Keep plants, trees healthy with fall and winter watering


By Liz Haynes

SUN Columnist

Quick facts:

• Water trees, shrubs, lawns and perennials during prolonged dry fall and winter periods to prevent root damage that affects the health of the entire plant.

• Water only when air and soil temperatures are above 40 degrees F with no snow cover.

• Established large trees have a root spread equal to or greater than the height of the tree. Apply water to the most critical part of the root zone within the dripline.

Dry air, low precipitation, little soil moisture and fluctuating temperatures are characteristics of fall and winter in many areas of Colorado. There often can be little or no snow cover to provide soil moisture, particularly from October through March. Trees, shrubs, perennials and lawns can be damaged if they do not receive supplemental water.

The result of long, dry periods during fall and winter is injury or death to parts of plant root systems. Affected plants may appear perfectly normal and resume growth in the spring using stored food energy. Plants may be weakened and all or parts may die in late spring or summer when temperatures rise. Weakened plants also may be subject to insect and disease problems.

During drought conditions

Limited summer rainfall and water restrictions can deplete subsurface soil moisture. Upon digging, people may find little moisture at eight- to 12-inch depths where most tree roots are located. Paying special attention to fall watering is important for trees to mature buds and enter dormancy in a healthy condition. Consider tree watering in addition to whatever general landscape sprinkling local water restrictions allow in fall months.

Colorado horticulture experts recommend watering underneath the branches within the circle bounded by the drip line. Water to a depth of 12 inches. Trees should receive ten gallons per inch of trunk diameter measured at knee height. This amount can be reduced by that supplied by general lawn watering or if rain or snow is received. Water trees three times per month in September. Cut back to one or two times per month from October through March, two times monthly for young trees and for evergreens.

Mulch within a circle bounded by the drip line to a depth of four inches, allowing six inches of space between the mulch and tree trunk. A mulch circle of any area will be beneficial whether it extends to the dripline or stops short of that.

Many water application methods can be used. Consider soaker hoses, soil needles or soft spray nozzles. On hard or compacted soils, soak, wait, and soak again to avoid water runoff. Be especially careful with soil needles, also known as deep root feeders. Some people insert these well below a 12-inch depth, placing water out of reach of tree roots. Soil needles should be inserted at an angle to a depth of six to eight inches. Leave the needle in place for three to five minutes with water turned on low to moderate pressure. Water the area under the branches in at least 12 sites for a medium-sized or larger tree. Disperse water sites evenly within the circle bounded by the dripline. For new trees, water all four sites at least three feet from the trunk (stem).

In dry years, established shrubs will need additional amounts of winter watering. Apply five gallons for a small shrub (less than three feet), and 18 gallons for a large shrub (more than six feet) on a monthly basis from October through March. Newly planted shrubs will require more winter water, twice monthly using these same amounts at each watering. Be sure to mulch shrubs to retain moisture.

Drought injury

Woody plants with shallow root systems require supplemental watering during extended dry fall and winter periods. These include European white and paper birches; Norway, silver, red, Rocky Mountain and hybrid maples; lindens, alder, hornbeams, dogwood, willows and mountain ash. Evergreen plants that benefit include spruce, fir, arborvitae, yew, Oregon grape-holly, boxwood and Manhattan euonymus. Woody plants benefit from mulch to conserve soil moisture.

Herbaceous perennials and ground covers in exposed sites are more subject to winter freezing and thawing. This opens cracks in soil that expose roots to cold and drying. Winter watering combined with mulching can prevent damage.

Lawns also are prone to winter damage. Newly established lawns, whether seed or sod, are especially susceptible to damage. Susceptibility increases for lawns with south or west exposures.

Watering guidelines

Water only when air temperatures are above 40 degrees F. Apply water at mid-day so it will have time to soak in before possible freezing at night. A solid layer (persisting for more than a month) of ice on lawns can cause suffocation or result in matting of the grass.

Plants receiving reflected heat from buildings, walls and fences are more subject to damage. The low angle of winter sun makes this more likely in south or west exposures. Windy sites result in faster drying of sod and plants and require additional water. Lawns in warm exposures are prone to late winter mite damage. Water is the best treatment to prevent turf injury.

Monitor weather conditions and water during extended dry periods without snow cover, one to two times per month.

Newly-planted vs. established plants

Newly-planted trees are most susceptible to winter drought injury. Woody trees generally take one year to establish for each inch of trunk diameter. For example, a two inch diameter (caliper) tree takes a minimum of two years to establish under normal conditions.

Trees obtain water best when it is allowed to soak into the soil slowly to a depth of 12 inches. Methods of watering trees include: sprinklers, deep-root fork or needle, soaker hose or soft spray wand. Apply water to many locations under the dripline and beyond if possible. If you use a deep-root fork or needle, insert no deeper than 8 inches into the soil. As a general survival rule, apply 10 gallons of water for each diameter inch of the tree. For example, a two-inch diameter tree needs 20 gallons per watering. Use a ruler to measure your tree’s diameter.

Newly planted shrubs require more water than established shrubs that have been planted for at least one year. The following recommendations assume shrubs are mulched to retain moisture. In dry winters, all shrubs benefit from winter watering from October through March. Apply five gallons two times per month for a newly planted shrub. Small established shrubs (less than three feet tall) should receive five gallons monthly. Large established shrubs (more than six feet) require 18 gallons on a monthly basis. Decrease amounts to account for precipitation. Water within the dripline of the shrub and around the base.

Herbaceous perennial establishment periods vary. Bare root plants require longer to establish than container plants. Perennials transplanted late in the fall will not establish as quickly as plants planted in spring. Winter watering is advisable with late planted perennials, bare root plants, and perennials located in windy or southwest exposures.

Information provided by Dr. J.E. Klett, Colorado State University Extension Horticulture Specialist and Professor, Horticulture and Landscape Architecture; and C. Wilson, Extension Horticulture Agent, Denver County. 


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