Researching Maintenance

Give your tree some PEP You recently reported on the use of PEP for wound dressing on trees. What is PEP and where can I get it?--Wisconsin

PEP is short for polyethylene plastic, a common plastic used for many purposes. When I spoke with Dr. Robert Blanchette, the University of Minnesota plant pathologist who conducted this study with graduate student Dennis McDougall, he stated that the specific type of plastic was not particularly important. In fact, neither was the plastic's thickness or color (clear vs. black). The key factor was preventing moisture loss. Thus, most plastics would be suitable as long as they were thick enough to withstand the elements for a couple of weeks while fastened to the tree with duct tape. (Blanchette pointed out that trees gain no benefit by keeping plastic wraps in place for longer than this, but he noted that you should apply them the same day of the pruning or wounding.) In his study, Blanchette used 2-mil plastic, roughly the thickness of a garbage bag. It isn't clear whether speeding wound closure necessarily reduces the chances of infection. Regardless, Blanchette currently is conducting additional tree-wound research by studying the effects of wound dressings that contain microorganisms antagonistic to wound-infecting pathogens. These treatments may effectively limit decay-causing microorganisms while wound closure takes place.

Pre-emergents for narcissus Will pre-emergents harm narcissus?--Pennsylvania

The question would be better phrased as "Will properly applied pre-emergents harm narcissus?" After all, almost any material can cause phytotoxicity if you apply it incorrectly. However, assuming that the applicator is willing to follow label instructions, numerous pre-emergence products are reasonably safe on narcissus.

Bensulide, metolachlor, napropamide, oryzalin, isoxaben, propionamide, pendimethalin, prodiamine, trifluralin and combinations thereof list narcissus on their labels (see "Turfgrass Chemical Update: Herbicides," Grounds Maintenance, January 1997). These products constitute most of the chemicals available for pre-emergence control of weeds in ornamental plantings, so you actually have a wide selection from which to choose. In addition, most of these chemicals list many other ornamentals, including bulb species such as tulip and crocus, so they should be safe on many mixed plantings as well.

Be sure to read label instructions closely; many restrict use to certain times or growth stages. Plus, in some cases, one brand's label may list bulbs while another with the same active ingredient does not.

Rust in young turf I have noticed that lawns installed on newly constructed home sites have more of a tendency to develop rust than established lawns. The topsoil at these new sites is rather poor. Is the rust developing because the grass is young and susceptible or because of the poor topsoil?--Pennsylvania

Two factors may operate in such situations, and your question touches on them both. First, young turfgrass plants are more susceptible to rust. In addition, rust thrives in low-nitrogen conditions such as often exist on new sites with poor soil. Thus, turf seedlings planted on new sites are primed for rust infestation. However, even sod is more susceptible on nitrogen-poor sites. Dr. Douglas Brede, a turfgrass researcher with Jacklin Seed, notes that newly planted sites often have little reserve soil nitrogen to offer young plants. Thus, the seedlings--already inherently susceptible to rust--are even more prone to infection when they begin to use up the nitrogen present from applied fertilizer. Brede states that turfgrass often is able to "outgrow" rust, but it cannot do so if low-nitrogen levels limit its growth rate. Thus, maintaining adequate nitrogen levels is doubly important to seedling turfgrass.

Concern exists with some people that composted lawn waste may contain pesticide residues. If that's true, using them in vegetable gardens, for example, may pose a health risk. This concern has gained greater attention as more cities use composting as a way to dispose of yard waste.

University of Nebraska researchers recently conducted a study, partly funded by the City of Lincoln, to determine which pesticides were present in lawn-waste compost. The researchers tested compost from Lincoln's municipal recycling program and found that residues of eight pesticides were present. Seven of the eight were present in levels below health-based screening values (HSVs) used to determine risk. The eighth was dieldrin, which exceeded its HSV by a small margin.

Obviously, dieldrin is not your usual lawn pesticide. Neither are the other pesticides they detected. In fact, all these chemicals--DDT, DDE, DDD, chlordanes, heptachlor, heptachlor epoxide, dieldrin and endrin--have been severely restricted or completely banned for many years. The environmental persistence illustrated by this study is, in part, what led to their discontinuation. Conversely, currently registered products, including those used for lawn care, were absent from the compost.

