Juan Carlos Cervantes
Extension Educator – White Pine County
Aspens are beautiful additions to any home landscape but have also have needs
Winter is upon us and even though it is not yet well advanced, thinking and talking about gardening helps one to endure the short days of light and the discomfort of the cold bitter wind.
This brief write up about landscape trees, Aspen (Populus tremuloides) to be exact, is one such attempt to distract the mind from the winter season. Aspen trees make beautiful additions to the home landscape because, provided it is planted in the right location, it adds pleasing visuals of color and movement to the home landscape and when the wind blows it gives off an agreeable sound.
These mental images alone, those of you who have aspen trees in your home landscape know to what I am referring, are reward enough during dark and cold days to discuss aspen trees.
Escape from winter alone is not the sole reason behind this article. This past summer I had several community folk drop by my office with leaf samples from their aspen trees in hand seeking answers to the cause(s) of yellow leaves and scorching of the leaf edges.
There were enough people, with no apparent connection to each other, with the same problem who came seeking advice to warrant writing this informational piece in the hope of reaching all those folks with aspen trees in their yards who face the same issue with their aspen trees but for whatever reason did not visit their local extension agent.
The simple reason this educational piece on aspen trees has relevance to the broader community is because aspens, albeit hardy and ubiquitous throughout the United States, is not a tree for all places. This fact alone warrants writing about aspen trees to the broader community because I see them I many yards.
It is true that aspen trees have the widest natural range of any tree in North America; they can thrive in a broad range of Hardiness Zones (1-7, https://www.arborday.org/images/zones/zones1-7.png); they can be found from sea level to the timberline; they can be found in 47 degrees of latitude and nine different time zones (longitude); they can tolerate heat, cold, wind, and minimal rainfall; yet they can still turn a beautiful gold color in the fall; they grow rapidly (24” in height/year) and are great as ornamental or shade trees. What is not to like? It is hardy and beautiful so most people assume that it must grow well everywhere, right? Well no it does not. Nonetheless with some planning and effort you can help your aspen trees thrive in your yard.
Ideal Conditions for Aspen trees
Aspen grows well in acidic, loamy, moist, sandy, well-drained and clay soils. It prefers abundant moisture but not without good draining, which means soils with good pores to facilitate gas exchange and root growth. . Aspens thrive in higher elevations, that is why you find natural aspen groves higher up in our mountain ranges, but can and do well in lower elevations with proper care. Aspens will thrive in full sun or partial shade, meaning it prefers a minimum of four hours daily of direct unfiltered sunlight.
Common challenges for Aspen Trees in White Pine Home-Landscapes
Soils in our valleys are tough on aspen trees because they are generally poor and with many areas heavy on clay, which interferes with water infiltration and drainage. Alkalinity also tends to be too high and some areas may have salinity levels that exceed aspen tolerance limits. Water tends not to be a constraint for landscape aspen trees because people irrigate; in fact, from my observation on the few samples brought in to the extension office overwatering is a large part of the problem contributing to yellow leaves, scorched leaf edges, or yellow leaves with a network of dark green veins.
Diagnosis is iron chlorosis
Coming up with a diagnosis for the samples I observed, and the high likelihood that this phenomenon is common in local home landscapes, is fairly straightforward given that soils in these lower elevation landscapes tend to be high in pH, high in clay content and therefore easily compacted from paws, feet, lawn equipment, and water saturation. High pH levels along with soil compaction is often points to iron chlorosis as the cause of entire leaves turning yellow or white with the outer edges scorched as well as entire leaves within the same branch turning brown from dead plant cells. In fact, it is uncommon for an individual branch or one half of a tree to be chlorotic or dead while the remainder of the tree appears normal. In some areas vegetation from the entire landscape may be affected by iron chlorosis, while in others only the most susceptible plants, like aspen trees, show iron deficiency symptoms.
Yellow leaves are a symptom of lack of chlorophyll, the green pigment responsible for photosynthesis (sugar production) in plants. Any reduction in chlorophyll during the growing season can reduce plant growth and vigor. In severe cases, or if iron chlorosis persists over several years, individual limbs or the entire plant may die. Without too much ado, iron chlorosis is a common occurrence in soils that are alkaline (pH greater than 7.0) even if the soil iron is not limiting. It is the high pH that causes chemical reactions to bind iron and make it unavailable to plants. Even if you were to apply iron shavings or rusty nails to the soil to correct for the deficiency, the high pH will cause the iron to immediately form solids and thud become available to plants. Iron deficiency and chlorosis, as stated earlier, are further aggravated by restricted air movement due to compaction, plastic mulching, and water saturated soils, conditions typically found in home landscapes. Additionally, home landscapes are low in top soils, with exposed lime enriched subsoils, which further aggravate iron deficiency. This is not say that all yellow leaves are symptoms of iron chlorosis but given all the prerequisite conditions for iron chlorosis we have pointed out that are found in White Pine County home landscapes, it is the most likely culprit. But to be sure, as stated in the last newspaper gardening write up, it is best to have your soil tested to rule out other possible causes because high pH soil also causes manganese deficiencies with similar yellowing of leaves but the good news is that treatment for iron chlorosis often ends up ruling out other problems.
