Should I Water in the Winter?

With the rains that have fallen so far this winter, how often should trees, shrubs and lawns be irrigated? Irrigation frequency is always an important question, because it ties in directly with the challenge faced by desert gardeners as they strive to keep a balance between keeping plants healthy and saving a precious resource. Because every drop of water in the desert has value, it is important to understand and apply the principles of good irrigation management.

Plants of all kind need water to survive. Even plants with a low water requirement, like cacti and agaves, need water from time to time, especially in these times of drought. Trees with lush leaves, like a mulberry, will need more water than the desert-adapted mesquite with its small, well protected leaves. Apples, grapefruit, peaches and other bearing trees carrying a full load of fruit need more water than the same trees at rest. Violate a plant’s basic need for water, and they will simply not survive in our harsh environment.Water is essential to plants in many ways. Plants use water to cool themselves on warm days. They use it for transport; that is, to help move nutrients from the roots to the trees and energy captured in the leaves to the roots.

Water also keeps individual plant cells turgid, that is firm enough for the cells to maintain their shape and function. Water moving from the roots to the leaves through the process of transpiration eventually evaporates in the leaves. Water vapor inside the leaf escapes through tiny holes, called stomata, into the atmosphere. This movement of water from the inside to the outside of leaves acts like a miniature evaporative cooler that keeps the plant tissues cool.

Dissolved nutrients, like nitrogen, iron and magnesium, move with water from the roots to the places in the plant where they are used. Likewise, sugars produced through photosynthesis in the green tissues of the leaves and stems are moved from their source of assembly to the various parts of the plant, including the roots, where food supplies are needed. Movement of these sugars is made easy because the sugars are dissolved in water. Individual plant cells, especially those with relatively thin cell walls, such as those found in leaves and other tender tissues, are dependent upon water to help them keep their shape. We are all familiar with plants that wilt because they are short of water.

The demand for water within a plant to carry out its normal life functions is not the only factor that determines plant water use. The environment in which the plant is growing also plays a significant role. Relative humidity, the amount of water vapor in the air, and temperature play key roles. In the desert, times of high temperatures and low humidity like we see in June and early July, mean high plant water demand while in times of cool weather and moderate humidity, the demand is much less.

So, what does all this have to do with how often we irrigate our landscape plants? If we understand the basic water needs of plants and couple that with an evaluation of the weather, we can draw a fairly accurate estimate as to how much water a plant will need at a given time.

During the winter time, with cool temperatures and enough humidity to slow water demand, all plants will need less water than they do during the summer months. If the plants lose their leaves as part of winter dormancy, they will use little water at all. If you are still irrigating the same now as you did in the summer, you are definitely giving your plants more than they need.

During moist winters, such as the one we are now enjoying, when rainfall events are frequent and drop lots of moisture, we probably could go the entire winter without turning on the irrigation system even one time, provided we successfully filled the soil profile with a previous irrigation. Remember, we want to ensure that the entire root zone is moist, not sloppy wet, but moist. As a contrast, during dry winters it may be necessary to irrigate from time to time to keep the roots of our plants moist. However, in any case, the plants will still not need as much water as they do during the hot, summer months.

Proper irrigation during the winter means that we need to do a balancing act. We always keep an eye on the weather and frequently monitor soil moisture conditions, so that we can make proper irrigation management decisions.

If we continue to receive appreciable rainfall, we might be able to cut back somewhat on our frequency of irrigation, especially in shallow-rooted plants like turf and bedding plants. Even with a good rain, however, I doubt that we will receive enough moisture to help us with our trees and shrubs unless we have previously filled the root zone.

When you irrigate plants, have a plan. Do not just set the automatic irrigation system and forget about it. You have to change the system settings with the seasons. If you are irrigating with a hose, do not irrigate the same way in the winter that you do in the summer, or vice versa. Know when it is in your plant’s best interest to irrigate before you start running water.

The best way to tell when it is time to irrigate is to dig down into the soil with a shovel, trowel or probe to a depth of about six inches. Take a handful of soil from that depth and squeeze it in the palm of your hand. If the soil feels moist, leaves a wet imprint on your hand and, or remains in a hard ball after you release your grip, do not water. There is plenty there for your plants for the next week or so. If the soil is still slightly moist, but the ball begins to crumble when you let go, it is time to irrigate.

Plants use much less water in the winter time than in the summer, but there remains a small requirement that must be managed correctly. By carefully balancing soil water levels around our landscape plants, we can keep them both healthy and productive all year long.