It's Citrus Worry Season again

These Washington navel oranges are coloring up quickly, but likely won't be ripe until late December or even later. (Photo: Kathy Morrison)

For gardeners, the ripening of the mandarins heralds the beginning of a new season: Citrus Worrying.

Temperatures have dropped, and frost warnings are already part of the conversation.

Ironically, popular mandarins are among the most cold-hardy of citrus varieties, Owari Satsumas especially. The fruit can endure cold down to 28 degrees F. before it is damaged.

Sweet oranges of all varieties also are likely to withstand cold, to 28 degrees, as are kumquats. But limes, lemons, citron and Oroblanco grapefruit are among the least cold-hardy, to just 32 degrees. (For specific varieties' cold tolerance, see this terrific citrus variety info chart from the citrus wholesalers at Four Winds Growers in Winters.)

So you have your frost blankets ready, right? Here are other things to keep in mind about citrus this month and into winter:

-- Citrus ripens only on the tree, so leave it there even if it has changed color. My Washington navel oranges are turning color a tad early this year. But they generally don't ripen until after Christmas, so I will keep on eye on them through December. How to tell when an orange is ripe: The rind starts to soften just a bit. If you're unsure, pick and taste one!

-- Store ripe citrus on the tree, as long as there are no freezes in the forecast. The exception to this is mandarins, which should be picked as soon as they are ripe.

-- If branches loaded with ripe citrus fruit are bending under the weight, prop the branches up so they don't snap off. A few 2-by-4s are useful for this; for my dwarf navel I've even used tall tomato cages in a pinch. (Thinning fruit in summer will prevent this situation, of course, but there's always a cluster of fruit you didn't catch early.)

-- If frost is in the forecast, water the root zone of your citrus tree before temps drop. Wet soil conducts heat better than dry soil and will help those frost blankets protect the tree. (Do make sure the frost blanket or other covering extends to the ground or you're defeating the purpose of the covering.) You might want to rake any mulch away from the dripline so the soil can absorb daytime heat and be a better heat reservoir.

-- Don't water the branches and leaves of the tree, by the way -- the frozen water will just add to the weight of the branches.

-- If you want to use old-fashioned Christmas lights to help protect your tree, put them on early and leave them there. It's no fun trying to string them on the tree on a cold late afternoon just as the sun is going down.

-- Citrus fruit, of course, is less hardy than the plant itself. If you lose fruit to cold, that doesn't mean the tree is dead. It may be months before you can assess how damaged the tree is, so don't prune off anything: Any apparently dead leaves and branches will protect the rest of the tree during the cold season.

-- Young trees are more susceptible to freezing, so be sure to protect them even if the forecasted temps wouldn't threaten a mature tree.

-- Along a fence or the side of the house are the best places to park citrus in containers. The ambient heat from the house will help keep it warm; the fence is a good place to attach the frost covering. I group my potted citrus together, too.

For more on citrus care, check out the Sacramento County master gardeners' website at The Home Orchard , which includes several links to citrus topics. I also recommend the "Growing Citrus in Sacramento" Garden Note #127 .