While good management techniques can partially compensate for the problems caused by a bad soil, an argument can be made that an athletic field is only as good as the soil that’s under it. Soil is important because it provides something for the grass plant to root into and is thus critical for stability. It should provide a resilient surface that’s capable of tolerating the traffic and heavy use typical of sporting events. Soil also serves as the reservoirs for water and nutrients and should have adequate, but not excessive, amounts to ensure the grass grows correctly. Roots also need air — thus, an ideal soil for plant growth has a ratio of solids (50 percent) to liquids (25 percent) to gases (25 percent).
What is Compaction and Why is it a Problem?
Compaction, basically, is the pressing together of the solid particles in the soil so that soil occupies a smaller volume. As a result, the soil has a larger proportion of solids and less space for liquids and gasses. This has a tremendous impact on the root system of the plant, and therefore, also the ability of many plant species to survive or thrive.
Soil particles are generally categorized by size into three categories: sand, silt and clay. Sand has the largest particle size. Because of this, there are larger spaces between particles, resulting in larger pores and more rapid movement of water both into the soil (referred to as infiltration) and out of the soil (percolation, or drainage). Clay is the opposite, as it has very small particles, a large capacity to hold water and nutrients and drains very slowly. In addition to these inherent differences, clay soils, that are often what we have when managing native-soil fields, tend to be more susceptible to compaction.
Weeds as Indicators of Compaction
Certain species of plants are more capable of adapting to compacted soil conditions. Unfortunately, our desired athletic turfgrass are not among them. Many of the plants that can survive repeated mowing and compacted soil are some of our common weed species.
What follows is a description of some of the weeds that are most frequently observed in compacted soils, as well as strategies for their management. But it should be stated that herbicidal strategies for the management of these weeds should be considered “Band-Aid” fixes. If these weeds are present in large numbers, they serve as indicators that your soil is compacted. Management practices, including proper core aerification and topdressing programs, should then be followed in an attempt to alleviate the compaction and therefore make the desirable turfgrass more competitive:
- Eleuisine indica is a tropical, warm-season summer annual grass. It’s somewhat similar in appearance to crabgrass in that
the ligule is a membrane surrounded by a fringe of hairs. But goosegrass will lie with the stems distinctly prostrate, radiating from the root system, and the sheath tends to be white. For this reason, goosegrass is sometimes called silver crabgrass. Another way to differentiate it is to look at the seedhead. Crabgrass flowers and seeds are attached individually to the flower stem, whereas goosegrass flowers and seeds are attached in distinct clusters. This results in a zipper-like appearance to the seedhead from a distance.
- It’s important to know the difference between crabgrass and goosegrass. While they can both infest a turfgrass area, if you have more of a problem with goosegrass on your field it could be because of a compaction problem. Also, for control, not only are the active ingredient recommendations for herbicides different, but also the recommended application timing. Goosegrass has shown resistance to the dinitroanaline herbicides, including pendimethalin and prodiamine. Because of this, cool-season turf herbicides such as oxadizaon and dimethinamid-p are recommended. Also, goosegrass germinates a bit later than smooth crabgrass, when soil temperatures have reached the low 70s (Fahrenheit). Goosegrass may also be controlled in cool-season turfgrass with repeat applications of postemergence herbicides such as fenoxaprop or topramazone.
- Poa annua is a cool-season turfgrass with complicated taxonomy. There’s an annual variety (Poa annua annua) and a perennial variety (Poa annua var. reptans). This isn’t something humans invented, like different cultivars of grass; it occurs in nature.
- Annual bluegrass can be identified usually just by observing that it’s flowering. While every species of grass will flower at least once per year, annual bluegrass can be seen in flower just about any time of the year that the temperature is above 40 degrees. It’s (mostly) a winter annual, meaning that there will be a primary flush of new germination in the fall and a primary flush of flowering in April. But, since it’s opportunistic, there are always plants that are going to seed. In the absence of this though, we can use morphological features to identify it as well. It’s primarily a bunch-type plant, with a boat-shaped ligule and a prominent membranous ligule.
- Annual bluegrass can be controlled by use of preemergence herbicides. But this strategy isn’t usually desirable because the application timing of the herbicide is in the fall, when we’re usually trying to overseed athletic surfaces. Control can be 70 percent or better, but the annual bluegrass that does germinate tends to be more than capable of producing enough seed to perpetuate the infestation. Plus, if you also have the perennial biotype of Poa annua, preemergence herbicides are going to be less effective.
- Postemergence herbicides have been developed for selective removal of annual bluegrass. Depending on your sport or play, this may or may not be an option. Ethofumesate, for example, works best if applied sequentially in the fall, while amicarbazone works better if applied sequentially in the spring. (Both herbicides will interfere with overseeding efforts). Mesotrione has also been used, but with mixed results. In some cases, control can be improved by combining mesotrione and amicarbazone. Rates and timing recommendations vary by turfgrass species — follow the label carefully.
