Backyard gardeners with citrus trees are enjoying their freshly squeezed orange juice and lemons for cooking. But as spring approaches, now is the time to pull off the leftover fruit, fertilize, prune and get the trees ready for next year's crops.
The UC Master Gardeners will hold a free class on Thurday, March 16 to educate the public on caring for their citrus trees. Master Gardener Candace Simpson will talk about pruning, fertilizing and watering, frost protection, and identifying and managing pests and diseases.
First of all, she warns, it won't work to save those seeds from the fruit you are eating now to try to grow new trees. It's much better to buy a new tree at a nursery, one that has been grafted.
"The reason they graft," Simpson said, "is to select root stock for compatibility with soil types and disease resistance."
The minimum size should be a 2-gallon pot with a plant that comes just above the knee.
On Saturday, March 4 at the Palo Alto Demonstration Garden, Simpson and other gardeners will plant a kumquat and a mandarin tree, both of which are frost-tolerant. The planting will be at 10 a.m. at 851 Center St. in Palo Alto.
If you already have mature citrus trees in your yard, Simpson offers this advice: The main thing that needs protecting from frost, she said, is the tree's graft union, where the graft comes into the main stock of the tree.
By now, Simpson said, fruit should be mostly harvested and the fruit that is still on the tree should be picked. That way, the tree will properly prepare for next year's harvest with an empty tree and will produce a maximum harvest.
The main enemy facing citrus trees in this area is a pest called a citrus leaf miner. This bug feeds off of new growth on citrus trees. Because of this pest, master gardeners have recently changed their fertilizing and watering advice: The old advice was to fertilize in early spring and again at the end of September. But the new advice is to fertilize only once, in early spring, and to do the pruning then as well.
"Once you prune and fertilize, and the new growth starts, you will get some citrus leaf miner, but that's it," Simpson said. "The bug only comes out for new growth."
The amount you fertilize is dependent on the size of your tree, so seek advice before doing it. Citrus fertilizers include the many trace elements (like iron, zinc, manganese etc) that citrus trees need, Simpson said. There are both organic and synthetic complete citrus fertilizers available in garden centers. Usually all nutrients except nitrogen are in good supply in our local soils, so one can assume that a more general-purpose fertilizer will also work, especially for growers who regularly use compost as mulch.
A full-grown, average size, fruit-bearing lemon, orange or grapefruit tree needs about one pound of nitrogen per year. If you buy a fertilizer that is 10 percent nitrogen (N), you would need 10 pounds of that per year to supply a pound of nitrogen. Smaller trees would need less in proportion to their size. Young non-fruit-bearing trees of any type need much less, about one-tenth of a pound of nitrogen the first two years (doubled each year after that).
Citrus trees are heavy feeders because they keep their leaves all year and have a heavy fruit load, Simpson said. They are also heavy water users. In winter, the trees are trying to support the crop of fruit, so they need to be watered even with the occurrence of heavy rains. In summer, the tree is pulling a lot of water out of the ground. The master-gardener watering guidelines are to water about two times a month in summer, making sure to get soil wet down to two feet. It is best to water evenly throughout the growth period, especially for thicker rind, larger oranges. Uneven watering will cause the rinds to split.
Besides the citrus leaf miner, other pests can attack citrus trees. They include white flies, aphids, mealy bugs and scale. These pests suck on leaves and exude sticky stuff, which then attracts a dirty black fungus to grow on the leaves. If you are vigilant, you can catch the white waxy material (called honeydew) being exuded by pests and wash it off.
"It usually starts on one branch. Sometimes you might want to prune it out," Simpson said.
She said there is one kind of scale that can kill a citrus tree. Scale is an insect that covers itself with a protective shell like a dome. Eggs hatch underneath the dome, producing "crawlers." A tree infected with scale will look like it has white pustules on the bark. In the crawler stage, you can spray with light oil.
"If you keep these things in check, natural predators can take over and eat the pests," Simpson said.
Citrus tree-planting demonstration
Palo Alto Demonstration Garden, March 4, 10 a.m. at 851 Center St.
Class on caring for citrus trees
Palo Alto Rinconada Library, 1213 Newell Road. March 16, 7- 8:30 p.m.