How cold is too cold for my citrus tree?

But lemons, citron and Oroblanco grapefruit are among the least cold-hardy, to just 32 degrees. Some lime varieties can handle temps down to 30 degrees, others to 32. (For specific varieties' cold tolerance, see this terrific citrus variety info chart from the citrus wholesalers at Four Winds Growers in Winters.)

Location also is a factor. A tree is an open area, with no fences or building nearby, will be more susceptible to cold than one in a small backyard with shrubs nearby -- the ambient heat adds to protection.

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

– Watch and track the forecasts! With the changeable climate, frost can sneak up on gardeners who are busy with holiday activities. I often check my phone’s weather app, then cross-check it against the National Weather Service’s Sacramento area forecast. Adjust the site for your zip code, and remember that your garden may be in a microclimate that runs warmer or colder than even a half-mile away. Frost, for the record, occurs when temperatures fall into the mid 30s and winds are calm.

-- Citrus ripens only on the tree, so leave it there even if it has changed color. My Washington navel oranges are turning color early again this year. But they generally don't ripen until after Christmas, so I will keep an 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 is no freeze (32 degrees or below) 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 my dwarf navel I repurpose my tomato cages for this, since they can support more than one branch. (Thinning fruit in summer helps prevent this problem  – and I did thin this year –- but there's always a cluster 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 on 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. Ask me how I know this.

-- If you do 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. Grouping pots together also helps.

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.