogens, are much more likely to be infected by a new pathogen if already stressed from environmental conditions or previous infections. Such stress diminishes resistance to pathogens. Therefore, a healthy garden is less likely to experience severe insect or disease problems.

Sanitation is also very important. Too much foliar litter or fallen fruit provides homes for overwintering or breeding insects and fungal organisms. Some rot diseases cause the dehydrated infected fruit to hang on trees as "mummies" from which the disease emerges when new fruit appears the following summer. All foliar and fruit litter should be removed as it accumulates, particularly from specimens that exhibit active pathogens.

Tolerance of minor insect infestation promotes functional ecological activity in the garden by providing sustenance for predatory insects. As long as the predators are well-fed without totally eliminating their "host" insects, they will remain. Some garden enthusiasts release ladybugs or mantises in the garden as supplemental predators.

Conversely, nonspecific insecticides severely disrupt this process by eliminating predators. Target insects may proliferate afterward because the predators do not. This could be compared to removing all rabbits and foxes from an ecosystem. Without foxes, the rabbits proliferate, because they breed like ... rabbits, and because there are few foxes to control them. This prolific breeding is necessary to accommodate for the quantity normally eaten by foxes. However, foxes have few predators and do not need to reproduce so prolifically. Consequently, their population recovers slowly.

More information concerning IPM is available from the West Valley Clean Water Program by calling 408.354.5385.

Tree of the Week: Australian willow

Three weeks ago, I discussed a small and manageable willow substitute, the mayten. Now I would like to discuss a willow that is not a willow at all--the Australian willow, Geijera parviflora. This is one of those trees that some people like and some don't. I personally think it lacks aesthetic appeal. However, it exhibits so many redeeming qualities that it is a very appropriate tree for many situations.

The trees, once established, tolerate dry conditions and severe exposure that might scald other trees. It's roots are typically deep and agreeable to nearby concrete. Consequently, the Australian willow is very suitable as a street tree. The main limbs curve out and away from the trunk before they curve upward, and may require considerable structural pruning for clearance over traffic.

The narrow, 3- to 6-inch-long leaves hang down from the delicate smaller branches. The finely textured, evergreen foliar canopy produces light shade below. Australian willows are typically less than 20 feet tall and wide but can get larger. Growth is slow or moderate, but enhanced with regular (not too much) irrigation. Root rot can occur if the soil is too saturated. Insect problems are very rare. Despite the delicate appearance of the Australian willow, it is otherwise very resilient.

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Horticulturist Tony Tomeo can be contacted at 408.358.2574 or at LGHORTICULTURE@aol.com.

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