Tracking the Natural Range of Trees by Annual Precipitation Rates

When I started preparing for my 2014 tree care presentations earlier this year, I realized I needed to rework one of the most important parts: explaining why irrigation is so important for Boise's urban forest. Many folks simply cannot grasp that the vast majority of these trees would not be here without supplemental water (whether or not this is sustainable, good, bad or meaningless is a rich topic for a future blog).

I blame this on the suspiciously apocryphal story of how Boise got it's name. It goes something like this: French fur traders exclaimed "Les Bois!" as they crested over a rise and saw the poplar lined Boise river below. When I hear this I can't stop imagining Herve Villechaize in a bearskin cloak, shouting "De trees! De trees!

My presentation strategy this year is to shock and awe my audience with a spread sheet. That's right, I'm going to Blind Them With Science! 

The idea is pretty straight forward. Most of the trees in our urban forest are native to the hardwood forests of the eastern Unites States. Those forests start to disappear past the Mississippi right along a magical line of demarcation where the precipitation drops below 20 inches a year. Here are the facts of life: Mountain Home gets an average of 7.62" of precipitation, Meridian gets 10.94", Boise 11.73". No water. No trees. *

Growing up first as a Marine Corp brat and then later a Forest Service brat, our family slowly moved westward from North Carolina, to Indiana, on to North Dakota and then zig-zagged across the basin and range interior west. It was a slow motion lesson in how precipitation shapes ecology.

For my presentation, I speed up this lesson by picking a common shade tree, the Silver Maple, and track it's natural range. By finding the western edge of the range and comparing that to the annual precipitation of that edge, we can infer the low end of the Silver Maple's annual water needs.

So why just pick a couple data points? I decided to find towns and cities with the highest and lowest precipitation in every state (courtesy of weatherDB). The data backs up what we know to be true, that is, precipitation begins to taper off in Nebraska, Minnesota, Kansas and Oklahoma. West Mineral in eastern Kansas, for instance, has an annual precipitation rate of 46.89", while Coolidge on the far western edge gets 16.82".

So take a look at the numbers. Basin and range topography starts to mess with the gradual decrease in precipitation as you move west**, and there are a couple unexpected anomalies (what's with Bell City, Louisiana 30 miles from the Gulf of Mexico?), but the numbers support my assertion that an urban forest in the arid west composed primarily of eastern hardwoods is by definition an artificial construct, albeit a beautiful one.

	Alabama		13.54” (Jones)	70.73” (Brooklyn)
	Alaska		3.8” (Prudhoe Bay)	156” (Whittier)
	Arizona		3.3” (Yuma)	23.91” (Superior)
	Arkansas		41.83” (Peel)	58.82” (Sulphur Springs)
	California		2.85” (Anza)	84.36” (Smith River)
	Colorado		7.04” (Monte Vista)	30.82” (Idaho Springs)
	Connecticut		42.74” (Bridgeport)	58.83” (West Hartford)
	Delaware		41.59” (Little Creek)	45.91” (Cheswold)
	Florida		42.97” (Salem)	70.73” (Century)
	Georgia		43.57” (Keysville)	80.96” (Pine Mountain)
	Hawaii		11.96” (Kailua Kona)	240.02” (Volcano)
	Idaho		7.62” (Mountain Home)	42.43” (Elk River)
	Illinois		33.35” (Arlington Hills)	50.21” (Joppa)
	Indiana		36.58” (Buck Creek)	51.17” (Rome)
	Iowa		26.38” (Larchwood)	40.91” (Russell)
	Kansas		16.82” (Coolidge)	46.89” (West Mineral)
	Kentucky		38.64” (Flatwoods)	57.73” (Cosplint)
	Louisiana		14.93” (Bell City)	67.26” (St. Bernard)
	Maine		33.6” (Saint Agatha)	56.72” (Cherryfield)
	Maryland		37.36” (Flintstone)	50.2” (Oakland)
	Massachusetts		42.55” (Orange)	53.42” (Avon)
	Michigan		41.14” (Lacota)	23.82” (Iron Mountain)
	Minnesota		20.65” (Kennedy)	36.5” (Whalan)
	Mississippi		50.22” (Lake Cormorant)	68.88” (Wiggins)
	Missouri		33.57” (Mound City)	52.26” (Lesterville)
	Montana		7.2” (Red Lodge)	37.96” (Saltese)
	Nebraska		14.21” (Henry)	34.84” (Douglas)
	Nevada		3.66” (Luning)	23.74” (Glenbrook)
	New Hampshire		38.03” (Plainfield)	98.87” (Randolph)
	New Jersey		40.01” (Ocean City)	52.39” (Dover)
	New Mexico		8.28” (Sanostee)	30.28” (Mescalero)
	New York		30.8” (Springwater)	62.69” (Bearsville)
	North Carolina		39.96” (Wagram)	90.51” (Balsam Grove)
	North Dakota		13.98” (Grenora)	25.2” (Abercrombie)
	Ohio		31.48” (Farmer)	47.71” (Harrison)
	Oklahoma		15.79” (Felt)	56.14” (Broken Bow)
	Oregon		8.92” (Boardman)	122.28” (Depoe Bay)
	Pennsylvania		33.05” (Tioga)	54.21” (Brownfield)
	Rhode Island		43.58” (Block Island)	51.42” (Smithfield)
	South Carolina		39.96” (Tatum)	80.96” (Mountain Rest)
	South Dakota		14.52” (Ludlow)	31.34” (Jefferson)
	Tennessee		73.49” (Gatlinburg)	41.01” (Kingsport)
	Texas		8.93” (Fort Hancock)	64.2” (Orange)
	Utah		5.23” (Monument Valley)	34.42” (Brian Head)
	Vermont		33.64” (North Hero)	55.84” (Manchester)
	Virginia		35.28” (Timberville)	52.78” (Tyro)
	Washington		7.42” (Royal City)	115.62” (Amanda Park)
	West Virginia		33.02” (Kirby)	59.87” (Snowshoe)
	Wisconsin		23.82” (Dunbar)	37.86” (Belmont)
	Wyoming		6.48” (Big Piney)	22.11” (Moose)

