Protecting mature trees in a xeriscape conversion

Xericoasis would be a pretty stupid blog title if I wasn't at least casually interested in water conservation and zone sensitive landscape design. But the truth is, I'm getting increasingly annoyed at what I see as a "kill a tree, save the planet" attitude in a lot of xericape conversions.

It happens all the time in the urban forest of Boise's North End. Somebody decides that the Kentucky Blue grass in the front yard is sucking up too much water to be sustainable. Fair enough. But then they cap off the sprinkler system, rototill and remove the grass, install weed fabric and spread (God forbid) six yards of perma-bark over the whole area. They finish off the conversion by planting a handful of weedy looking natives and run single drip emitters to each plant.

I usually come on to the scene three to five years later when the eighty year old shade tree in their front yard starts dropping dead branches on the roof and they want an arborist to tell them why...

stressed tree in parking lot "xeriscape"
Tack isn't one of my strongest personality traits, but I do my best to politely explain to the client that:

1. the rototiller destroyed a large amount of the tree's fine water absorbing roots
2. the "perma-bark" superheats the soil
3. the weed fabric creates a hydrophobic dead zone, and,
4, the minuscule amount of water the drip line provides might keep a one gallon Eriogonum alive, but it won't do much for a tree that receives 30-60" of rainfall in its native range

Invariably, I hear the comment that that , "well maybe we shouldn't plant shade trees in our climate". Horse Puckey, I say. The benefits of our urban forest and it's role in such things as mitigating the urban heat island effect is well worth the investment of water it takes to sustain it. Besides, many properties in and around Boise are blessed with abundant non-potable irrigation water- a vestige of our valley's agrarian heritage.

Poor water coverage often seen in drip irrigation  systems
In Willowglenn installations, we often forego drip systems for a high efficiency broadcast sprinkler like Hunter's MP rotator nozzle. Efficient broadcast sprinklers more closely emulate natural rainfall, and encourage lateral root development. I've seen too many trees blown over because some genius thought that putting a bubbler at the base of tree was sufficient. It may keep the tree alive for awhile but it never develops decent lateral roots that would give it stability in the wind.

Integrating existing shade trees into a xericscape conversion can be done, but you have to be smart about it. The fact is, that 80 year old shade tree might have done OK hanging out with the turf grass, but turf grass and trees actually have quite different irrigation needs. Turf grass requires frequent watering but most trees prefer infrequent deep watering- a great starting point for the design of a mildly xeric design.

Choose an organic mulch and plant your xeriscape bed with a range of plants that provide four season interest. Take care to try and not damage the roots of your existing tree as you plant the shrubs and perennials and keep an eye on the tree for signs of stress.

How's this for an idea: let's keep our trees and save the planet. Brilliant.