Friday, January 15, 2010

To Mulch or Not to Mulch Citrus Trees in a Freeze?

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 current 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!"

 When adding mulch beneath a tree, pile it three or four inches thick. The mulch should extend from near the trunk (but not touching the trunk) to beyond the outer canopy of the tree; that's where the majority of the tree's feeder roots are located. Why shouldn't that mulch touch the tree trunk? Placing mulch next to the bark of the tree encourages crown rot and allows easy access for mice, gophers and voles, which might girdle the bark around the base of the tree.


  1. Thanks for this Info! I plan on adding 2 lemon and 2 lime trees to our yard. When do you feel the best time to plant would be, After danger of frost?


  2. The best time to plant citrus here in the Sacramento Area is spring, after all danger of frost. Start looking for citrus at nurseries then. Shameless plug: Cedar Seeger, one of the principals of Four Winds Growers (they grow and sell citrus trees to most of the nurseries in CA), will be on the radio shows on Sunday, Jan. 24, 9-11 a.m. Also streamed at and That will be a good time to get your citrus questions answered!

  3. Thanks Fred!
    I have marked it on my calander! Thanks for your advice!

  4. Hey! How about you move that big pile of mulch in my direction! I could use some!

    OK, I could use a lot.