*OK, that's a bit too simplistic. We do have riparian trees that cling to the banks of our waterways like a mercy seat, but the only native, non-riparian, deciduous tree in our area is the shrubby, brutish Celtis reticulata.

**Idaho City, for example, is 36 miles northeast and 1200 feet higher than Boise and receives 27.95" of precip. compared to Boise at 11.73".

Clearance Pruning Mature Trees

Back in the prehistory of the last millennium, Stacy and I headed down to Florida for our honeymoon.  I snapped a shot of this southern live oak (Quercus virginiana) in St. Augustine.  The ancient tree looks as if it's about scoop up and run off with my new bride.

Low, lateral branches like this are a kid's best friend.  Which is why, a Boise forester explained to me, city trees are limbed up long before they can develop these low branches.  A pity.

A pity too that upon occasion, mature trees need to be trimmed because of clearance issues.  Better to take out just low limbs than an entire tree though, and better to perform the trimming in such a way as to minimize the stress on the tree.


A new driveway is scheduled to be installed right under the large, low limb of this ash.  Rather than make the Airstream slated to be parked there do the limbo, the owners asked me to trim up the tree (arborists call this "raising the canopy").

Removing large limbs on a tree creates open wounds that make the tree more susceptible to disease and pests which can then lead to decay.  It's critical that pruning cuts are performed correctly to expedite the healing process. This healing process is accomplished by a process that the late Dr. Alex Shigo called the "Compartmentalization Of Disease In Trees" (CODIT).  Cells form walls around the wound, effectively quarantining the area to prevent the spread of decay to the rest of the tree.      

So, where and how do you make the cut?  Let's take a look.

Preparing for the first cut.  (Yes.  I will get off the branch first, smarty-pants.)

It's very easy to see the demarcation between the branch and the trunk of the tree in this photo.  That line is called the branch bark ridge, and it's where the bark from the trunk joins the bark of the branch.

Nestled in the branch bark ridge is another area called the branch defense zone.  Cells in this area are responsible for the growth of woundwood that will ultimately grow over the cut.

Woundwood callous is close to sealing off pruning cut

If the branch collar is damaged (often by flush cuts), the woundwood will not grow from the damaged area.  This prolongs the tree's susceptibility to decay.

These branches weigh hundreds of pounds so the standard three cut process ain't gonna work.  I trim the branch in small sections, working my way in from the tip until I have a piece small enough to hold.  An undercut a quarter of the way up, and a final cut from the top (carefully trimming along the branch bark ridge), and viola!  A clean cut.

Remember, not all branch bark ridges are perpendicular to the trunk of the tree.  Take a close look before you cut!

Half a day later...

From start to finish.

Saguaro Canyon, Idaho

Out of all the bizarre, pre-recession themepark residential developments in the Boise area, Saguaro Canyon in Meridian gets my pick for the best.  All those Tuscan villas, the Carolina antebellum neoclassical architecture, and Sun Valley inspired landscapes with aspens full of borers aren't even in the same league.

Sadly, you'll not find a single saguaro at Saguaro Canyon, but you will find a rare collection of specimen sized yuccas and Joshua Trees strangely interspersed with more commonly found Deodar Cedars and Weeping Sequoias. 

An ambitious experiment in landscaping with plants of questionable hardiness, this is what Saguaro Canyon looked like shortly after it was completed.

Ferocactus wislizenii and friends....

At the entrance you're greeted by a who's who list of Sonoran/Chihuahuan superstars.  And, just to make sure you get the subtle vibe of the place, massive slabs of Arizona sandstone were installed as a backdrop.

A sexy night time shot a little further in illuminates Yucca  filamentosa x thompsoniana, Yucca brevifolia,  Cylindropuntia imbricata,  Agave havardiana,  Fouquieria splendens and Opuntia x somethingoranother.


I stopped by a few weeks ago in the bitter cold to see how our southwest friends were holding up.  By this point, I figure that we should be able to get a decent sense of what will survive and what won't.

