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How cold is too cold for my citrus tree?

Age and variety of tree, plus location, make a difference

With overnight temps in the 30s, we look outside and worry. Will frost harm my lemon tree? Will we lose the navel orange crop? What about the little lime tree planted earlier this year?

Well, it depends. Different varieties are more cold-hardy than others. Location makes a difference, as does the age of the tree. And usually the tree itself is more impervious than its fruit.

Interestingly, the Owari Satsuma mandarins ripening now are among the most cold-hardy of citrus varieties. 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 and blood oranges. Yuzus are the hardiest citrus, withstanding temperatures down to 24 degrees.

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.)

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 it 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.

-- 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 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 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 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.

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Garden Checklist for week of June 23

Get to work in the mornings while it’s still cool.

* Irrigate early in the day; your plants will appreciate it.

* Generally, tomatoes need deep watering two to three times a week, but don't let them dry out completely. That can encourage blossom-end rot.

* Let the grass grow longer. Set the mower blades high to reduce stress on your lawn during summer heat. To cut down on evaporation, water your lawn deeply during the early hours of the morning, between 2 and 8 a.m.

* Tie up vines and stake tall plants such as gladiolus and lilies. That gives their heavy flowers some support.

* Dig and divide crowded bulbs after the tops have died down.

* Feed summer flowers with a slow-release fertilizer.

* Mulch, mulch, mulch! This “blanket” keeps moisture in the soil longer and helps your plants cope during hot weather.

* Avoid pot “hot feet.” Place a 1-inch-thick board under container plants sitting on pavement. This little cushion helps insulate them from radiated heat.

* Thin grapes on the vine for bigger, better clusters later this summer.

* Cut back fruit-bearing canes on berries.

* Feed camellias, azaleas and other acid-loving plants. Mulch to conserve moisture and reduce heat stress.

* Cut back Shasta daisies after flowering to encourage a second bloom in the fall.

* Trim off dead flowers from rose bushes to keep them blooming through the summer. Roses also benefit from deep watering and feeding now. A top dressing of aged compost will keep them happy. It feeds as well as keeps roots moist.

* Pinch back chrysanthemums for bushier plants with many more flowers in September.

* From seed, plant corn, pumpkins, radishes, melons, squash and sunflowers.

* Plant basil to go with your tomatoes. 

* Transplant summer annuals such as petunias, marigolds and zinnias.

* It’s also a good time to transplant perennial flowers including astilbe, columbine, coneflowers, coreopsis, dahlias, rudbeckia, salvia and verbena.

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