Growing the Desert Garden

Welcome to the Desert Garden, with garden coach Tyler Storey, where we talk about everything having to do with gardening and landscaping in the Desert Southwest. From composting to Cercidium and agaves to arugula — we'll cover everything you want to know to grow your own beautiful Desert Garden.

Friday, June 20, 2008

From the Inbox: Pruning for Monsoon Season

From a Correspondent:

I read your article titled Unhealthy Obsession: Pruning Trees which prompts me to ask, how is one supposed to prune in preparation for monsoon storms? In April of 2006, I planted an Ironwood, a Palo Brea, and a Guajillo, which I'm training to be more tree than bush/shrub-like. In October of 2007 I planted a Palo Verde, and a Foothills Palo Brea trees. They're all under 7 feet tall and the last two are under two feet tall. None are staked. I water deeply 2-3 hrs, every two weeks (in summer) per your guidelines. The 2 small trees are watered for shorter periods and at more frequent intervals – until they're established. Is this correct? I'm uncertain how to prune correctly to ensure I don't lose them to monsoon winds, but prune conservatively as you recommend. Only the Ironwood, Palo Brea and Guajillo have been pruned. I pruned primarily to remove branches which crossed, grew vertically downward, and would create a safety issue or eventually be too low to walk under. The Palo Brea is my biggest concern because it's developing nicely, but with a wide canopy I fear will flail wildly during the monsoons. I know NOT to 'lion-tail' trees and that isn't the look I want to achieve anyway. I want them to grow naturally so they ultimately become beautiful focal points in my desert-landscaped yard. I DO want them lush with full canopies, but how do I prevent them from being toppled when they're still so young?
Karren
Maricopa, Ariz.

Karren,

Thanks for the question and for providing such detail. I think I could teach people how to care for trees just by telling them "Do what Karren does." Planting small trees in the Spring or Fall, not staking, watering deeply, and pruning only as necessary, are the best steps you can take to make certain that you have healthy, vigorous, and storm-resistant trees.

From what you describe, I wouldn't worry about your trees making it through the monsoon season storms. Trees suffer blow-down for two main reasons: because they have a weak or unbalanced canopy structure, and because they have a weak or shallow root system. Correct pruning takes care of the former, and correct watering takes care of the latter.

So, let's look at the details. Planting small trees is the first correct step. Small trees, in containers of 15-gallon size or less, are more likely to have a good balance between the root system and the canopy size. If a tree's canopy is too large for the root system to support, then blow-down is likely and staking becomes necessary. Staking causes its own problems; trees develop strong trunks by swaying in the breeze and incorrect or over-long staking can prevent the trunk from becoming sturdy. It's not uncommon for staked trees to fall over once their stakes are removed.

Next comes water. This is the one area where you might check to be certain you're doing things correctly. All trees, regardless of size or variety, should be watered to a depth of three feet every time they're watered. The idea here is that we want the roots to grow down deeply into the soil to anchor the tree. Most roots won't grow any deeper than three feet, so watering more deeply is a waste. You can use a soil probe to check how deep your water is going.

The next element to preventing blow-down is to water widely. You want to be certain that your trees have wide root systems that anchor them into the soil. One of the chief causes of blow-down, especially with Mesquites, is a very narrow root system supporting a wide canopy. Think of your watering pattern as a donut with the tree in the middle of the donut hole. You want your donut's inside and outside edges to be several feet on either side of the drip line (the edge of the canopy). So, while it sounds like your watering time is probably good, do check the depth and the width.

And, at last, comes pruning. They key here is to go slow and steady. All of the trees in your list are native desert trees with naturally low-breaking or multi-trunk forms. As you prune them up to form a higher canopy, continue to do it slowly, over a number of years. Lower branches on trees help to form a strong and sturdy trunk. If we prune off too many lower branches all at once, the trunk stays thin and spindly, and the trunk is unlikely to ever gain the girth that is needed to support the canopy. As you prune up, your ideal is to "clean" no more than 18 inches of trunk a year, less when the tree is quite small. Leave those branches to develop the thick trunk and you'll have a strong and sturdy tree.

As always, we can do everything perfectly and nature still might throw us for a loop. One of the characteristics of monsoon-season storms is the occasional micro-burst; if your tree gets hit with one of those, it's going to go over. But for now, don't worry about it. If it does happen, it will make a good story, one that with careful management can get even better over the years.

I hope this helps,

Tyler

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