Rain, Frost, Freeze? Welcome to Late November, Citrus Tree Owners!

Heading out for grandma's house for Thanksgiving Week? We are now officially entering the shivering season for the Central Valley, Bay Area and low foothills. Late November through mid-February is the most critical time here for protecting frost-susceptible plants.

Not a rare, white weeping fig. More like "frozen fig"

Don't let your citrus trees and other frost sensitive plants (such as the freeze damaged weeping fig, above) get caught by a "Turkey Day Surprise" while you're away. Morning lows will be dipping into the upper 20's and low 30's. The National Weather Service is predicting rain for Northern California for Tuesday and Wednesday, with snow levels dropping to near 2000 feet in the Sierra foothills. When the rain blows out Wednesday night, more colder air is coming in for Thanksgiving, Friday and Saturday. Overnight lows are forecast to dip below freezing, with frost and a freeze highly likely...depending where you live, and where you're citrus trees reside. 

All gardening is local. That thermometer on the window or door frame of your house? It's showing the temperature at that particular point on your property. And, it's probably showing a reading that is getting a boost from the heat that's escaping your house. On the other hand, the temperature where your citrus trees are located is probably lower. Putting a high/low digital outdoor thermometer near your citrus trees, and comparing it to the morning reading on your house-mounted thermometer may raise more goosebumps on your arms. At our place, it is not uncommon to see a six or seven degree lower temperature at 7 a.m. next to our citrus, despite the fact that the nearest Meyer lemon tree is only 20 feet away from the house. Get yourself one of those thermometers and check the readings in your own yard on these shivering mornings. That thermometer on the side of the house might say 33 degrees...but the one out at your frost-sensitive plants might say 27, enough to turn the insides of your lemons to mush.

Frost Cloths Protecting Lemons, Mandarins, Oranges

Several days before an expected frost (temperatures dipping down to 32 degrees) is the time to gather the necessary implements to protect your citrus trees, including giving the ground beneath them a good soaking (moist soil is better than dry soil at moderating the temperature beneath the tree).

Most gardeners first thoughts about protecting their citrus trees during a frost or freeze is, "protect the fruit!"

Four Winds Growers, the Winters-based wholesale grower of many excellent varieties of citrus, offers the Citrus Variety Information Chart at their website, FourWindsGrowers.com. 

Included in that chart is extensive information about each citrus variety, including suitability for indoor growing; its bloom and fruiting seasons; its recommended summer heat level to produce good fruit; and, its minimum tolerable temperature for preservation of fruit quality.

  The chart points out that lemons, limes and citrons are most sensitive to frost, while sweet oranges, grapefruit, tangerines and calamondins are intermediate. Kumquats and Owari Satsuma Mandarins are the most frost-tolerant, braving temperatures into the twenties (that would classify as a freeze).

From that chart, here are the temperatures (in degrees Fahrenheit) at which citrus fruit damage may occur. 

Sweet Oranges

Washington Navel Orange  28
Trovita Orange  28
Cara Cara (Pink) Navel Orange 28
Lane Late Navel Orange  28
Robertson Navel Orange  28
Shamouti Orange (Jaffa Palestine)  28
Valencia Orange  28
Midknight Valencia Orange  28  

Blood Oranges
Moro Blood Orange  28
Sanquinelli Blood Orange  28
Tarocco Blood Orange  28
 

Sour Oranges
Bouquet De Fleurs Sour Orange  28
Chinotto Sour Orange (Myrtle-Leaf)  28
Seville Sour Orange  28
Bergamot Sour Orange  32
 

Mandarins
Gold Nugget Mandarin (Patented)  26
Tango Mandarin (Patented)  32
Owari Satsuma Mandarin  24
Dancy Tangerine  32
Clementine Mandarin (Algerian)  28
Murcott Mandarin  32
California Honey Mandarin  32
W. Murcott Mandarin  32
Kinnow Mandarin  32
Kara Mandarin  32
Page Mandarin  32
Piie Mandarin  32
Kishu Mandarin  32
 

Lemons
Improved Meyer Lemon  32
Eureka Lemon  32
Lisbon Lemon  32
Ponderosa Lemon  32
Variegated Pink Lemon  32
Yen Ben Lemon  32  

Mediterranean Lemons
Villafranca Lemon  32
Genoa Lemon (Gea)  32
Limonero Fino Lemon  32
 

Limettas
Millsweet Acidless Limetta  32
Marrakech Limetta  32
 

Limes
Bearss Seedless Lime (TahitiPersian)  30
Kaffir Lime (KiefferThaiWild)  32
Meican Lime (Key)  32
Thornless Meican Lime  32
Meican Sweet Lime  30
Palestine Sweet Lime  30
Rangpur Lime  32
 

