Same Depth, Less Frequent: Irrigating in Winter

Question:

Now that winter is here in Las Cruces, I'm wondering what the frequency of watering should be and the best time of day or night to have the water come on?

-        Rob M., Las Cruces, New Mexico

Answer:

It seems #itscomplicated is a hashtag I could use every week. Knowing how much water to apply in your landscape is hard enough in the summertime when demands are high, but it can be even more difficult to know the right amount of water needed when many plants are bare and it can be easy to forget.

Most plants need less water in colder months. This is partly because dormant plants are not actively growing. Lower temperatures also reduce transpiration of water through plant tissues. When deciduous plants drop their leaves, photosynthetic rates also drop, as do water requirements.

That does not mean, however, that no water is needed at all. In our high desert climate, warm winter days, along with cold, drying winds, trigger some transpiration, which further dries the soil. Plus, many plants, like rosemary and pine trees, do not lose their “leaves” at all, so they continue to transpire, even if at a slower rate than in hot summer temperatures. Mulch is key! Mulching helps insulate plant roots and maintain soil moisture in both winter and summer. Not to mention, mulch makes a great weed barrier.

The most widespread rule of thumb is to water less frequently in the winter months, but always water to the same depth. If you water landscape plants for, say, 30-minute intervals once a week in the hottest months and then back off to only 10-minute intervals once a week in colder months, roots will die back. This invites stress-related symptoms like insect problems, diseases, diminished performance, and even plant death.

But hold on, there is a caveat. While always watering to the same depth is the best rule for irrigation efficiency, one drawback is the possibility of salt buildup in the root zone, which can be damaging. (Someone please send in a question about salt toxicity and tolerance, so I can cover that another week.)

Judith Phillips, a landscape designer and garden writer in the Albuquerque area, pointed out that plant irrigation needs in winter depend largely on when they were planted. Even desert-adapted plants will need more frequent irrigations if they were installed this summer or fall. The following watering guidelines are from the Arizona & New Mexico Getting Started Garden Guide by Mary Irish and Judith Phillips and are geared toward desert-adapted, established plants (more than 1 year, or 3 years for trees). December through March, water trees, shrubs, and warm-season grass every 45–60 days; groundcovers and vines every 30–60 days; cool-season grass every 30 days. Annual plants tend to have smaller root systems, so water every 10–14 days during bloom. The recommended watering depths, which should be kept constant throughout the year, are 24–36 inches for trees; 18–24 inches for shrubs; 8–12 inches for groundcovers, vines, and annuals; and 6–10 inches for turf.

Determining how long your irrigation system or watering hose needs to be on in order to get the correct moisture depth is up to you. One way to do this is to push a long screwdriver or piece of rebar down into the soil. It will move easily in moist soil and stop when it reaches dry soil.

As far as the best time of day to water in the winter, it seems that the biggest concern is with damage to irrigation systems, which are more likely to freeze overnight if not drained completely. Standing ice is also a hazard issue, and ice violations can be grounds for fines. I hand-water perennials in my garden once every 6 weeks or so on warmer days by setting a timer and moving the hoses from one planting area to another.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page (@NMDesertBlooms)

Please copy your County Extension Agent and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/