August 14, 2008

What’s the Secret for Sweet Navel Oranges?

By Robert H. Schuler

Q: I live in Pomona and have a 10-year-old navel orange tree that produces fruit that is inconsistent in sweetness. The fruit is good every year, but some years (three years ago, for example) the fruit was outstanding in sweetness. I follow the directions for fertilizing and sprinkle some citrus pellets and try and deep water every month. Can you explain why the sweetness varies from year to year and can you suggest anything to ensure consistent sweetness? - Don Durboraw, Pomona

A: Navel oranges can ripen anytime from late November through February each season. The growers use a Brix index system for Total Soluble Solids especially in citrus and in the grape vineyards to know the peak harvest time. The longer the fruit is on the tree, the sweeter the orange will be. The pH of the soil should be above 6 and the heat of the summer also adds to the sweetness of the orange (that is no problem in our area).

Citrus does not do well if planted in a lawn area. A bad freeze in the winter could damage the fruit. Watering a day or two before the freeze and again after will help, because moist soil holds more heat than dry soil.

Your deep watering practices (roots can be 2 feet deep) are good, but don't let the soil below 2 inches deep dry out. Water 2 feet from the trunk to 2 feet outside the tree's drip line.

A mature tree can reach 15 to 20 feet. Nitrogen fertilizer is required at the rate of one pound of actual nitrogen per tree. Using (33-0-0) ammonium nitrate would require 3 pounds or 5 pounds of ammonium sulfate (20-0-0). In the Inland Valley, citrus trees do not require additional phosphorous (P) or potassium (K), unless a soil tests indicates it to be needed (usually this is done every two to four years if necessary).

The nitrogen application is required in winter or early spring for flower and fruit setting. I find that half the amount of fertilizer recommended for the winter/spring application and one- quarter application for the summer and one-quarter for the fall works best.

The consensus of the experts I contacted in our area confirmed the above information and noted that besides the nitrogen, our soils need the addition of micronutrients including zinc, manganese and magnesium. This can be applied by chelated formulations or foliar spraying periodically every two months.

The sweetness of the orange depends on the length of time the orange is on the tree, proper watering, nitrogen fertilizer and adding micronutrients to the soil.

Q: I have brown, burnt-looking edges on leaves of my small deciduous trees that are planted in container pots. I am watering almost every day and fertilizing every two weeks. Do I need to add something that will alleviate this problem? - Dawn Nyhus, Loma Linda

A: Near the end of the summer, mineral salts can accumulate at the edges of the leaves and turn them a rusty brown color. If you are fertilizing your plants that often, I would suggest you use one- quarter to one-half of the recommended strength in the spring and summer, about every three weeks.

Give the plants in the containers a good flushing using water only each month and continuing through the fall. Make certain that plenty of water comes out of the container's drainage hole. The new leaves next spring should be fine.

If other plants and trees in your garden have brown leaf edges, deep water at least 6 inches over a few days. We cannot anticipate that we will have a good rain before November.

Q: My cymbidium leaves are turning from dark green to yellow green. Should I be feeding them something this time of the year? - Terry James, Rialto

A: Cymbidium leaves that are yellow green indicate that you have them in the right sunlight. If they were dark green, they need more sunlight. They do not like to dry out, so keep the soil moist. They need good drainage and don't let them stand in water. In their natural habitat, they get rain almost every day during the growing season.

Cymbidiums like lots of fertilizer. Use a timed-release fertilizer pellet that is designed to last nine months, such as 14- 14-14. Add these pellets to the pots in the spring and again now. You can also add some high-bloom formulation (high in phosphate) fertilizer with your watering system during the next three months.

Do you have a garden question? Contact the Master Garden Hot line in San Bernardino at [email protected], call (909) 387-2182 or e- mail Master Gardener Robert H. Schuler at [email protected]

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