Try this first -cost effective ways to prevent iron chlorosis
The best treatment of iron chlorosis is prevention because all other treatments are not easy nor are they cheap. The first step to prevention is to avoid planting cultivars which are sensitive to high pH levels but given that we are dealing with established aspen trees, there are some inexpensive treatments you can perform before you resort to more expensive treatments. First among these is to avoid saturated soil conditions for your aspen trees. If your trees are downgrade in your lawn and you water your lawn regularly, install drainage away from your aspen trees; you can also aerate the soil around your aspen tree, making sure you cover soil to the edge of the leaf drip; avoid using plastic sheeting as a mulch around aspen trees because it restricts oxygen movement into the soil; if you are using fertilizers with high phosphorous, switch to fertilizers with lower phosphorous content; avoid damage to the trunk with the weed whacker or mower; mulch around aspen trees, starting a few inches from the trunk to the drip line (outer edge of crown, if possible, it may not be realistic to put a lot of grass under organic mulch); and lastly avoid chemicals around your aspen trees.
If you first fail, try again –more costly methods to treat iron chlorosis
Soil treatments: Use soil applications to treat individual trees in early spring. A mixture of equal parts iron (ferrous) sulfate and elemental sulfur can produce lasting results and is relatively inexpensive. Select an inorganic iron source with a high concentration of iron and one that is derived from iron or ferrous sulfate. Read labels to determine iron concentrations and forms in different products.Treat small areas by making holes 1 to 2 inches in diameter and 12 to 18 inches deep. Space the holes 18 to 24 inches apart around the area within the drip line of affected trees. Fill each hole with the iron sulfate-elemental sulfur mixture to within 4 inches of the soil surface. Read recommendations for the quantity of the ferrous sulfate-elemental sulfur mixture required to treat plants according to their size. Make holes with an auger or soil probe that removes soil to reduce compaction. Avoid damaging large, woody roots when making holes. Also, check with local utility companies if making holes in the vicinity of underground utility lines.
Foliar treatment: Foliar applications are made directly on the leaves of affected plants during the growing season. These treatments produce a quick response, often in a matter of days. Response to foliar sprays, however, is often incomplete (spotty control) and temporary. Repeated applications of foliar sprays may be required if chlorosis symptoms persist or as new foliage appears. Foliar sprays are difficult to apply to large trees, however. Iron chelates are quite effective as foliar sprays. Follow label recommendations that come with these products. A 0.5% solution of ferrous sulfate applied to foliage also provides some control and is less expensive. A 0.5% solution is formulated by dissolving 2 ounces of ferrous sulfate (20 to 22% iron) in 3 gallons of water. Foliage should be sprayed in the evening or on a cool, cloudy day to prevent leaf burning. Add a few drops of liquid soap or wetting agent (available at farm supply stores) to help the solution adhere to the leaves. Repeated applications of foliar sprays may be needed if chlorosis symptoms persist or as new foliage appears.
Trunk injection or implantation: Iron compounds in dry or liquid form can be placed directly into holes drilled into a tree’s lower trunk. Systems also are available that use plastic tubing and tees, capsules of various types, or a hypodermic-like tool to place iron materials into the tree. Though these techniques can be quite effective, they injure the tree’s trunk and should be used with care. Minimize injury by using methods and formulations that require small holes (some systems use holes as small as 1/8 inch diameter), and avoid any treatment that would require injecting a tree more than once every few years. Commercial injection formulations are available as liquids or powders and should be used according to directions. Look for formulations that contain ferric ammonium citrate (iron citrate) or ferrous sulfate. Holes should be made with a sharp brad-point bit to ensure quick uptake and reduce injury. Pay particular attention to manufacturer recommendations on hole placement, angle, depth, and diameter. Studies have shown that uptake is better and more evenly distributed if holes are drilled near the soil surface on the outside of root flares. Covering or capping holes can be done for cosmetic reasons, but will not reduce the chance for decay or speed healing. Wound dressings should not be used. Injection treatments generally are most effective if applied in the early spring during bud break, but follow label directions for particular products. Treatments later in the year often will not be as effective and may not last as long. Effects can be expected to last for two or more years, after which retreatment probably will be necessary. Avoid injecting materials on hot, dry, windy days since leaves may blacken or burn, though such damage is usually temporary and not serious. Make sure the tree is well-watered for several days before and several weeks after injection treatments.
Gardening Guide for High-Desert Urban Landscapes of Great Basin Regions in Nevada and Utah, Heidi Kratsch, Horticulture Specialist, University of Nevada Cooperative Extension. SP-13-09
City of Reno Urban Forestry Commission. (2011). City of Reno Approved Street Tree Species List. Reno, NV.
Dirr, M.A. (1998). Manual of Woody Landscape Plants. Champaign, IL: Stipes Publishing. Rick Heflebower, Horticulture Agent, Utah State University
UNCE does not endorse any particular brand-name product to treat iron chlorosis
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