IN THE GARDEN DON'T WORRY ABOUT FALLING AVOCADO LEAVES.

Byline: JOSHUA SISKIN

Q: I have an avocado tree that is quite old. I started it from a seedling in a glass jar and eventually planted it outdoors. It is huge now, but it is very messy as the dried leaves fall year 'round.

Our lawn sprinklers water it regularly. Is it getting too much water?

The tree does produce dozens of large avocados, although many fruit fall off when they are still small.

-- Harriet Jones,

Northridge

A: Continuous leaf fall is integral to the physiology of evergreen trees, whether pines, live oaks, redwoods or avocados. An evergreen, physiologically speaking, is a plant that is always covered with foliage but which also loses its leaves on a continuous basis in order to make room for new growth. Each year, approximately one-third of foliage is shed. All evergreens tend to lose leaves abundantly in response to stress, whether induced by drought, freeze, flood or disease. I would imagine that this summer's blazing and continuous heat has been particularly hard on tropicals such as avocado, whose large and floppy foliage may fall as insurance against water loss.

Heavy leaf fall in any plant signals present or impending water stress. Deciduous trees are those that lose their leaves all at once, such as peach, plum, apricot, maple, crape myrtle, poplar or birch. In their native cold winter climates, it is impossible for these trees to draw water out of the earth during those months when the ground freezes.

The winter dormancy of these trees protects them from death, since their flimsy leaves with their watery sap would be frozen solid if they were still attached to stems and branches. The viscous resin in the sap of cold-climate conifers, by contrast, acts as a protecting anti-freeze during winter months during which needles or scales persist. Deciduous desert plants, such as Mexican palo verde, mesquite, ocotillo and certain acacias lose their leaves in response to summer's drought.

The fact that you are getting plenty of large avocado fruit testifies to the health of your tree. Those avocados that do not reach full size before falling off the tree develop from flowers that were never pollinated, either because of inclement weather that spoiled the pollen or lack of bees in your neighborhood. Commercial avocado growers place beehives among their trees in order to maximize pollination and increase the size of their crop.

I don't think you are watering too much, as avocados need to be kept well-soaked throughout the summer. I would not despair over fallen avocado leaves, as their accumulation and decomposition on the soil surface has been shown to retard the development of a root fungus (Photophores cinnamomi) that is deadly to avocado trees everywhere.

Q: My princess flowers (Tibouchina urvilleana) are blooming beautifully. They are unruly and about 15 feet high. When can I prune them, and how much can I cut them back?

-- Stanley Schwartz,

Sherman Oaks

A: Princess flowers, which are actually large shrubs at maturity, bloom throughout the year in regal purple. In the Valley, they grow best with morning sun exposure. Flowers are produced on shoot terminals which means that to keep plants densely flowering, they should be cut back on a regular basis, at any time, by one-third to one-half or more. As these plants age, they begin to lose their charms, especially when they receive too much sun. Unless well-fertilized after being cut back, new growth will be less than lush and flowering will be diminished.

TIP OF THE WEEK: I received the following e-mail from Alan Pollack of Woodland Hills: ``I was delighted that you encouraged the use of a barn owl box for the control of gophers and other rodents. I designed plans for such a box that I will mail out free of charge to any interested readers. If we ever get cooler weather, I will even build the box for $60, with proceeds going to Wildlife Care of Ventura County, of which I am a volunteer.'' Pollack can be reached by e-mail at Alpat62 (at)aol.com.