We have a 4-year-old dwarf mandarin tree that blooms every spring. It gets little fruits and then they all fall off.
The tree is in the full sunlight all day long. We water every day and fertilized it with Miracle-Gro that contains magnesium and iron.
Johann Huber, Rocklin
According to UC Master Gardener Veronica Simpson, mandarin fruit is tender. Hot winds and excessive heat during flowering and fruit set cause fruit drop and sunburn.
Citrus need moist soil, but overwatering is not good. Drainage is very important. Checking your soil for drainage is a must.
Does it hold the water too long or drain quickly? Try testing the drainage by digging a small hole and examine the soil in the rooting zone to about 1 foot deep.
Soil lightens in color when it is dry. Feel the soil. It should hold its shape when squeezed or rolled into a ball when wet. If it does not, it's too dry.
These same guidelines apply if your tree is in a container. It's important to determine the amount of water it needs.
As for fertilization, many home gardeners don't fertilize their citrus at all, 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-size mature orange, lemon and grapefruit tree (15 to 20 feet foliage diameter) planted in the ground, should be fertilized at a rate of about 1 pound of actual nitrogen per tree per year. This same formula applies to mandarins.
To determine how many pounds of fertilizer to use, divide the desired amount of actual nitrogen by the perentage 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 that is 21 percent nitrogen, divide .25 by .21, which equals approximately 1.2 pounds.
For help in measuring, a 14-ounce soup can of ammonium sulfate has about 1 pound of actual nitrogen.
Because adequate levels of nitrogen are required during flowering and fruit setting, late winter or early spring fertilizer applications broadcast over 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.
Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties.
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