From the City Forester: Is It Time to Water My Tree?

Just as the cool air seeping into my bedroom windows makes me snuggle deeper under the warm covers, that same cool air is letting our trees know that their rest is coming – fall is here, winter soon to follow.  Leaves turn bright hues of yellow and red, or in some cases dull brown, as the trees pull back nutrients and prepare to drop their foliage.  Those fall colors sometimes show up early in the season, late summer, and when that happens it’s a sign of stress.  Usually, almost always, the stress is too little water.

Trees are big plants with lots of leaves; therefore they use a lot of water.  Living in a desert as we do presents some particular problems to us in our tree care efforts.  Smart use of water is the key to growing trees (and getting all the great benefits that trees give us!) without breaking the bank or draining the aquifer.  Sometimes, smart use means more use.

What do I mean by “more use”?  Let us step back a minute, to my sentence above: big plants use a lot of water.  If we under-water our trees, they don’t grow well and may even go into decline spirals, ultimately ending in the death of the tree.  A dead tree doesn’t give us those benefits, and so all the water we used getting the tree to a mature size is lost, wasted, if we don’t keep that mature tree alive. 

There are at least two ways to think about under-watering trees.  One has to do with how deeply and broadly we apply water; think of this as the water footprint.  The other has to do with how often and during what seasons we water.  How much, how often?

How much turns out to be a really hard question to definitively answer in broad terms.  Younger, smaller trees need fewer gallons of water than larger, mature trees.  If the tree is growing well, it will benefit from a bit more water every year than it got the year before.  Of course, there are limits, but don’t expect a young tree that thrives on ten gallons of applied water per week to be happy on that amount five years from now!  Instead of thinking about how many gallons of water, think in terms of what amount of soil got moistened.  Most tree roots grow out beyond the tree’s own canopy, and are largely found with about two feet of the surface.  Try to apply water to the soil in a wide ring around the tree – the inside of the ring is about half the distance from the trunk to the canopy edge, and the outside of the ring is the same distance beyond the canopy edge.  Adjust for the reality of your site, and that gives you the wetting target.  However you apply the water (soaker hose, multiple drip emitters, slowly puddling garden hose, etc.), apply enough so that one day after watering, the soil has some moisture down at least 18 inches. 

How often is a little easier to answer, but there are still important nuances to consider.  If the tree has recently been planted, its root system will not yet have grown out into the surrounding soil very much.  In that case you want to water it like it’s still in the nursery – a little bit every couple of days.  If it’s an established tree, the ideal is probably about every two weeks.  If you can water a broad ring (or portion thereof) to about 18 inches deep, watering every two weeks should keep the tree reasonably happy.  On real sandy soils, you might want to do this every week; on heavy clay soils, every third week.

What about fall and winter?  The standard recommendation is one broad, deep watering per month during the dormant period of the tree.  This keeps the roots in good shape, and allows them to quickly provide water as the new leaf buds begin expanding in the early spring.  A lot of drought stress could be avoided by paying more attention to winter water needs.  Deep snows and long, slow rains count, but light dustings and brief showers don’t penetrate deep enough to store much water in the soil.

Lastly, consider mulching around your trees, especially newly planted young trees.  Organic mulches like bark, wood chips and pecan shells cool the soil without heating the air around the trees, and as these materials decompose they promote the formation of tree root friendly soils.  It is preferable to avoid using weed fabric below the mulch, as that presents a barrier to the infiltration of the decaying organic matter into the soil ecosystem.  Weeds often grow with their roots on top of this material anyway.  For weed control, nothing beats diligence and a good hoe!

Don’t forget we live in a desert!  With very few exceptions, any tree we plant will need ongoing irrigation for its whole life.  However, the many and very real benefits of trees in our urban environment make that water investment a very solid investment, in our quality of life now and into the future.