Grapefruits
Oroblanco Grapefruit  32
Rio Red Grapefruit  28
Star Ruby Grapefruit  28
Chandler Pummelo  28
Cocktail Grapefruit  28
Chinese Grapefruit  28
Melogold Grapefruit  28
 

Kumquats
Meiwa Kumquat  28
Nagami Kumquat  24
Indio Mandarinquat  26
Centennial Variegated Kumquat  30
Nordmann Seedless Nagami Kumquat  28
Marumi Kumquat  26
Eustis Limequat  32
 

Citrons
Buddha's Hand  Fingered Citron  32
Etrog Citron (Ethrog)  32
 

Other Interesting Varieties
Minneola Tangelo  28   
Australian Finger Lime  32   
Yuzu  24   
Calamondin  32  
Variegated Calamondin  32

But what about the overall health of the citrus tree? How low can temperatures go during a freeze event before the tree is toast? 

 "I consider 22 degrees to be terminal for citrus tree cambium cells," says Cedar Seeger of Four Winds Growers. The cambium layer is the growing part of the tree, the cells that are producing new wood and healing wounds. It is located just beneath the bark.

And that's for a citrus tree in tip-top shape: good health, with moist soil around it during a freeze. Cedar uses the example of a Meyer lemon tree:

Blanket + Tomato Cage for Citrus Protection

"We often have a two to three hour dip to 28 degrees after storms; and if the above conditions are met, even Meyer lemon trees can survive, albeit not to happily, without protection. 28 degrees for four hours probably won't kill the tree, provided the rootstock cambium doesn't freeze. It will defoliate and lose twigs. At 24 degrees things start to get dicey. That's when the blanket, frost cloth, bonnet and/or the old-style, large outdoor Christmas lights that give off some heat will work wonders. Remember, those blankets and bonnets need to go to the ground in all cases."

Chandler Pummelo, Pummeled by 2010 Freeze

You may recall Thanksgiving Week of 2010, when morning low temperatures dipped well below freezing for six days in a row, led by a citrus-killing 27 degree morning on Thanksgiving.








When a large, cold-air mass moves in from the north after a storm in the winter, that is called an advective freeze. The one that sticks out in most gardeners' memories here was the freeze of mid-December 1990, when nighttime temperatures fell into the teens for several days in a row, with a couple of days that didn't climb above 32 degrees. To add even more injury, a second cold snap hit near the end of the month, with temperatures dipping into the mid-20's. Many of the most susceptible (frost intolerant) landscapes were completely lost; some nurseries never recovered.

"Our first year in the citrus business here in Winters was 1990-91. My wife, Mary Helen, and I have a Masters in Disaster," says Cedar Seeger. 

Which is why Cedar is an adherent of watching the dew point, the temperature at which saturation has been reached, when water vapor condenses into water. The lower the dew point, the more danger of cold damage to your plants. One good online source for dew point temperatures is the National Weather Service's Tabular Forecast Page  , which offers a forecast for two days in advance (that link is for Sacramento). 

"A good watering going into an advection night is mandatory. If the ground and surrounding grass is wet, it creates a micro dew point environment around the trees. In a dry, cold La Nina winter such as we're about to get, it is important to remember the dew point concept. At 22 and below, it's full on emergency response, pile straw, hay around trunks, anything, lights, covering," says Cedar, a man who learned these lessons the hard way.  But he is not an adherent of running sprinklers during a freeze.

"My experience with overhead sprinklers is that they more often than not freeze up, and then it's all over. And you are risking branch breakage on that ice-entombed citrus. Yeah, it can work, but screw it; it's messy, risky and a lot of work. Use the large Christmas lights and mid-weight frost covers. But pay attention to the trunk / rootstock. If that freezes, it's a goner."

Citrus trees most at risk to fatal damage from a frost or freeze are the young trees. It is vital that they be covered completely when a heavy frost or freeze is predicted, and provide protection for the trunk, bud union and rootstock area. That can include trunk wraps, newspapers, old carpeting. If possible, move small containerized citrus closer to the house, preferably next to a south or west-facing wall to maximize reflected heat.

In the book, "All About Citrus and Subtropical Fruits", author and grower Lance Walheim also suggests applying a copper-based fungicide to the trunk and then mounding or banking soil against the trunk and lower limbs. Just don't leave it on too long; fungal rots can develop (that copper can only work for a limited time). He advises leaving that soil next to the trunk from Thanksgiving until February, or March, in colder areas.

Don't be fooled by the predictions of a warmer and wetter El Nino winter. Mornings can still get too chilly for unprotected citrus. Welcome to Turkey Weather season!