The researchers tested soil from home yards to determine the source of the pesticide residues and found many of the same chemicals (these products were common termiticides in the past). Noting that residues of these chemicals in municipal compost tend to rise early and late in the season, the investigators suggest that aeration and dethatching--more common in the spring and fall--may bring soil upward so that it mixes with lawn clippings and becomes part of the waste stream. The soil tests also revealed the presence of two currently used pesticides: pendimethalin and chlorpyrifos. Why aren't these chemicals also found in compost as are the older compounds? Lacking the same persistence, they apparently do not make it through the composting process without decomposing.

Is there any danger from using compost that contains residues of these older compounds? Not according to the researchers. After all, if soil is the original source of the residues, what harm is there in mixing the compost back into the soil? To confirm this, the researchers grew carrots and radishes in compost-amended soil. Analyses of these vegetables, after peeling, showed no residues. In fact, the researchers suggest that using compost in this manner could actually dilute the chemicals present in the soil as well as speed up their decomposition by enriching the soil.

Research consistently demonstrates that typical fertilization practices do not generally pose a serious threat of nitrate leaching in dense, healthy turfgrass. Turfgrasses, due to their high nitrogen-utilization efficiency, trap nearly all mobile nitrates susceptible to leaching. However, no one had studied differences between species, and between cultivars of a species, until researchers conducted a recent study at the University of Rhode Island. Several factors affect leaching potential, including soil characteristics, precipitation levels and fertilization rates. Could leaching potential also vary by turfgrass species and cultivar?

The researchers looked at 10 cultivars each of Kentucky bluegrass, tall fescue and perennial ryegrass to see if differences in soil nitrate levels and leaching potential could be attributed to genetic differences between species and cultivars. They applied 3 pounds per 1,000 square feet (50 percent from ammonium nitrate and 50 percent from urea/methyl urea) per hectare per year, split over three equal applications in April, June and November.

Using soil lysimeters in established turf stands, the researchers periodically sampled soil water over a 2-year period and tested the samples for nitrate content. Using percolation models to estimate when leaching (due to heavy precipitation) would have carried nitrogen below the root zone, they estimated how much nitrate-nitrogen would be lost to leaching. Presumably, high nitrate levels in the soil water would be due to low uptake by the turfgrass roots, resulting in more mobile nitrates susceptible to leaching.

In general, results were consistent with other studies that found only small amounts of nitrogen are subject to leaching in turfgrass soils. Nearly all samples in this study tested well below drinking-water standards for nitrate content. More to the point of the study, the researchers found significant differences between not only species but cultivars as well.

Tall fescue had the lowest leaching potential, but perennial ryegrass was a close second. Kentucky bluegrass was relatively high, but most of the measurements were well within drinking-water standards. Nitrate levels in perennial-ryegrass soil water averaged more than 2.5 times those of tall fescue. However, Kentucky-bluegrass levels averaged more than 2.5 times as much as those of perennial ryegrass. Average calculated nitrate losses due to percolation amounted to about 7 percent of applied nitrogen in bluegrass, 2 percent in ryegrass and 0.8 percent in tall fescue. Among cultivars within each species, similar differences occurred. For example, the highest Kentucky-bluegrass nitrate reading was more than four times that of the lowest Kentucky-bluegrass reading. The tall-fescue and perennial-ryegrass averages varied less but still significantly.

Soil-water sampling took place over 2 years, and the researchers found significant variation occurred in nitrate leaching during this time. Generally, the periods when leaching estimates were highest corresponded to heavy precipitation or periods of low turfgrass growth when roots were not actively taking in nitrogen. Thus, fertilizer-application timing can affect leaching potential. The researchers are not sure what accounts for the cultivar differences. However, they suggest that because of these variations, breeders should document nitrogen-use characteristics of turfgrass cultivars. Most cultivars probably are well within acceptable limits. For example, in this study, the leachate of only one variety (a Kentucky-bluegrass cultivar) averaged higher than drinking-water standards allow, and only by a small margin. However, the use of varieties that lower the chance of leaching could help allay environmental concerns in unusually sensitive sites.

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