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Rain, Rain – Don’t Go Away!

Article By Joran Viers, City Forester

Rain, Rain- Don't Go Away!

The heavens open up and the rain falls. Hallelujah, we all cry in unison! Rain is good, rain makes the plants grow. Trees all over town are feeling relieved, or so I imagine (despite what some have suggested, I don’t really commune with trees at that deep metaphysical level). Of course, with happy trees you get weeds. Our local weed palette is filled with things like pigweed and kochia, which can grow enough to make seeds even in a dry year, but in a wet year like this they kick into overdrive. Puncturevine, a.k.a. goathead, pops up overnight, seemingly blooming the day after that. Sand bur proliferates at the edges of sandy softball fields. I’ll take that trade, though – all the plants we do like in our landscape love the rain as much as the weeds do.

Thirsty Trees

Even with the rains, though, some trees suffer a lack of water. Trees planted in very compacted soils and/or soils of limited volume simply can’t get enough of that fleeting but fundamental resource. Trees with a plastic or fabric layer under the mulch that covers the root system may not find that water, as the water finds its own path of least resistance to be on top of and over that layer, never entering the soil. A little apple tree I planted this spring has turned up its leaves, which are drier by the day even though I’ve watered that site. I think the Bermuda grass around it simply isn’t sharing. That’s how Bermuda grass is, selfish and spiteful; that’s why it does so well as a turf grass, too! Once the young tree has suffered too much, it can’t recover, the damage is done and it’s permanent. The other apple tree I planted is doing real well; it doesn’t have a Bermuda grass lawn surrounding its young root system so maybe I’m on the right track.

Understanding Your Trees

Finding the right track is a tricky and often unsuccessful process, I hate to admit. When a medical doctor examines a patient, they have the advantage of years of specialized training, millions of dollars’ worth of research, and patients who can give them feedback. Most of us tree experts have some advanced training, but not the kind of intense program that medical school offers. We work with patients of many different species, not all of whom respond the same to stressors and treatments. And, our patients don’t speak to us in a direct way; we can’t ask them to rate the pain on a scale of one to ten, or how their leaves are feeling. So we have to back up a bit, take a broader view, and try to understand the hints a particular tree in a particular site may offer us.

How do Trees Communicate?

Trees have a limited number of ways they can express stress and related problems. A very common symptom is leaf damage – wilting, browning, etc. While very obvious, this kind of symptom may not tell us much. For instance, wilting leaves do suggest a lack of water – in the leaves. But why is it happening? Maybe the soil is too dry and more irrigation is needed. Maybe so much irrigation has been applied that the soil is saturated to the point all oxygen has been driven out, which causes the roots to not work well at their key task of bringing water in (roots need oxygen in every cell for metabolic processes to occur). Maybe there is enough, and not too much, soil moisture but it is high in salts, making it impossible for the roots to take in due to osmotic pressures. Maybe all of that is fine, just fine…but somebody damaged the trunk in a way that is cutting off moisture flow. The point is, there are a lot of ways to arrive at wilted leaves! As an aside, beware the landscape care company that can point to vague symptoms and tell you the tree has this or that specific disease. Even the State Plant Pathologist down at NMSU can’t tell by looking, and she is the expert. Verify any claims by consulting with the local Cooperative Extension Service office here in Bernalillo County.

Bagworms

Sometimes leaf damage has a very specific and obvious cause. Bagworms, for instance, have been wreaking havoc around town this year. These cryptic caterpillars spin a silken bag to hide in, embedding the outer surface of the bag with old leaf bits, small bark chunks, and other plant material. It’s a very effective camouflage, and even more effective protection. I’ve tried to tear the little pouches open to expose the critter inside, and I can’t. Cutting them open with a sharp knife works, though, and if you do that in the fall or early spring you may find a mass of yellow eggs inside some of the bags. That’s how they pass the winter, as dormant eggs hidden in leaf-covered sleeping bags.

Under the Bark

Often, leaf damage looks really scary – the tree turns ugly, all chewed up and brown where it should be green. However, leaves are meant to be expendable and the damage is usually less than it appears. More worrisome is damage to the vascular tissues of the tree; certain insects and diseases attack these important water and sap moving vessels. Because the xylem and phloem (science words of the day; look ‘em up!) are found underneath the bark of the tree, they are not visible and so damage can go unnoticed until the whole tree appears to crash, declining quickly from apparent health to apparent death. The bad part of this is that most of the vascular-attacking organisms can’t really be controlled once they are into their favored environment. Prevention becomes the best control, and prevention mostly means keeping the trees healthy in the first place - which brings us back to rain.

The Benefits of Rain Water

Water is life, especially for non-desert trees forced to live in a hot, dry climate like ours. Rain is the best water source; it’s clean, it’s not alkaline like our ground water, and sometimes (if the rain storm involved lighting) there is even a small dose of growth-stimulating nitrogen in the rain, a by-product of the intense electrical activity of a lightning strike. Rain falls evenly across the landscape (at a small scale, that is – we’ve all seen those New Mexico rain storms that hit this side of the street but not that side!), moistening the soil uniformly. Roots tend to be distributed widely underground, so this wide distribution gives maximum access for the tiny root hairs to take in the life-sustaining water.

Be Grateful, Be Prepared

But we do live in a desert, and rain like we’ve had is the exception, not the rule. Enjoy it while it’s here, appreciate that the plants in our landscapes are enjoying it, and keep the irrigation hose handy. Let’s not let the rain’s positive impact evaporate when things dry out. Trees provide so many benefits to us in our built environment; we need to give back to them to keep them green and growing.

Joran Viers

City Forester

Parks and Recreation Department

City of Albuquerque

505-857-8627

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