Q: As part of my IPM program, how do I best determine a threshold for acceptable weed cover on an athletic field?
– John Brant, supervisor of sports and recreation grounds, Ohio University
A: This question is common, and not easy to answer with one blanket statement. On certain fields, the threshold may be zero tolerance. This is because safety and playability come first. On other fields, the goal may not be to eliminate all weeds, but to keep the number low enough as to prevent unacceptable visual appearance or an uneven playing surface that increases risk to athletes. There may even be some areas, such as general lawns and heavily used recreation fields, where all weeds are tolerated.
The answer is further complicated because it depends on what species of weeds you deal with. Spurge and knotweed, for example, may produce green cover in the summer; but since they are annuals, those areas will be bare soil from October until spring. This may be acceptable if the field is used for summer sports but not for fall sports. Another consideration is how much damage the weed infestation does to the turfgrass. Sedges are unsightly during the summer, but, unless it is a very severe infestation, this species tends to not thin the turfgrass and reduce playability in the fall. On the other hand, species such as crabgrass and goosegrass will result in stand loss.
Since it’s not an exact science, the most recommended course of action is to monitor your weed populations and collect data. Collecting data can be a challenge, so maybe have two approaches – weeds that spread (clover, weed grasses) could be assessed as a percentage of the grass cover, while tap-root weeds (dandelion, plantains etc.) are physically counted. Collecting data helps everyone understand the issue better, and can be used to set future goals. In essence, data is power! Collecting data can done in a couple of ways; (1) Subjective assessment. For example, a turf manager determines percentage weed cover. Subjective data can sometimes be viewed as skewed, since it’s based on personal opinion. Or, (2), objective assessment, where the turf manager counts with a point quadrant, or generates numbers from digital imaging, etc. Objective assessment results in measurable and trackable data, no matter the assessor. In addition to weed numbers, keep data on types of weeds and information on how they affect the fields (do they diminish aesthetics, affect ball roll or bounce, compromise athlete safety and/or performance?).
One final thought from weed scientist, Dr. Dave Gardner: Many intramural and school athletic mixes now contain micro-clovers. Controlling unwanted weeds in a turf:clover stand will need careful consideration. Unfortunately, there are no herbicides that control the undesirables and leave the clover alone. A novel approach may be to control those undesirables when the clover is dormant, which may be either in the heat of summer, or from November to April.
Resources: STMA’s Playing Conditions Index (PCI) assessment form can be found on the Members section of the website. Ohio State’s Field Evaluation Document can be found online, or you could make your own.
Pamela Sherratt is sports turf extension specialist at The Ohio State University. Her Q&A column appears in SportsField Management magazine.
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