Issue: November 5, 2005

Landscapes need irrigation in winter

Question:

Do I have to water outdoor plants in the winter? They aren't growing, so they shouldn't need water.

Answer:

Most outdoor plants aren't visibly growing in the winter; however they aren't dead and still need water. Their root systems may continue to grow even after the top is dormant, but we can't see the roots grow. Water is needed for root growth.

Plants do need much less water in the winter because water loss from the plants is greatly reduced, not totally stopped. Evergreen trees and shrubs still have leaves and continue to use water (at a reduced rate). Deciduous trees and shrubs have lost their leaves but their twigs contain water, which is slowly lost to our dry New Mexico air. This water must be replaced. Grass leaves are often brown and dry, but the crowns of the plants are still living and need water. Some gardeners overseed their summer lawns with winter grasses that remain green, and they certainly need water. Even the perennials we can't see (bulbs below ground and other plants that die to the ground each winter) need water to live and resume growth in the spring. Even though plants aren't actively growing, they need water to remain alive.

The plants described above are using water at different rates, but most can be maintained in adequate health if they are watered once a month, except the green winter grass that will need to be watered more often. This means applying enough water to moisten the root zones of the plants. The root zones of trees and shrubs occupy a fairly large volume of soil and require soil moisture to a depth of at least two-to-three feet. Dormant perennials have smaller root systems and often require soil moisture to only a depth of one foot. If there has been sufficient natural precipitation in the form of rain or snow, additional irrigation will not be needed; however, if it has been dry for a month or if the natural precipitation has not been sufficient to moisten the soil deeply enough, supplemental irrigation will be needed.

When the ground is frozen, the water in the soil has frozen. It is not possible to irrigate through this layer of ice, so supplemental irrigation must be supplied before the ground is frozen or after it thaws. Frozen ground does indicate moisture in the soil, but perhaps not enough for the plant to survive the winter. Deep watering before the ground freezes is the best way to assure adequate water when the ground will freeze for long periods. In regions where the ground freezes only briefly, irrigation once a month during the winter is the best practice.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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

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