Trees Under Attack!
How to care for and protect our urban forest, Part 2.
Trees are an integral part of our urban landscape. They cool our homes, shelter us from cold winds, and are particularly pleasant to picnic under. Yet sadly, we take them for granted. We assume they get everything they need without any intervention from us. In the most technical sense, this is true. Trees make their own food, and in nature, pick the best spots to grow.
But in an urban setting, this is not the case. Water, sunlight, nutrients, space: We often don’t give these factors a second thought when we buy, plant, and live with trees—until the tree starts to get yellowing leaves, dead limbs, or worse. Then, finally, we may notice, but sadly, it may be too late.
Last month in CATALYST, James Loomis (“Garden Like a Boss”) discussed the peril our green canopy is facing due to damage caused by under-watering and provided helpful guidance on correct watering techniques. That isn’t the only treecare issue, though.
It may seem improbable, but trees can suffer from an over-abundance of water, even in this desert climate. Water displaces oxygen in the soil. Tree roots need oxygen as much as they need water. When soil is too wet over a period of time, roots become water-logged, decay and die. When a tree is habitually over-watered, it dies.
Soil can play a role. Heavy or clay soils hold more moisture and drain more slowly, whereas sandy, loose soils don’t hold any water and may prompt more frequent watering. So, pay attention when you water. Does the water puddle on the surface? Does it seem to drain away quickly? Digging down a few inches, a day or so after you water, can help you understand the characteristics of your soil.
Plunging a screwdriver into the soil is another handy trick: It goes in easily (or relatively so) when and where the soil is damp. If you can get a screwdriver in 4 to 6 inches, the soil is adequately moist, so hold off on watering. But this trick does not work in very sandy soils, so, you’re stuck digging.
If you aren’t interested in getting dirty, there are signs to look for that indicate over-watering: perpetually damp or smelly soil; smaller, yellowing, or scorched leaves; and shallow surface roots.
What can you do about over-watering? Well, if you’re the cause, knock it off. It’s important to apply enough water to sustain a tree, so you may not want to give a tree less volume of water (see last month’s article), but you may need to water less frequently. You may find that a downspout is directing rain run-off directly to a tree. Redirecting the downspout or digging a furrow to redirect the flow can help protect the tree. Also, stop fertilizing the tree, if that’s been part of your tree care ritual, at least until the tree recovers.
Trees planted in well-watered lawns are in particular danger of being over-watered, as the preferred watering regimens for trees and lawns are very different.
Some strategies to mitigate potential harm: First, remove the lawn away from the base of the tree, ideally to the extent of the canopy. Second, direct irrigation spray heads away from the tree so that the watering pattern is more disbursed. Not over-watering your lawn will also help.
Complicating the issue is that not all tree species have the same water needs. For instance, willow, birch, and sycamore need more water; oak, Kentucky coffeetree and limber pine need comparatively less water. So, get to know your soil and your trees. Check out websites like slcgardenwise.com or cwel.utah.edu for watering guidance.
The dreaded power trimmer
There is probably not a bug, be it crawling, sucking or burrowing, that can do damage to an urban tree more quickly and thoroughly than the dreaded power trimmer. Whether you call it an edger or weed whacker, when you get them near trees, they are mechanized tree-destroying machines. And unlike some urban tree threats (think drought or disease) this threat is entirely in our hands. Literally. In our hands.
You might think of trees as impervious. After all, they’re big and strong. They’ve been around awhile. They’ve got that bark that protects them from the vagaries of weather. But that bark is not armor. And it can be damaged.
Think of bark like you would skin. It keeps stuff that should be inside, well, inside, and outside stuff, well, okay, you get the idea. That skin is flexible, expandable, and porous; so is bark. And while skin is pretty tough, it can get bruised or cut. So can bark. And if that cut is deep enough, blood vessels and bone can be damaged. The similarity with trees continues. Bark protects the vascular tissue of a tree, called the cambium layer. This layer holds the conduits that move nutrients and moisture between roots and leaves. It is what keeps a tree alive. Under this cambium layer is the heartwood. It is disused cambium tissue that has hardened, and what gives a tree its ability to be erect.
Here’s an irony: You may follow all the guidelines and tips about watering appropriately, but if you damage the cambium layer or gird the tree, it cannot take full advantage of the water and nutrients you have painstakingly applied.
Bark can be an inch or more in depth, or as thin as 1/16th of an inch. Either way, there isn’t much between a tree and the rapidly rotating metal or nylon blades of a power trimmer. The damage can happen fast, and if it happens frequently, it will be permanent and devastating. Because after you cut through the bark, you begin to cut through the cambium layer. When these pathways are destroyed, the tree is denied critical nutrients and water. Even if the tree can recover from some of the woundsw, the tree still may experience decline and dieback. If the damage goes around the entire base of the tree (something called girdling) the tree will, in all probability, die.
Preventing damage to trees from trimmers is very easy: Keep them far away from your trees. If a tree is planted in lawn, make certain no grass abuts the tree trunk, so that a tree cannot be struck by either a lawn mower or trimmer. This will keep the tree safe, and be easier on the person mowing. Also, it’s a good thing to mulch around a tree (about 3-4 in. in depth), but avoid piling the mulch against the trunk.
And so, tree-loving urbanite , now you know what to do to be a good steward of our urban forest: If the tree in question is already planted, you can provide enough water to sustain health and support growth without over-watering, and by avoid mechanical damage that can prevent a tree from taking advantage of that appropriately applied water.
However, if you’re the one planting the tree, you can begin by making better choices about species and locale before you visit the nursery and purchase a little plastic tub of tall, leafy goodness. Let’s chat about that soon, shall we?
Stephanie Duer has been planting trees around the Salt Lake Valley for the last 20 years as a professional landscaper. She is currently the water conservation manager for Salt Lake City. Prior to coming to Utah, she helped clients create sensible, beautiful and sustainable landscapes throughout the west.