Thursday, March 7, 2013

Dry Soils and Tree Watering

Dry Soils and Spring Watering of Trees,

Posted by David Whiting, Dept. of Horticulture & LA, CSU
March 7, 2013

I was out digging some winter carrots from the garden on Saturday and was amazed at how dry the soil is.  This could have implications for our trees, shrubs, and perennials.  We often talk more about the winter watering of trees and shrubs than what many of us actually get out and do winter watering.  The reality is that many landscape trees and shrubs are not winter watered, and seem to get by, sometimes.  A few of years ago, I kept telling myself that is rather dry and I should get out and water.  However, being busy, I did not make the time and I paid the price with dieback on an arborvitae.

Winter burn on evergreens, paying the price of knowing that the soil was dry but not making the time to winter water.

So while tradition speaks of winter watering, what the science say about it.

Fall watering – If we go into the winter season with dry soils (Colorado is normally dry in the fall without landscape irrigation), woody plants can lose around 20 degrees of hardiness.  That is a tree, which would normally go down to -20 degrees F, would be damaged at 0 degrees.  A tree that would normally go down to -10 degrees F, would be damaged at 10 degrees F.  Even without extreme cold, winter injury can occur on woody plants due to a dry fall/winter season. 

The take home message is that it is very important to fall water woody plants before turning off the irrigation system. 

Southwest bark damage is a symptom of drought.  It is most common on trees surrounded with hardscape features.  The hardscape reduces soil moisture levels, and reduces root spread potential.

Spring watering – Absorbing roots (feeder roots) grow in multiple flushes through the growing season.  That is a flush of root growth followed by a maturing periods with reducing water and nutrient uptake, followed by another flush of growth.  In our climate, around five flushes of absorbing root growth occur per year. 

The first flush of root growth will occur when soil temperatures rise to the mid- 40s.  However, this will occur only if soil water is available to support the growth.  If the soils are dry, growth of these absorbing roots will be delayed until moisture becomes available.   With a dry spring, we could miss the first of five flushes of absorbing root growth!  In dry years, the lack of soil moisture will directly reduce absorbing root growth potential for the season, and thus seasonal photosynthesis, tree vigor, and growth potential.

The take home message is to start spring watering trees when soil temperatures reach the mid-40s.

In dryer soils, absorbing roots also cycle into maturity more quickly, reducing water uptake potential.  If the absorbing roots overly mature (due to dry soils preventing the next flush of root growth) future growth flushes could be delayed for weeks, months, or even years!



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