GARDENING : CALIFORNIA-GROWN CITRUS TREES BENEFIT FROM COLD NIGHTS.

Byline: Joshua Siskin

If you live in Los Angeles and have room for only one fruit tree, you might consider planting an orange, grapefruit or a lemon. If you make the citrus decision, you will have a ready supply of vitamin C for most of the year, since citrus can flower over a long period of time, yielding a steady crop of fruit for several seasons.

In addition, the fruit of many citrus varieties can be stored on the tree, since after ripening it will not drop on its own and retain its freshness for several weeks or more.

Curiously, this ability to hold onto the tree is not a natural tendency. If you go to the part of the world where all citrus species originate - tropical China and Southeast Asia - you will find that citrus fruits immediately fall from the tree after ripening. In their equatorial habitat, there is no reason for citrus trees to hold onto their fruit, since the warm and moist conditions required for seed germination are ever present. With this in mind, never allow citrus seeds to dry out if you plan on germinating them.

In addition to staying on the tree after ripening, there are other advantages California-grown citrus has over citrus cultivated in its native climate. Because of the unfluctuating temperatures in the tropics, citrus rinds may remain green even as fruit matures on the inside. This makes it impossible to determine which fruit should be picked. Some harvested fruits will need more ripening, and some will be overripe.

When people from equatorial countries arrive in California, they are astonished by the glowing oranges and yellows - which we take for granted - or our backyard citrus. It is our daily temperature fluctuations and colder nights, as compared to the tropics, that give our fruit its ornamental quality.

The large temperature difference between day and night is also responsible for a more balanced flavor in California citrus. Tropical citrus fruit is much sweeter because it has less acids, but not necessarily tastier, than citrus grown where temperatures fluctuate. In fact, our citrus is richer in both acids and sugars and thus has a rounder, fuller flavor than that of tropically grown fruit.

Some homeowners mistakenly believe that the amount of watering affects the taste of citrus fruit. There are two possible explanations for the fact that your oranges, grapefruits or tangerines have no taste: 1) You have a tree that was grown from a seed (seedling trees, as opposed to named, grafted varieties often have insipid fruit). 2) You have a named variety, but it is inappropriate to our climate. Many grapefruit varieties, for example, need the kind of desert heat they would get in Palm Springs in order to sweeten up; in our climate, they look beautiful on the outside, but have no flavor. Desert grapefruit with pink pulp also may have white pulp when grown in Los Angeles.

To see whether your soil is appropriate for growing citrus, conduct the following test, as recommended by Julius Sauls, extension horticulturist for Texas A&M University: ``Dig a post hole 3 to 4 feet deep and fill it with water. All water should drain from the hole within 24 to 36 hours. Soils requiring more than 48 hours to drain completely should be avoided, unless raised planting beds are used.

One of the most common questions about citrus concerns splitting of the fruit. This phenomenon occurs just before the fruit is fully mature, immediately prior to or during the harvest season, which, for many varieties, is in the fall and winter. Splitting usually is brought on by a heavy rain, following a period of drought. This year, for instance, we had our first rain after a record number of consecutive rainless days. A sudden influx of soil moisture encourages rapid growth of the interior fruit, which outpaces growth of the rind, which causes the rind to split.

The way to prevent fruit splitting is by a consistent fertilization and irrigation regime. For mature trees, extension specialist Sauls recommends monthly fertilization between February and October, sprinkling four cups of a fertilizer that contains 8 percent to 13 percent nitrogen or two cups of a fertilizer that contains 17 percent to 21 percent nitrogen with each application. In the San Fernando Valley, citrus trees should be deep-watered once or twice a week from February to October, depending on the weather, and proportionately less as cool and rainy days begin.

Tip of the week: Citrus trees may be pruned any time of the year without damaging them. However, to minimize crop losses, February is recommended as the month for pruning, since March is when flower buds begin to develop on many varieties. Citrus trees can be thinned or topped, whether to encourage light penetration or make it easier to harvest the fruit. However, it is not wise to remove more than a third of the canopy at any one time.