Wringing water from trees

enlarge this image

Above: Water a shade garden, you water a tree. Best to learn how much water the tree will take and how to be sure your garden gets its share. For instance, did you know that big flare roots, such as the visible sections of white pine roots (bared in an interesting project we can tell you about another time) are not themselves very effective at absorbing water? As they grew they gave up that duty and now serve as transport channels for the water collected out beyond the branch ends. There, water gathering's done by young, opportunistically produced root tips. 

It's a ruthless battle in which the most leaves wins

Keys to growing a good garden under trees:

Overcoming low light,
Alleviating water shortage and
Choosing plants for the fewest inter-species antagonisms.


Here's what we know about water use by trees

and how you can wrangle a fair share for the garden plants.

Recognize big tree root zones
An ungodly amount of water, going up
Water a lot, still not enough!
Useless to cut established tree roots
Exceptional root pruning
More answers,  mulch and timing
Summary of watering recommendations
Case study, under two maples

Recognize big tree root zonesHarneyfailedBedNotedSS.jpg

We looked at this site (right) in Failures tell tales and will use the Norway maple just outside the garden's west edge to describe tree root zones and water use.

This tree has a 40' branch spread, but a root zone perhaps 100' across. That's a conservative estimate; roots extend 1.5 to 2 times farther than branches, or more.

An ungodly amount of water, going up

The U.S. Environmental Protection Agency has calculated that a tree absorbs a bit more than a third of any rain that falls on its root zone. It may take every drop of a gentle rainfall, less of a downpour.

So consider a one-inch shower that falls on our 40' maple's territory. It amounts to 4,900 gallons and the tree can very quickly draw in 1,700 gallons. It can do that day after day. At that rate, from the 240 square feet of bed shown in this picture, the underlying roots can accept 50-60 gallons at a pop.

In addition, there's a silver maple just south of the bed, almost certainly drawing from that same plot.

Water a lot, still not enough!

Now, lay out a generous irrigation system of 50 or 60 trickle emitters, each releasing between 1/4 and 1 gallon per hour -- 15 to 60 gallons of water oozes onto the bed in an hour. Given the trees' appetites, the system must run for several hours a day or its whole output is going to the trees.

Again, that's a conservative estimate. We know from personal observation that one 3/4" diameter root of an elm that slipped over the liner of our 8' x 10' pond was lowering the pond 2-3" per day -- that's 50 gallons. One root.

"Steven, what's the deal with the pond? Is the waterfall flowing over the edge? I added three inches of water just yesterday..."

Tree roots in the bed under this Norway maple will have gone into high gear in terms of growth, simply because the garden was watered. Each root will have produced a greater number of ephemeral hair roots to make the most of the bounty.

Below, left: An explosion of roots at the tip is typical of many tree species. We took this root cutting from the slightly depressed trench at the edge of an irrigated garden, where water tends to accumulate. The root grew modestly branched tips until it reached that trench. There, the presence of extra moisture enabled this extravagant growth.

Below, right: Some species' roots are more opportunistic than others. Here are two roots, a silver maple root on the left growing into the bed and a magnolia root on the right growing out (arrows indicate direction of growth). We exposed and photographed just where they grew, at the edge of the magnolia's bed. Both have been cut before (at the top edge of the cardboard backing) as we maintained the bed edge. Notice how both branched at that point, but many more roots developed from the cuts to the maple root, than on the magnolia root. Also, notice how the maple has produced new root tips even from older wood. The maple's the more serious competitor -- bet on it!

RootTipsMagno0996s.jpg RootTipsAcerMagnN8177s.jpg

Useless to cut established tree roots

While you have a picture in mind of the root-load in this area, you should be able to see why cutting tree roots is not an effective way to gain more water for other plants in an area. If you cut a 3/4" root that was extended to the far edge of the root zone, branched and producing most of its hair roots there, it will now branch from the cut, within the bed. Growing on the energy provided by hundreds of thousands of leaves, it will outpace the growth of any garden plant root.RootCollTipCut0941s.jpg


All of these roots were cut and then grew new tips. They come from one garden but eight different tree and shrub species. The different amounts and patterns of root growth demonstrate their species' genetic programming. An arrow indicates the location of an old cut from which a proliferation of tips developed.

