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.

November Frosts, Freezes Ahead? Probably.

 The typical Sacramento-area frost season (when temperatures dip to 32 or below for short periods of time) is fairly short: primarily, December and January.

 

 

However, November frosts do happen here with regularity. Freezes, too.

The earliest frost date for Sacramento was on a November 4, back in 1935, when the morning low fell to 30 degrees. The latest frost date recorded was on March 27, 1898, with a low of 32.

 

In 2011, a surprise cold snap on the morning of November 5 sent some areas in Sacramento County to freezing.

 Two years ago, there was a 2010 Thanksgiving surprise: the morning low temperatures in the suburbs of Sacramento dipped into freezing territory. 28 in Elk Grove. 24 in Rancho Cordova. 23 in Folsom. The temperatures in Rancho Cordova and Folsom stayed below 28 degrees for 7 hours that morning. That's a citrus-killing, perennial-punching hard freeze.

Freeze-Pummelled Pummelo

Not a Happy Hosta Thanksgiving

 

 

What is cold? Some definitions:

Frost: temperatures dip to 32 °F (0 °C) for short periods of time. Occurs with fair skies and light winds.

Freeze: temperatures at or below 32 °F

Hard Freeze: temperatures below 28 °F for several hours.

 Fruit-laden citrus trees could be threatened by very cold mornings in the weeks (or days) ahead. Some planning tips for the upcoming cold mornings:


Before a frost:

• Identify cold spots in landscape by monitoring with a thermometer that registers high and low temperatures.

• Identify plants at risk: citrus, succulents, tender perennials, tropical and subtropical plants.

• Have supplies ready: sheets or frost cloths, lights, wraps for trunks, thermometers, stakes or framework to hold covers off foliage.

• Prepare tender plants: avoid fertilizing and pruning after August to minimize tender new growth. 

• Plant insurance: In September and October, take cuttings from frost sensitive perennials; keep cuttings in a sunny, indoor area.

• Rake away mulch to allow soil to warm up during the day and radiate heat at night into plant.

• Monitor weather forecasts and note how low temperatures will be and for how long. 

Pipe Wrap: Cheap Frost Insurance

When a frost is forecast:

1. Move potted plants to a warmer spot next to house or under patio cover, especially on south side.

 2. Check that plants are well-watered since dry plants are more susceptible to damage, and moist soil retains heat better than dry soil.

3. Cover plants with a row cover before sunset to capture ground heat radiating upward at night, but remove covers daily if it is sunny and above freezing to allow soil to absorb heat.

4. Add heat by using outdoor lights: hang 100 watt drop lights or Holiday string lights to the interior of the plant. Use the old C7 or C9 large bulbs, not new LED lights which do not give off heat.

5. Wrap trunks of tender trees if hard freeze is expected, using towels, blankets, rags, or pipe insulation.

6. Harvest ripe citrus fruit. Generally, both green and ripe fruit are damaged below 30 degrees, but there is some variation by species (refer to the chart in UC/ANR Publication 8100, "Frost Protection for Citrus and Other Subtropicals").

7. Winterize your gasoline-powered garden equipment. Gas can go bad and screw up your engines if allowed to overwinter, unused. Drain the tanks or turn off the supply valve and run the engine until it stops. For containerized gas (or gas still in equipment) add a stabilizer. Run the engine for 10 minutes or so to make sure the stabilized gas is thoroughly mixed into the engine.

When a Freeze or Hard Freeze is Forecast (temperatures remain at or below 28 degrees for several hours)

  1. Wrap any exposed plastic water pipes; use a cover for outdoor faucets. Turn off the water supply to outdoor irrigation faucets, if possible. Allow those faucets to drain.

 





 

 

2. Disconnect garden hoses and lay them out straight...away from driveways!


3. Adjust your pool, spa or pond filtration timers so that they are running when the chance of freezing temperatures is greatest, between two and nine a.m. Moving water is less susceptible to freezing.

4. For dish-shaped fountains: Turn off and let drain to the holding tank below ground. Remove any standing water in the dish.

Frosty the Fuchsia

After a frost:

1. Identify damage: dark brown or black leaves and twigs.

2. Wait to prune out damage until after danger of frost is past, and new growth begins in spring.

3. Make sure the backyard birdbath isn't frozen over in the morning. Daily fresh water for dogs and cats is also a good morning habit.