IN THE GARDEN TREES FLOWER HEAVILY IN VALLEY'S SUMMER HEAT.

Byline: JOSHUA SISKIN

I was taken aback this week by the appearance of flowers where I was not expecting to see them. In the first case, flowers were covering trees which seemed all but dead last winter and. In the second, trees that never bloom in August were doing exactly that.

Last winter, while driving in Sylmar, I shuddered at the sight of several mature crape myrtles that had been hacked back to the trunk. There were half a dozen woody stubs above the trunk of each tree that testified to the previous existence of well-developed branches. The owner, it seemed, had gotten carried away in the process of pruning his crape myrtles. Such a shame, I thought, since after they lose their leaves each fall, these trees are admired for their branching structure and mottled bark.

Now every crape myrtle branch was gone and only two pillarlike trunks remained. All winter long, whenever I gazed at this arboreal carnage, I wondered what could possibly have motivated such radical, overzealous pruning.

But now, in the dog days of summer, I suddenly came upon these trees again and did a double take.

Those very same crepe myrtles that had been pruned into trunk-pillars last winter were heavy with purple flowers. Their sudden beauty illustrated two laws of pruning. First, pruning is an invigorating procedure; the more radically you prune, the more dramatic the new growth or regrowth will be. Second, plants that bloom on the current season's growth - whether roses or crape myrtles - can be cut back severely in the winter without compromising their bloom.

Of course, you have to know when flower buds are produced on your shrub or tree of choice to know when to prune it. Otherwise, you could make a costly mistake. Take a peach tree, for instance. Although shoots on your peach tree may have grown six feet or more during the spring and summer of this year, no flowers or fruits will be seen on these shoots until the spring and summer of 2003. If, during the coming winter, you were to prune all your peach shoots back to the trunk, you would not see any flowers or fruits next year. The flowers and fruits, which you have seen this year were produced on shoots which grew during the spring and summer of 2001.

Before pruning a flowering plant, it makes sense to learn when it produces flower buds and after how much time these buds actually open. If you have a plant whose identity is in doubt, observe it for a full year before making use of the pruning shears.

Idaho locusts (Robinia ambigua ``Idahoensis'') are among the street trees recently planted along Ventura Boulevard in Sherman Oaks. Usually these trees have finished blooming by the first week in July, but I have marveled at their display of magenta blooms throughout the month of August. Their late blooming may be explained by their recent planting combined with our several-weeks-long heat wave.

Flowering out of season is typically a response to stress. A plant thinks it is going to die and therefore desires to make flowers and seeds before its demise. Being planted in the summer is stressful on any plant; that strain combined with the Valley's August heat will result in stress that is sufficient to bring out the flowers on spring-blooming species such as the Idaho locust.

TIP OF THE WEEK: The most popular Valley hydrangeas, known affectionately as mop heads and lace caps (Hydrangea macrophylla), may be pruned through August. They should not be pruned later than that since next year's flowers will sprout from buds on this year's growth. If you are in doubt about pruning your hydrangea, just leave it alone. Plants usually come into their own when they reach several feet in height.