More about differentiating between plant roots in What's Coming Up 106, page 13.


Stimulated to branch repeatedly within a relatively moist bed, the root once cut may never grow back out to forage far fields but stay put, increasing the overall drain on the bed's moisture.

So, if a tree root is in your way as you dig or plant in a shady garden, spread your perennial's or annual's roots over it, or shift to plant next to the root. Don't cut it.

Exceptional root pruning

The one exception, the time when root pruning can make a difference, is root pruning if the tree is young when you start and if you also prune that tree regularly to stay small.

Cutting roots without limiting the crown, too, is a losing battle. More leaves simply make more roots and increase the pull that moves water from soil into roots.

So keep that young tree's roots out of an area by drawing a line early on and trenching along that line annually. Also limit the tree's leaf surface with regular pruning.

There is more about root pruning in  Growing Concerns 640: ...Root pruning.

Here's a Parrotia tree root, pruned six months before the photo was taken. Also shown, the tree's growth rate during that interval. You can see all the new root tips that have developed from the cut. We can see that the branch growth rate is normal for the tree. So we know the tree is responding well to the treatment and staying healthy.


More answers, from the third and fourth dimensions: Expanding root space, and respecting important times

Use a very finely shredded bark mulch or compost in a shady garden, and renew it in fall. Don't wait until spring. Plants adapted to shade take advantage of a solar window to grow very early in the year. They root into last year's leaf litter, drying it and making it their own because it's then unattractive to tree roots. This all happens before tree roots begin vigorous growth each year, as the leaf buds open.

Weed in fall and don't allow a single weed to remain, especially near garden plants' crowns. Even tiny weeds are big water users and fierce competitors when they grow in close.

Summary of recommendationsDonnellyV1B4Aft241s.jpg

  • Water slowly and copiously every day. Let it trickle 'round the clock, or water every morning, all morning. No way can shady perennials compete with a tree during peak hours in the afternoon, when water literally runs up the tree, drawn by evaporation.
  • Start watering early in spring before the maples flower, spot watering emerging perennials.
  • Do not tolerate any weeds, not one. Even the tiny ones are killer competitors for scarce moisture.

Above, right: Did you notice the weeper hoses in this photo from the related article,  More light under trees?

Below: The garden in our case study. Between the photos, two tree prunings, many changes in watering and almost three growing seasons have passed. (The shade remained unchanged, contrary to the effect presented by different camera angle. The left hand image was taken looking east in the morning, the other looking west at 4 in the afternoon when sun reaches in for about 90 minutes.).

 Vahlb1B4Aft505s.jpg Vahlb2B4Aft506s.jpg

Case studies, under big trees

We planted this backyard garden under a big silver maple and a younger red maple. (Younger means growing faster and sometimes using more than its average share of water.) We helped the owner learn how to care for the garden, stopping by when there were problems. We traced most trouble to drought stress -- mildew prone plants succumb to mildew when dry; blooms abort; leaves discolor. At almost every turn, we prescribed more water.

We knew we were making headway when, by the end of the first summer, each of the owner's calls to us began, "First, I have the water turned on. Second..."

The following spring we installed a system of weeper hoses and convinced the owner to use the pressure regulator (most people don't understand the purpose of the funny washer with a tiny opening), fill the hoses in spring then turn them down to the barest trickle and simply leave them on all summer. Then the garden "took." He kept a record and told us he didn't use much more water that year than when he had been throwing it around in the air.

Often, we waste a lot of water to wind and evaporation, and also in run-off when we apply it too rapidly.

Below: In this garden, too, trees dominate: One Norway maple, two huge oaks and three birch trees grow in one city back yard. Soaker hoses left on almost continuously did the job. Two summers passed between the images, with overhead watering in play the first year, soakers in year two.

JordanNewB4Aftr498s.jpg Jordan2YrsB4Aft499s.jpg