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