Garden Detective: Why do oranges stay sour?

Published: Saturday, Sep. 7, 2013 - 12:00 am

I have an orange tree that is loaded with fruit every year. The oranges are full size, but they are sour. A gardener told me that they need to be fed frequently. This I did and the fruit is still sour. Perhaps I didn’t feed it enough. What advice do you have?

Charles Moy, Folsom

According to UC master gardener Lorraine Van Kekerix, there are many varieties of oranges with a wide variety of sweetness and tastes. Some oranges are sour rather than sweet and are grown primarily for marmalade, or as ornamentals, since people don’t return to pick them more than once.

Sour oranges may also be coming from the root stock. Many orange trees are grafted onto more disease-resistant and/or dwarfing rootstock and sometimes a shoot grows from below the graft and produces less tasty or sour fruit. Oranges could also be sour if they are planted in shade.

Since your oranges have always tasted sour, check to see whether they are growing on a root sucker. If not, the way to get sweet oranges would be to plant a sweet orange tree as described below. Nurseries in the area may have citrus tasting in the winter and spring if you want to taste the fruit before you buy a tree. Contact your local nursery to ask if they have a citrus tasting.

To obtain the best citrus fruit, the tree needs to be planted in an area with good drainage and no standing water near the base of the tree. If you live in an area with clay soil, you can plant the tree in a raised bed to provide good drainage.

Citrus also need full sun to ensure high-quality fruit. For plants, full sun means at least six hours of direct sun a day. If the area where the citrus is planted gets less than six hours per day of direct sun, the tree may grow, but is not likely to produce good quality fruit. For citrus, sun all day helps ensure ripe fruit. After a warm summer to help produce the fruit, some citrus varieties require a period of cold weather to sweeten.

To protect from frost and freeze, citrus trees may also need protection or heating when night temperatures drop much below freezing. You can increase the temperature of your citrus on cold winter nights by using old-fashioned Christmas tree lights or by covering a small tree. The cover should be removed during the day. A sheet or blanket will block the sunlight the plant needs, and the plant could burn when it gets hot under a plastic tree cover.

Citrus trees like regular deep watering in the summer – typically once every two weeks unless we have an extended period of days over 100 degrees. Although citrus fruit production is best with heat and sun, citrus grow best with cool roots. If you apply a 2- to 3-inch layer of mulch under the tree (stopping a few inches short of the trunk itself), it will reduce evaporation from the bare soil and keep the roots cooler.

Citrus do best with a slightly acidic fertilizer. Citrus/avocado fertilizers are commonly available in garden centers. In areas with hot summer growing seasons like we have in Sacramento, it is best to fertilize your orange tree in two or three small doses in the spring. That way the fertilizer does not cause the tree to sprout tender new shoots when they could be killed by a late winter frost or at the hottest time of year.

Full grown citrus like yours need about 1 pound of actual nitrogen per spring growing season. To determine the amount of fertilizer to apply, take the percent of nitrogen (first of the three numbers on the label, for example 20-0-0). For this example, divide 1 pound of actual nitrogen by .20 to calculate that 5 pounds of fertilizer should be applied. If you apply the fertilizer in three doses, you would apply 1.66 pounds for each dose. Apply the fertilizer as directed on the package.

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