Droughty winters prompt questions about winter irrigation in orchards.  The questions seem simple: Should I irrigate this winter?  If so, when should I irrigate? How much water should I apply?  The answers to these questions aren’t quite so simple because they depend upon specific management philosophies and conditions.

Should I irrigate this winter?

Some reasons to irrigate during a dry winter:

• Orchards with moist soils tend to be warmer and reduce the risk of winter kill during severe cold snaps. So far, this has not been too much of a concern this winter.
• Winter irrigation works toward refilling the root zone to field capacity. Field capacity represents a condition where the soil is wet but still aerated.  Any more water will drain deeper below the bulk of the root system.
• Achieving a full winter soil moisture profile should delay need for the first crop irrigation after leaf-out. This may achieve warmer soils that foster root growth and better aeration to discourage root diseases.  It may also improve orchard access to manage important bloom and foliar diseases.
• In theory, winter irrigation is banking water for use later in the growing season. It has potential to help lessen the amount of pumping during the summer and drawdown of groundwater levels.
• Winter irrigation may be more important in orchard soils that have very low water infiltration rates (silt and clay soils). These soils require more time for enough water to infiltrate deep.

Some reasons not to irrigate during a dry winter:

• Minimize the winter energy bill. Then again, if water is successfully banked for later use, perhaps it will lessen the summer energy bill.  Costs for pumping are usually less in the winter than summer.
• The uncertainty of rainfall. “Fantastic February’s” and “Miracle March’s” have occurred in the past to overcome dry starts from November through January.  The risk is, if it rains a lot after significant amount of winter irrigation, it’s possible to be faced with overly wet spring conditions.
• Winter irrigation can potentially be a source of leaching nutrients such as nitrate-nitrogen that are mobile in the soil. This can be managed by timely in-season fertilizer applications at proper rates.
• Does it really spell dooms day if the decision is made not to irrigate during a dry winter? Maybe or maybe not.  This is where knowledge of site specific conditions becomes important.  It may be more important to winter irrigate orchard soils high in silt and clay content.  They have slower water infiltration rates and higher water holding capacity which requires more time and water to refill.

If so, when should I irrigate?

When December is dry, like this year (precipitation was virtually absent), begin to gradually refill the soil profile with occasional irrigations.  In fact, some winter irrigation occurred in orchards this past December, particularly in almonds.  Track the rainfall received, irrigation water applied, and keep an eye on forecasts.

A reasonable strategy is to substitute irrigation for the shortage in rainfall on roughly a monthly basis beginning in December until enough rainfall has been received in combination with irrigation to refill the soil profile five feet deep.  The table demonstrates the idea.

Different lengths in the orchard dormancy period will also influence decisions.  Since almonds bloom earlier than prune and walnut, there is less time for winter irrigation.  Do not to postpone winter irrigation too late so that it bumps up against the period when the trees begin to break dormancy with the emergence of new roots.  Ideally, complete any winter irrigation in almonds by the end of January, in prune by mid February, and in walnut by early to mid March.  This gives the irrigation water time to infiltrate and re-distribute in the soil profile so that the soils are refilled and re-aerated for an optimum environment to encourage root growth.

How much water should I apply?

Winter irrigation is only given consideration when rainfall is in short supply.  How much irrigation water is needed is determined by how much water the soil profile can hold and adjusted for additions of rainfall.  Most orchard soils will hold between 1 to 2 inches of available water per foot of depth.  So, 5 to 10 inches of winter irrigation should go a long ways towards refilling a five-foot root zone when rainfall is very low.  Leftover moisture from last season’s post-harvest irrigations and effectively stored winter rainfall will also reduce the amount of winter irrigation needed.

Most micro and mini sprinkler systems will apply between 1.25 and 2.0 inches of water per 18 to 24 hours of irrigation.  So two or three winter irrigations, at most, should help.  Drip irrigation applies water at lower rates and wets a smaller fraction of the orchard floor so this needs to be considered.

Soil moisture monitoring can definitely help determine the need for winter irrigation.  Checking soil moisture by hand is a very basic method to evaluate soil moisture conditions.  There are many online stores where soil augers can be purchased (try: AMS samplers; Forestry Suppliers; Ben Meadows).  The USDA, NRCS also offers a nicely prepared publication with color pictures titled “Estimate soil moisture by feel and appearance”.

There are a wide variety of soil moisture sensors that can also be used.  Refer to the article “Soil moisture sensor selection is confusing” for more insight.