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.

WTF? A Freeze in February? In California? Along the Coast?

 Areas of Northern and Central California, including the coast, may see record breaking low temperatures Friday through Sunday. If your local weather forecast is calling for morning lows below 32 degrees, here's a last minute checklist:


 • If it hasn't rained, water plants thoroughly, especially container plants.
 
• If possible, move sensitive container plants next to a south or west facing wall.
 
• Cover citrus and other sensitive plants with burlap, row cover fabric or sheets (be sure to keep the sheets dry). Tent plastic sheets over the plants; don't let plastic touch plant leaves. A light bulb placed in such a plant can offer a few degrees of protection.
 
• If using an anti-transpirant polymer coating material such as Wilt-Pruf or Cloud Cover, apply at the warmest time of the day, or at least six hours before an expected frost. However, research conducted at Washington State University concluded that these products may actually be detrimental to certain plants during a freeze. If using these products, thoroughly water the soil around the plant before applying.
 
• Disconnect hoses and drip lines, removing end caps. Lay out straight.
 
• To prevent broken grass blades, don't walk on a frozen lawn.
 
• Remove the lowest sprinkler head to drain.
 
• Cover unprotected faucets and pipes, including any spa or pool equipment.
 
• If temperatures are predicted to fall to the low 20's: prevent frozen attic pipes. Let lukewarm water trickle out of the indoor faucet farthest from the inlet. Also, let faucets with pipes running along an outer, north facing wall trickle during the night. 

• Ideally, add insulation to attic water pipes.
 
• Open cabinet doors to get more heat to the pipes. Close the garage door if water pipes pass through the garage.
 
• Setting your thermostat nightly at 55 can add needed heat to the attic pipes.
 
• If leaving the house for a vacation during an expected freeze, turn off the water to the house, and open up the faucet farthest from the inlet. Be sure to turn off your water heater.
 
• To prevent cracking tile, run your pool and spa equipment during the freezing hours.

• Don't forget about your pets during a prolonged freeze. Bring them indoors at night. Move or replace their drinking water. Break up any frozen water in bird baths.

Mulch under Citrus Trees: Pros and Cons

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."

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 to improve drainage.

Holiday Lights for Frost Protection: Which to Choose?

 
"For frost protection of citrus, use the large, C9 Christmas/holiday lights..."

Now, there's a phrase we've been tossing around a lot lately.





To generate as much heat as possible into a citrus tree's canopy on a night when the temperature dips below freezing, stringing those C9 lights around the tree's interior is a good idea. Christmas lights work better than a single, large light bulb in the center of the tree canopy. It eliminates cold corners and edges. Those bulbs can add four degrees of protection. The combination of bulbs with a frost cloth will give you seven or eight degrees of extra warmth (be sure not to let the bulbs rest against the fabric or the leaves).
   
But after doing a little research, does it come as a surprise that not all C9 holiday lights are created equal?

The original C9 incandescent bulbs generated about 10 watts per bulb. Now, the selection of C9's (and the slightly smaller C7's) vary greatly: from 10 watts to less than 1 watt per bulb.

C7 (5 watts)

C9 (9 watts)

LED C7 (less than one watt)

If you're buying C9 lights for frost protection, be sure to read the label. You want as much heat as possible.


The output among various style C9 and C7 holiday lights from one online retailer:  
.6 Watts - LED C7    
1 Watt - LED C9   
2.5 Watts - Low Energy C7  
3.7 Watts - Low Energy C9
5 Watts - Traditional C7 
7 Watts - Traditional C9 
10 Watts - Traditional C9


And the information on the package can be tricky. One box of Christmas lights I found in the garage was labeled "C9 Style". Turns out, only the covering of the bulb was C9 in style; the output of the bulb itself was .75 of a watt.


Besides being festive this time of year with your citrus, you can extend the colorful light show to your cold frames.
  
