At the first hint of cold weather, we unhook the hoses from their faucets and winterize the irrigation system. But your landscape still needs water. Foggy mornings, heavy frost, and cloudy skies often give a false impression that plants have adequate soil moisture.
Looking at the Mesonet records for the OSU/OKC campus area of western Oklahoma County, we only received 2.10 inches of moisture since Oct. 1. Since Nov. 12, we have received zero inches of precipitation. Of the 2.10 inches received, 1.77 inches came Oct. 13 and the November rain came Nov. 11.
The low amount of moisture, coupled with higher than average temperatures and windy conditions, can lead to serious problems in the landscape.
Winter watering can be crucial to having healthy plants in the landscape. During the summer months we are clued in to water stress by observing wilting and dropping of leaves. Plants that experience winter drought can't tell us until the next year when they fail to thrive. Winter drought can lead to root injury or death.
Symptoms of drought injury can range from the top of the tree or shrub being dead the following spring or they may leaf out and flower just fine in the spring, relying on stored food reserves. Once that energy supply runs out, plants weaken and start dying back. Even if a plant isn't killed outright, it is made more susceptible to insect and disease attack.
Woody plants with shallow root systems require supplemental watering during extended dry winter periods. These include birches, maples, and ashes. Evergreen plants that can benefit are junipers, arborvitae, euonymus, yews, and magnolias. Woody plants benefit from mulch to conserve soil moisture.
The best thing to do is to water the landscape in the late fall. The most critical time to water is just before cold weather hits or during periods of extended warm weather. It is interesting to note that not so many years ago, horticulturists thought it was good practice to insure dormancy by drought-stressing plants in the fall to decrease the chance of winter injury. Research since then has indicated that the reverse is true ... so now we try to ensure plants aren't drought-stressed in the fall.
As you apply water this winter, allow the water to soak in slowly to a depth of 12 inches. Water when the ground is not frozen and the temperature is over 40 degrees. Since winter watering is more labor intensive to you, apply the water where it counts the most, in the root zone. Consider that established trees have roots that go out at least as far as the tree is tall and usually further. It is in the “drip line” and just beyond where most of the water should be applied. The “drip line” is an imaginary vertical line that is perpendicular to the longest side branches of the tree and perpendicular to the ground. Water applied at the tree trunk base is wasted because there are no water absorbing roots there.
Watering recently planted trees and shrubs is a different story. Their roots do not go out as far yet. In this case you will want to water the root ball zone and just beyond. The aim is to water where the roots are.
Keep in mind that even when we get rain, trees and shrubs planted close to the foundation and under eaves may still need watering. Located in these areas, they receive little precipitation and they lose more moisture than other plants because of their proximity to the structure and reflected heat from the walls.
Ray Ridlen is an agriculture/horticulture educator for the Oklahoma County Extension Service. His column addresses frequently asked horticulture questions. For more information, call 713-1125.