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