In trials conducted by Colorado State University, one 25 light string of C-7 (mid-size) Christmas lights per frame unit (four feet wide by five feet long) gave 6 degrees F to over
18 degrees F frost protection.  Lights were hung on the frame under the plastic and turned on at dusk and off at dawn. 

More Frost Protection Tips for Citrus

Besides the frost protection tips for citrus and other plants mentioned in previous posts, here are some relatively inexpensive tricks for raising the temperature a few degrees around your young citrus trees during this frost and freeze season.

Tubular pipe insulation. Designed to keep your attic or basement water pipes from freezing, these can also protect the trunks of young citrus trees. One to three year old citrus tree trunks are very susceptible to freeze damage. The tree could die back to the root stock; but the root stock will quickly send up new shoots, resembling your pre-existing citrus tree...except for the nasty thorns, thicker leaves and ugly, tasteless fruit that rootstock growth will produce. Been there. Done that. Including a nasty thorn into the forehead. 

 

Remove the insulation at the end of the expected frost season to prevent any trunk rot issues from accumulated moisture.

Lights can be used to warm the tree. Shine an outdoor-approved 100 watt lightbulb on the large limbs or trunk to warm the most mass. The larger, older Christmas bulbs will work, too. Newer ones, although more energy efficient, don't produce as much heat as the older ones.

Put that tomato cage to winter use as a framework around a small citrus tree. Any blanket, including plastic, can be used at night to protect the tree, and not risk transferring the bitter cold to the leaves (which might happen if you just draped a plastic drop cloth over a citrus tree). But as the University of Arizona points out, remove that blanket during the day:

"If you use sheets or blankets, remove the coverings every morning when the temperature under the covering warms to 50 F. Permanently covering plants with sheets or blankets for the duration of the winter can be harmful. Even if the temperature under the drape does not warm up enough to 'cook' the plant, it is likely to warm up enough to cause the plant to break dormancy, begin actively growing, and thus become more susceptible to frost damage. However, many of the frost cloths available may be left on for extended periods without risk of harming the plant."

And, simply throwing a covering over a citrus is not enough, adds the University of Arizona: "Completely drape the plant from top all the way to the ground. Do not allow any openings for warmth to escape. This procedure will trap the heat radiating from the soil and maintain a more humid atmosphere around the plant foliage. Optimally, the drape will be supported by frame which does not allow it to touch the foliage. DO NOT gather the drape around the trunk of the tree. The goal is to trap heat being radiated from the ground, so ensure that the drape touches the ground at least as far out as the drip line.

And that brings us to one of the drawbacks of a relatively new frost protection product on the market, the frost bonnet: sort of a high-tech garbage bag, but made of a spun polypropylene fabric that cinches together at the top and bottom.

I'm trying these plant protector bags this year, but I am wary for several reasons: they are rather inexpensive (two 40" x 45" frost bonnets sell for about $8).

 

The warning label, in small print, is not very reassuring. 

 

And, even though you can safely leave these coverings over the citrus plants throughout the winter (or can you?), they look like the remnants of the Rotary Club's Saturday highway garbage cleanup drive. 

Oh, and because I forgot to anchor them securely to the soil, they blew off in a heavy wind last weekend.

And, a few really inexpensive frost protection ideas:

Keep the soil under the citrus tree well watered. Firm, bare moist soil absorbs more heat and loses it more rapidly than soil that is dry or covered with mulch or vegetation, according to the University of Arizona.

Think twice before purchasing anti-transpirants or other products to spray on your citrus trees for frost protection. According to the University of Arizona: "A variety of chemicals have been marketed with claims of changing the freezing point of the plant tissue; reducing the icenucleating bacteria on the crop (thus inhibiting ice and frost formation), or affecting growth (extending dormancy). To this date no commercially available material has withstood scientific scrutiny."

And if there is frost damage? Wait until late spring before reaching for the pruners. The damaged parts will be more obvious by then, so you won't accidentally remove healthy parts of the tree.