Fall and Winter Watering

Fall and Winter Watering

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

When irrigation water is turned off and systems blown out in the fall, your landscape plants still need water.  Foggy mornings, heavy frost, and cloudy skies often give a false impression that plants have adequate soil moisture.  Relatively dry air and low or no measurable precipitation lead to dry soils.  This means that even in the fall and winter... trees, shrubs, and lawn grasses need water to avoid drought stress.  This is particularly true during the fall and early winter when there is little or no snow cover... or when there is an extended warm fall.

Fall and winter watering can be crucial to having healthy plants in the landscape.  During the summer we are clued in to water stress by wilting and dropping leaves.  Plants that experience fall and winter drought can’t tell us something is wrong until the next year when they fail to thrive the next year.  Fall and winter drought can lead to root injury or death.  These drought-injured plants may not show symptoms of the problem until the next season or even the next year.  In fact 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.

This type of “winter” injury seems to be more common than we realize in this area.  For the past several years, we have been losing many of our area birches.  This has been attributed to past winter droughts and cold temperatures without the benefit of insulating snow cover.  Birches are particularly sensitive to fall and winter drought.  Weakened by this, the birch trees that weren’t killed outright have become increasingly susceptible to attack by the bronze birch borer, which attacks and eventually kills weakened birch trees. 

Other shade trees are also susceptible to winter drought damage, especially those with shallow root systems.  This includes Norway maple, silver maple, linden, Colorado blue spruce, Norway spruce, and many other evergreens.  Shrubs are also vulnerable to winter drought damage, especially those growing up close to the house or in a warmer location.  This includes junipers, Oregon grape-holly, and euonymus.

So what’s the answer?  It’s simple.... water the landscape in the plants and the fall.  Yes, it will be work and you’ll have to use your domestic water but, the effort will be worth it when you see that your plants survive and others around you don’t do as well as yours.  I’m not talking about watering plants every day.  .... you’ll probably only have to water a couple of times in the fall to prevent damage. 

The most critical time to water is in the fall just before cold weather hits or during extended warm weather.  It’s interesting to note that not so many years ago, horticulturists thought it was a good practice to insure dormancy by drought stressing plants in the fall and this decreased the chance of winter injury.  Research since then has indicated that the reverse is true... so now we try to insure plants aren’t drought stressed in the fall.  The soil should be kept slightly moist down to a depth of 18 inches for most shrugs and a depth of 18 to 24 inches for trees.  Water only when the air temperature is above freezing and the soil isn’t frozen... which isn’t usually much of a problem during the fall but can be during winter dry spells.  Water early in the day to allow water time to drain away from the bases of plants.  (Frozen water next to the bark can physically damage trees and shrubs.)  Soaker hoses work well for applying the water slowly and where needed.

Since fall and winter watering will be more labor intensive for 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 “dripline” and just beyond where most of the water should be applied.  The “dripline” 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 don’t go out that 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.  This makes sense doesn’t it?

Keep in mind that even if we do get lots of rain during the fall, shrubs and trees close to the house foundation or located 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.