Watering Needs for Trees

(Question)

How is watering needs for a tree determined? It seems that the European beech tree in the neighbour’s front yard soaks up a lot of water, from the lawn. How would the balance between the tree and lawn, in watering be determined?
How is saturation(satiety?) determined, given the GTA annual rainfall?

Another neighbour has a long punctured sleeve connected to the downspout, which seems intended to take the water from eaves system. In theory this would work if it rained regularly in the west end GTA.

 

(Answer)

Watering is probably the most important aspect of ensuring the health of trees, both newly planted and well-established.  Stress from drought makes trees more vulnerable to predations by insect pests and diseases.  Drought conditions, i.e., very dry soil, can limit a tree’s ability to absorb water, and can cause smaller roots near the soil surface to die.  It is important for tree roots to absorb water right up until the end of the fall.  But how much is enough?  And how do you know?

This is a good question, especially with the GTA weather over these summer months which has veered between frequent, heavy bursts of rainfall to periods of extreme heat and drought.  Keeping an eye on the weather is something that homeowners should be doing in order to ensure the health of our gardens in general, and our trees, both mature and newly planted.   Determining the amount of rainfall or irrigation necessary to satisfy a tree’s needs is a complex subject, and there are equations available for use by arborists and other tree professionals, but for the home gardener, there are some general rules that, if followed, should help to keep our trees healthy.

Experts agree that deeper, less frequent watering is better for trees than lighter, more frequent watering.  We also know that when a tree is surrounded by a lawn, the turf will take up much of the water, leaving less for the deeper tree roots, so a longer period of deep watering will be helpful. This precept would apply to your neighbour’s European beech tree.  The European beech, Fagus sylvatica L. is considered to be a drought-sensitive tree, so in the absence of a good rainfall (a slow, steady rain rather than a quick but torrential downpour), your neighbour should consider a weekly watering.   The object is to water the roots of the tree, which is best done by running a hose slowly at soil level, at the drip line of the tree.  The “drip line” is the term that describes the area of a tree’s canopy or crown as if drawn in a circle around the trunk of the tree: when you see images of a tree and its roots, you will notice that the roots spread underground much like a mirror image of the canopy above the ground.  The outer edges of the tree’s roots are full of what are known as feeder rootlets, which are responsible for absorbing moisture and nutrients.   If you use a low sprinkler, you can place a measuring cup or other straight-sided container under the sprinkler and water until the level in the cup has reached one to two inches.  This is the equivalent of one to two inches of rain, which should penetrate the soil up to about six inches, and will reach these fine rootlets which are close to the surface of the soil.  Some experts recommend longer soaking which will go into the soil to a greater depth, especially for well-established trees.

Newly planted young trees, whose root systems are still developing, should receive regular watering, not only at the drip line, but also right around the trunk so that the root ball is well irrigated.  Some experts recommend watering every couple of days, depending upon the weather. Others suggest a weekly watering for as long as two or three years, while the tree develops a strong root system.  Transplanted older, large trees can experience significant stress from drought as well as from transplanting, and the nursery where they were purchased should advise here.  You may have noticed some of the newly planted city trees use root bags, which are a good way of delivering the right amount of irrigation.  Nurseries will be able to tell you whether your new tree is sensitive to drought.  Japanese maples, dogwoods, birches, and hydrangeas are all considered drought-sensitive.  Mulch, properly applied (that is, a layer of a couple of inches, in a circle several inches from the trunk of the tree) helps to retain moisture in the soil.

Your neighbour’s perforated sleeve is a great way of diverting rainwater from the downspout further and distributing it into the lawn or garden, away from the house foundations.  Another idea which is gaining rapidly in popularity is the rain garden, which is a landscape feature that helps to collect rain and melted snow, absorbing it into the soil and preventing it from entering storm drains.  If you are interested in how these work, here is a great description:    https://trca.ca/wp-content/uploads/2016/10/2138-STEW-Rain-Gardens.pdf

Here are a couple of websites which provide some useful information:

On irrigation of trees generally: http://www.mortonarb.org/trees-plants/tree-and-plant-advice/horticulture-care/watering-trees-and-shrubs

On the specifics of watering young trees:  https://www.extension.umn.edu/garden/yard-garden/trees-shrubs/watering-new-trees-shrubs/

Finally, the Toronto Master Gardeners has produced a guide to planting trees, and in partnership with the City of Toronto, a guide to growing trees and shrubs organically:   http://www.torontomastergardeners.ca/gardeningguides/planting-a-tree-a-toronto-master-gardeners-guide/ , http://66.209.177.85/~torontom/wp-content/uploads/2012/07/Organic-Trees-and-Shrubs.pdf

As home gardeners, it is up to us to be vigilant and keep our eye on the weather and on our trees and shrubs.   And as one of these articles reminds us, don’t forget the city trees that are on our boulevards.