- Polygonum aviculare is one of the first of the annual weeds to germinate in the spring, as early as late February. Its germination precedes smooth crabgrass by several weeks. Interestingly, because of the way the first leaves emerge from the soil, it’s not uncommon to hear folks mistake this weed for smooth crabgrass. These differences become clear in short order, since this is a broadleaf weed. Mature prostrate knotweed tends to form a mat and has a distinctive blue-green color. The stems are dark green as are the leaves, which are alternately arranged and vary in shape from oval to almost linear. There are small white flowers from late spring until frost, but these are not noticeable unless seen up close.
- Euphorbia maculata is another prostrate, matt-forming annual weed. Unlike knotweed, spurge germinates later in the year, often several weeks after goosegrass and may still be seen germinating during July. Knotweed and spurge are sometimes confused with each other, but there are some easy ways to tell the two apart. The stems of prostrate spurge may be either smooth or hairy and may be distinctly reddish in color. Also, the leaves can be variable — sometimes they’re a uniform green color and other times there may be a spot on the leaf, hence it’s other common name, potted spurge. To identify it and differentiate it from knotweed, notice that the leaves have an opposite arrangement, as opposed to the alternate leaves of knotweed. Also, if you break a stem, the sap of prostrate spurge is milky or whitish in color, whereas knotweed sap is clear
- There are quite a few herbicidal options for the control of prostrate knotweed and prostrate spurge, each with advantages and disadvantages. Since both weeds are annuals, there are preemergence herbicides that are labelled for their control. There are two issues with this, however. One is timing — knotweed germinates very early in the season and spurge tends to germinate late and may still be germinating in July. Most preemergence herbicides are effective for about 90 days, thus applying the preemergence early enough to control the knotweed runs the risk of there not being enough herbicide left for residual control of either late-germinating crabgrass or prostrate spurge. If you only have a problem with one of these weeds, this might not be a concern. But the bigger issue is that preemergence herbicides also quite effectively control our desired cool-season grasses, thus no overseeding operation can take place during the period that the herbicide is active. And, since both weeds tend to be more of a nuisance on bare soil areas, there’s a good chance that the strategy of using a preemergence product isn’t going to be compatible with your overseeding schedule.
- Because of the incompatibilities typically seen with turfgrass overseeding and use of preemergence herbicides, best control of spurge and knotweed is with postemergence herbicides. It’s typically recommended that a three- or four-way herbicide be applied to maximize control. For both weeds, combination herbicides that contain both 2,4-D and triclopyr tend to be more effective.
- Best control of prostrate knotweed and prostrate spurge is when they’re very young. There are two reasons for this. One is that the weed is easier to control when it is young. It can be difficult to achieve good control with postemergence herbicides after it has matured or hardened off due to summer heat and drought stress. The other reason to get them while they’re young is to prevent them from flowering. Both are annuals, meaning they come into the field from seed. If you can prevent the seeds from forming, you can start to reduce your population of these weeds.
- Plantago major is a fibrous, rooted perennial that forms distinctive rosettes of shiny leaves, oval in shape, with smooth margins and veins that run parallel to the leaf margin. It also has a distinctive flower spike.
- The important thing to remember when it comes to control of broadleaf plantain is that it’s a perennial. As such, it will have underground parts and structures from which the leaf tissue can regenerate. Spring time applications of postemergence herbicides may give some control. But there’s chance the weed may grow back from below ground after a period of 60 to 90 days. Best control of perennial broadleaf weeds, including broadleaf plantain, is with an application of a postemergence herbicide in the fall, preferably in November. Ester forms of the herbicide are most effective during spring months. Air temperatures should be in the 40s to 50s and soil temperatures should be in the 50s to 60s.
- About the time that you mow the turf for the last time (so it’s green, but no longer growing) is the best time to make this application. Herbicides applied in the fall tend to translocate below ground better than when applied in the spring, which improves the chances of getting long-term control. The other thing about the late fall application is that it shouldn’t interfere with your fall seeding operations, as any germinated turfgrass should be old enough to have been mowed three times prior to this application being made.
Author’s note: The list of weeds found in compacted soils varies perhaps by location, perhaps based on opinion. For example, I could have included information about path rush, bermudagrass or chickweeds. Perhaps plantain isn’t as frequently seen in compacted soil as some of these other weeds, unless you also have low fertility. The key — with compacted soil or any other condition — is to recognize that some weeds serve as indicators of cultural deficiencies that can be corrected, such as not enough fertility or not enough water. In the case of weeds that signal compacted soil, the take home message is that after you control the weed with an herbicide, you should try to correct the compaction that caused it to be more competitive with the turfgrass.