Let's fast forward five years after the installation was completed ...

On a cold and snowy day...

The sandstone is still there, as are the Deodar cedars, the weeping sequoias and spruce....

A leashed Yucca  filamentosa x thompsoniana trying to make an escape.

The lone Agave survivor.  A. neomexicana?  A. havardiana?  You tell me.  A Yucca baccata sulks in the background.

Yucca  faxoniana on the right.

Cylindropuntia imbricata looking to tangle with anyone dumb enough to amble by.

No sign of the narrow Fouquieria splendens (bummer) or even  the Opuntia polycantha. Unsurprisingly, Ferocactus wislizenii has also left for the great desert in the sky (it was a nice idea, though).  The oft planted Yucca filamentosa 'Color Guard' looks to have been added later.

Driving a bit further into Saguaro Canyon you start to see specimen sized Yucca faxonia as well as more Yucca  filamentosa x thompsoniana (or is it Yucca rostrata?)

Yucca brevifolia

The most gratifying part of the visit was seeing just how well the Joshua Trees are doing.

Scattered throughout the common area landscape, all the specimens look to be thriving. 

     A veritable yucca copse!

The clubhouse.

All I'm gonna say is that cultured stone looks better in photographs.

Another Yucca faxonia nestled against a south facing wall.  It's where I'd be if I was a yucca forced to live in Idaho.

Yucca baccata hanging with his pal Ephedra equisetina

Cercocarpus trimmed into tidy little snow cones. 

Why stop there?  Surely the Chrysanthemum nauseosus could be trimmed into fish and flying monkeys?







So, we've got a clear idea now of what will survive in southwest Idaho, right?  

Not really.  

Sure, I didn't expect the Ferocactus wislizenii or the Fouquieria splendens to overwinter, but what happened to the hardy Opuntia or the Festuca idahoensis?  I think this hints at some irrigation issues, but it's hard to pin down how soil, drainage and other factors played into the plants' success or mortality rate.

I'm pleased that the Idaho Botanical Garden now has a xeric test garden where these factors can be measured and controlled.  In a couple of weeks I'll be sharing a picture tour of that as well.

1001 Blogs You Must Read Before You Die

'Cause, what the world needs now is another folk singer like I need a hole in my head.
~David Lowery

In the spirit of New Year self-reflection, I pose this question: does the world really need another garden blog?  I've been writing in what could be kindly described as a leisurely manner since 2009, but I've yet to declare my raison d'etre.

I promise to keep this short, but let's backtrack to 1979...


I still remember the day my Dad brought home ELO's Greatest Hits.  The instantly accessible melodies and faux symphonics dovetailed perfectly with the classical music and Beatles I'd been listening to all my young life.  So when I first read that Jeff Lynne had once said that ELO was formed to pick up where I am the Walrus left off, it kind of made sense.  The problem is, he never said it.

But there it was again.  Reading through my new copy Stacy gave me for Christmas of 1001 Albums You Must Hear Before You Die, I noticed the review of ELO's 1977 Magnum opus, Out Of The Blue made the same reference to I am the Walrus.

There's probably not a great deal of harm done by misquoting some frizzy-haired musician who's managed to hang on to the same set of sunglasses since 1974.  The problem is, I see this kind of thing happening a lot in garden/landscape blogs: the dissemination of information that has a ring of truth to to it, but is patently false.  Blogged.  Reblogged.  Over and over again.

So am I some sort of self-appointed gatekeeper?  John the Baptist in the Wilderness? Nah, I'm just a punk who doesn't have a problem pointing out that the Emperor left his pants at home.

The first time this happened I was still under the naive assumption that everyone is seeking the truth.  I'd been chatting with some Dr. so and so at a local Ag supply outfit about soil conditioners when the conversation turned suddenly to his research into the recent "billbug epidemic in turf grass".  

Innocently enough, I remarked that I had heard of this infestation but had yet to see it in the hundreds of properties that we cared for.  I went on to tell him that, in those instances where customers thought they had billbugs, it turned out to be problems related to sprinkler coverage and exposure - both problems which could be mitigated by relying less on turf in landscape design.  He threw me out of his office.

Sitting in my truck in the parking lot, what should have been clear all along suddenly became obvious: tacking on a "Dr." to the front of your name doesn't make you any more of a truth disseminator than a contractor with stained Carharts.  Dr. so and so's agenda was simply to create a new revenue stream to the business.  New infestation = new product to sell.

I'm still on the look out for billbugs and I will continue to keep an open mind about these kind of things but, ultimately, I'm going to rely on the gardener's best tool: our own power of observation.

So does this world need another gardening blog?  It does if said blog has something interesting to say and speaks truth to power, money, misinformation, greenwashing, trendiness, elitism, pseudoscience and just plain bad taste.

But it's not all about being a contrarian. In future posts I'll flesh out what xericoasis actually means. I'll also continue to share thoughts and pictures from the field, spanning the whole arc of Willowglenn's services, as well as gardening, cooking (and eating!) in our own urban acre.

Here's to a fantastic 2014!