Winter Irrigation During Drought

Adapted from the article “Winter Irrigation During Drought” by Joseph H. Connell, UCCE Farm Advisor Emeritus, Butte County in the December 2008 Sacramento Valley Walnut News.

We know that during the winter months walnuts can be hurt by either too much or too little water.  It’s been a very dry fall so far and the deep soil profile in most walnut orchards takes significant rainfall to be recharged.  Cutting back on water earlier in the fall slows down the trees growth and helps harden them off.

If temperatures drop rapidly below freezing there can be the potential for cold damage to young vigorous non-dormant walnut trees.  Winter kill, which can affect both young and mature walnut trees, happens when extremely low temperatures occur during the winter months even if trees are fully dormant. Water stressed trees or trees planted in sandy soil are most susceptible to this injury.  Drought conditions during winter can make winter kill worse if we get sudden severely cold temperatures.

If we don’t know if it’s going to rain significantly, how do we best apply water during the winter to alleviate drought?  If you have water available, a light 1½ to 2 inch irrigation that simulates typical rainfall patterns is advisable.  If we’re still dry in December, then begin to gradually refill the soil profile with occasional 2 inch irrigations.

Keep an eye on rainfall forecasts and amounts of rain received in your neighborhood so your irrigation timing doesn’t end up creating a condition where the orchard is too wet.  Although unlikely if current conditions persist, saturated soils can kill roots from water logging or can increase the chance of crown or root rots developing.

Most of our walnut soils generally hold around one and a half inches of available water per foot of depth.  Trying to re-wet a 5 foot root zone in late February to early March if it hasn’t rained much by then isn’t a desirable situation. Check soil moisture as the winter progresses to see how deep the soil profile has been re-wetted so you can make sure you don’t have dry soil in the root zone come spring.  The ultimate goal is to make sure the soil reservoir is completely refilled either by rain or winter irrigations by the time your walnut trees begin to wake up next March.

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