Gardening : How to Care for Mature Citrus Trees

Perhaps no other tree has played such an important role in the Southland as the citrus tree. For many years the citrus industry was a major player in Southern California’s economy. And nowadays citrus trees play a nonpareil role in the landscapes and gardens of Southlanders.

Consider the virtues of this marvelous family of trees. Even if they didn’t bear luscious fruit, their evergreen, lush beauty and exotic fragrance grace landscapes like no other trees; the delicious, healthful fruit are a bonus.

Yet citrus trees can be frustrating to homeowners who don’t know how to care for them properly. Sickly appearing trees and sparse fruit population can be corrected with proper care.

To learn about proper care for home citrus trees, we visited with John Pehrson, a renowned California citriculturist who recently retired after spending 38 years with the University of California.

Pehrson offered the following advice for home-grown citrus trees--keep in mind that this advice pertains to mature citrus trees grown in the soil. Newly-planted trees and those grown in containers require special treatment.

Watering

Pehrson says that because citrus trees are evergreen they lose a lot of water through their leaves; therefore they need adequate soil moisture to remain healthy. The trees, however, should not be over-watered.

How often to water? As there are vast differences in soil types, and the trees require more water during our hot months and less during our cool season, there is no precise rule of thumb. Instead, Pehrson suggests that you regularly test the soil around your trees for its moisture content.

To test for soil moisture, Pehrson says that you should simply use a shovel and take a soil sample. Take your sample from around the tree’s drip line (imaginary soil line that circles the outermost canopy of foliage). Insert the shovel to its full depth and remove a handful of soil. Form it into a ball and squeeze it.

If the soil ball crushes and crumbles easily, the soil is dry and it’s time to irrigate. On the other hand, if the ball of soil stays intact--casts into a mold--then you should not irrigate.

Pehrson says that during irrigation it is best not to get the trunk of the tree wet as this can promote disease. Pehrson suggests digging a circular furrow around the drip line surrounding the tree. Letting the hose slowly run in the furrow will allow good coverage without wetting the trunk. The irrigation needs to be a long deep one; water should penetrate the soil from 12 to 18 inches (a probe inserted into the soil will determine how deeply you are watering).

Drip and trickle irrigation systems are also fine, however, the emitters should also be located away from the trunk area.

Pehrson says that in areas where the soil is salty, the soil needs a very deep watering (to a depth of three or four feet) every few months to flush away excess salinity.

Feeding

Although it is widely recommended that one use a balanced fertilizer on citrus trees, Pehrson says that most Southland soils have an adequate supply of phosphorous, potassium and trace elements, but that nitrogen is the most important element and it should be applied regularly.

For most Southern California soils, Pehrson particularly recommends ammonium sulfate because the sulfur in it helps counteract the alkalinity found in most Southland soils; as a bonus it is an inexpensive source of nitrogen.

As to how much to apply, Pehrson says that a fully grown, mature citrus tree needs 1 to 1 1/2 pounds of pure nitrogen per year. This can be misleading as the fertilizer label usually refers to the percentage of nitrogen found in the fertilizer. For instance, ammonium sulfate contains 21% nitrogen (label reads 21-0-0). Therefore to get one pound of pure nitrogen to the tree, you need to apply almost five pounds of fertilizer. When applying fertilizer, distribute it evenly around the tree and water it thoroughly into the soil.

Pehrson says that compost and other organic materials should also be applied to the soil occasionally to help improve the soil structure.

Pehrson suggests that instead of applying the total amount of nitrogen at one feeding, you spread it out over three smaller feedings: In early spring, early summer and late summer.

If after proper feeding and care, the foliage on your trees is pale or yellowish, there may be an inadequacy of trace elements in the soil, or they may be bound up in the soil and unavailable to the trees. Pehrson says that citrus trees grown in the foothill areas often lack zinc or iron because lime in the bedrock often prevents proper absorption. To correct this condition, Pehrson suggests feeding with a foliage micronutrient spray, which contains the trace elements. Most nurseries stock such sprays--follow label directions carefully.

Pruning

Unlike deciduous, fruit trees require little pruning. The most important pruning job is to remove dead wood. This should be done on a regular basis.

The only other pruning necessary is for shaping or training purposes, to eliminate sucker growth occurring below the graft line in the lower trunk and to remove frost-damaged wood. Frost-damaged wood should not be removed until the summer following the frost damage. This allows the tree to properly recover from the shock of the frost.

Pest Control

Good news for organic gardeners: Pehrson says that in most areas of Southern California, you can avoid spraying citrus trees with pesticides if you faithfully and regularly wash off the leaves of the trees.

Although aphids, mealybugs, mites and scale present problems to Southland citrus trees, Pehrson says that Southern California is rich in predatory insects, which keep pests under control. When you keep the foliage clean it allows ladybugs, various predatory wasps and other “good” insects to better control the “bad” bugs.

When the foliage is dusty and dirty, it impedes the predatory insects in their mission. In addition, by washing the foliage you will regularly wash aphids and other pests off the trees.

To clean the trees, you should use a spray nozzle that will provide a strong blast of water. Do this whenever the foliage accumulates dirt and dust.

Pehrson says that by keeping the foliage clean and letting the predators do their job, most spraying can be eliminated. He notes that while aphids, scale, mites, mealy bugs and others are annoying, they are seldom a major threat to the tree’s health. In fact, Pehrson says that these pests are necessary, at least in small numbers, to provide prey for beneficial insects.

However, if you feel you have an extreme pest problem that needs chemical control, Pehrson recommends you take a sample of the insects to a nursery where they can identify the pest and recommend the correct chemical application.