Q: I have a Moro blood orange tree that has lots of oranges but they don’t turn sweet. They stay very tart. They are juicy and sour. Is there anything I can do – and I’m thinking of next year – that will help sweeten the oranges?
Bea Lloyd, Sacramento
A: It may have something to do with where your tree is planted, suggests UC master gardener Annie Kempees. When planting edibles such as a vegetable garden or fruit trees, bushes or vines, it’s important that they not be planted in lawns when at all possible. Also, when edibles are planted among non-edibles, such as in edible landscaping, care should be taken to assure that the edible crops are receiving the needed water and fertilizer. Inattention to these factors may result in poor-quality produce that is barely edible or bitter.
Almost all citrus thrive by a few specific rules, Kempees said. Here in the Sacramento region they like full sun and regular moisture year round. “Regular moisture” translates to deep water around the dripline (likening the tree’s foliage to an umbrella where water drips off the edges) once every seven days until established, around 18 months to two years. This also means the ground around the tree, at the dripline, will be consistently moist but not wet.
However, our four-year drought requires citrus growers to be prudent when irrigating, and diligent in applying mulch from the trunk to the dripline. This may result in less-than-optimal crop production.
As for fertilization, many home gardeners don’t fertilize their citrus at all, or use compost/mulch around their trees and get quality fruit. Bear in mind that excessive fertilizer can run off into the gutters and pollute creeks and rivers.
If you choose to fertilize, the following is recommended: A fully bearing, average-sized mature orange, lemon or grapefruit tree (with foliage diameter of 15 to 20 feet) should be fertilized at a rate of about 1 pound of actual nitrogen per tree per year.
To determine how many pounds of fertilizer to actually use, divide the desired amount of actual nitrogen by the percentage of nitrogen on the bag, using a decimal for both. (For example, if you need to apply one-quarter pound of actual nitrogen, and you are using ammonium sulfate which is 21 percent nitrogen, .25 divided by .21 equals approximately 1.2 pounds.)
For help in measuring, a 14-ounce soup can of ammonium sulfate contains about 1 pound of actual nitrogen.
Because adequate levels of nitrogen are required during flowering and fruit setting, late winter or early spring broadcast fertilizer applications to the soil can provide the required nitrogen supply. Some references recommend dividing the nitrogen fertilizer into thirds (early spring, summer and fall) but UC Cooperative Extension specialists point out that high levels of nitrogen fertilizer are to be avoided for oranges and grapefruit during the summer and fall, as that contributes to thicker rind, lower juice content and regreening of Valencia oranges. On the other hand, lemons give a beneficial yield response to moderate nitrogen during the summer.
The “Moro” variety of blood orange is slightly more bitter and less acidic than other blood oranges. Harvest season is December to March. Young trees are slow to come into bearing, commonly taking a year or two longer than navel oranges, and have the tendency to bear heavily in alternate years.
According to dwarf citrus experts Four Winds Growers, Moro blood oranges need warm days to form sugars and chilly nights to develop their distinctive purple-red color.
Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to firstname.lastname@example.org. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:
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