Homegrown column Aug. 15 2009

My aspen tree appears to be stressed. The leaves look dry and are turning black around the edges; I don’t think water is the issue, but I’m not really sure. I guess I need to know exactly how much water it should be getting.
What could be causing this leaf problem, and what can I do to help the tree recover? I really don’t want to lose it.
— Leah

What you’re describing sounds like a leaf scorch. This happens when the leaf is losing water faster than it can be replenished through the roots and stems. The leaf gets a yellow, brown or black margin on it and it can work in to involve the entire leaf at times.

 To begin with, it’s a good idea to think about anything that would interfere with absorption of water by the roots and its transmission through the stems to the leaves. It could be a problem with watering. It could be a disease organism or insect killing off roots or portions of the stem’s vascular tissue. It could be a physical injury from bark damage to trenching or grade changes that damage roots. It could also be a function of the weather or the nature of the plant (more on that later!).

 When I talk to folks about leaf scorches and watering, their first reaction is usually “oh, I need to water more.” I suppose that makes sense on the surface (since the leaf isn’t getting enough water) but it often isn’t the solution to the problem; in fact it can sometimes make the problem worse. Watering problems are probably the most common causes of leaf scorches here in western Colorado, but it’s just not under watering that can be blamed. Over watering can be just as common a problem as under watering and cause the same symptoms. You see, roots breathe like we do. They need oxygen to survive and if the soil stays too soggy consistently, the roots can begin to suffocate and shut down and die off. This can obviously lead to water stresses in the plant, but not from lack of water, but because there’s too much water! I know it sounds kind of contradictory, but we see this all the time.

 It’s probably a good idea for you to do a little digging down to check what the soil moisture is like. Is it wetted deeply enough? I like to see water penetration 12” to 18” when you do water. You then need to try to let the soil dry SLIGHTLY before soaking it again. aspen aren’t terribly drought tolerant trees (though they’re more drought tolerant than most folks give them credit for) so watering regularly is a must; however, they can be over watered just like everything else.

 Also, take a close look at the trunk of the tree from the soil line up past the first several branches. Is there anything unusual there? Look for cracking, peeling bark; missing bark (especially at the base due to weed eater damage); sunken areas; discolored patches; holes; sawdust; bleeding; etc. Any of these things might indicate an insect, disease or physical problem. Has there been any digging or regrading of the soil near the tree in the past four or five years? A new sewer line or sprinkler system? Soil brought in or scraped away to regrade the area near the tree?

 Finally, we come to the last of the possibilities, weather and the nature of the plant. I think this is what’s going on here. aspens really are not well adapted for life down in the Valley.

Don’t get me wrong, there are tens of thousands of the trees growing down here (I’ve had them in my yard too!) but they’d much rather be up at 10,000 feet where it’s thirty degrees cooler and it rains every afternoon (but then wouldn’t we all!).

 It is common for the leaves of aspen trees to scorch to one extent or another during the summer. You’ll see a yellow to brown to black halo around the edge of the leaf.

Depending on the tree, its age, the care it’s getting and the weather this halo can be a faint coloring at the edge or it can work its way to the middle of the leaf with the majority of the leaf turning brown or black. The tree may look a bit haggard by the end of August, but it will sprout out fresh green leaves next spring. Concentrate on providing the care the tree needs and in time that leaf scorch will tend to at least look not quite as bad. Hope this all helps.

Dennis Hill is the nursery manager at Bookcliff Gardens, online at http://www.bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or e-mail .(JavaScript must be enabled to view this email address).


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