Citrus, Frost and Mulch: Yes or No?

I love mulch. That would be obvious to anyone who has ever talked gardening with me. Big piles, small piles, inches of mulch scattered everywhere. Love it!

 

The benefits of adding organic mulch (wood chips, shredded tree limbs, pine needles, compost, straw) to the top of your garden soil:

• retains moisture

• keeps soil temperature constant, reducing plant stress

• suppresses weeds

• gradually increases soil organic matter

• attracts beneficial organisms that improve soil fertility and porosity.

• Mulch encourages healthier plants, reducing the needs for pesticides and fertilizers.

• protects roots and plants from mechanical injury.

• On hillsides and around homes, it suppresses the spread of brush fires.

But a long-held recommendation from the University of California flies in the face of the "all mulch, all the time" rule regarding protecting citrus from the effects of freezing temperatures: "A cover crop or mulch can lower minimum temperature at night, posing an increased threat from freeze damage." 

So, our advice has been over the years, "rake away mulch from beneath citrus before an expected frost or freeze".

Now, the California Landscape Contractors Association is offering the opposite advice in a release regarding frost protection: "Mulching with a partially composted material is one the best ways to protect plant roots because it helps insulate the soil, reducing heat loss and minimizing temperature fluctuations. Protecting the roots is necessary in order for them to survive the cold." The CLCA also points out: "Be sure to check the mulching material about once a month to make sure that moisture is getting to the soil below. Avoid using weed block materials, plastic or other moisture barriers beneath the mulch so that water can get to the roots. You may also need to water some of the drier areas in mid-December or mid-January if you find the soil dry."

So, who's right? Sacramento County Farm Advisor Chuck Ingels says: keep on mulching!

"The CLCA is right on," says Ingels. "In our mild climate, mulch doesn’t protect the tree from cold because the soil and roots really don’t ever freeze. Mulch protects the soil for other well known reasons. Regarding that UC study: years ago I thoroughly researched this and wrote about it in "Protecting Groundwater Quality in Citrus Production". In a large orchard, the best orchard floor conditions for reducing frost hazards is bare, firm and moist soil. The sun hits the soil and re-radiates the heat at night, warming the air. Tall cover crops are worst because not only do those plants not hold much heat, but tall cover crops raise the level of cold air (cold air sinks), increasing frost damage potential."

"Perhaps with just a few citrus trees there may be some benefit in this regard," Ingels concedes. "But any difference is generally very miniscule. What happens on the surrounding five acres (asphalt vs. buildings vs. bare ground) affects the air temperature around your tree. So, mulch away!"

ROOT ROTS + MOISTURE + MULCH = TROUBLE FOR CITRUS TREES But wait a minute, here's another reason why mulch and citrus trees may not be the best of friends: phytophthora. The spread of this root and crown rot fungal disease may actually be aided by too much mulch beneath shallow rooted citrus trees, especially in moist, slow-draining soils. According to the UC Division of Agriculture and Natural Resources book, Pests of Landscape Trees and Shrubs, "phytophthora kills the roots and root crown area of infected plants...possibly causing mature plants such as citrus to grow slowly and gradually decline... Phytophthora may affect only small feeder roots or rootlets, major roots or all roots and the crown... Because mulch retards drying of the soil and excess soil moisture greatly contributes to the development of root rots, improper or excessive use of mulch may actually promote root rot development."
TO MULCH OR NOT MULCH CITRUS? A COMPROMISE.

 If your soil drains easily and is not compacted, you may be able to get away with adding a few inches of mulch beneath your entire mature citrus trees, but keep the mulch from touching the trunk; six inches away, at a minimum. But if you have clay soil, slow draining or compacted soil, or just want to play it safe, Debbie Flower, Professor of Horticulture at American River College in Sacramento, offers this advice to stave off decline in a citrus tree: place the mulch from about two feet away from the trunk to beyond the outer canopy of the tree.




For newly planted citrus, move the mulch to the outer edge of the canopy (and beyond).








This way, the shallow roots near the root crown (as well as the crown area itself) have less of a chance of getting root rot problems. 

Another strategy: plant citrus in raised beds or containers to improve drainage. And perhaps a string or two of the old style C9 Christmas lights for some added heat on frosty nights. And while you're at it, perhaps some insulating pipe wrap around the trunk.

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

Today, it was an inch of rain. Tomorrow, the rain clears out and it's supposed to blow to 40 mph in the Sacramento area. The next night...frost? The National Weather Service is predicting overnight lows Friday and Saturday to dip into the mid-30's. But depending where you live, it could be lower. All gardening is local. 

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.

Frost Cloths Protecting Lemons, Mandarins, Oranges

This is especially true for citrus tree owners, who are anxiously keeping an eye out on the upcoming weather forecasts.

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.


And Thanksgiving, by the way, is next Thursday. Welcome to Turkey Weather season!