One of the most important operations that you conduct as the manager of an athletic field is whether to seed or overseed your fields.

In college courses, you are taught that the best time of year to establish cool season turfgrass by seed is roughly the period from Aug. 15 to Sept. 15. Agronomically, this makes sense, because germinating grasses require warm soil temperatures, but are intolerant of sustained summertime heat. The idea is to get the seedlings rapidly germinated and then in fall and early spring provide a period of time in which to establish a good root system prior to the onset of summertime heat and drought stress.

Read more on the need to overseed.

Another very important reason for recommending autumn seeding is that competition from weeds is greatly reduced at that time of year. Crabgrass, for example, begins to germinate when forsythia is in bloom (mid-late April in the Midwest) but actually peaks in germination intensity in May to early June. Thus, a seeding operation that is conducted at that time faces severe competition from crabgrass seedlings.

In addition to crabgrass, numerous other annual grassy and broadleaf weeds have germination windows ranging from mid-spring to mid-summer. In the past, this presented a challenge to turfgrass managers because nearly every available herbicide option was not considered safe to apply to seedling turfgrass. But in recent years, several herbicides have been introduced to the market that, when used according to the label, are safe on seedling turf, making it easier to establish turf from seed or to overseed during spring and summer.

Figure 1. Control of crabgrass, goosegrass, yellow foxtail, yellow nutsedge, pigweed, and purslane was nearly 100 percent when mesotrione herbicide was applied at seeding. Perennial ryegrass was seeded into the area, lightly incorporated and then Tenacity was sprayed over the top. Seeding was conducted in early July. Photographs were taken 21 days after seeding.

Before considering herbicides that can be used at seeding, you should make sure that your cultural practices are optimized so that your new seedlings are as competitive as possible. Conduct a soil test to be sure that any nutrient deficiencies are corrected and to measure your organic matter level, cation exchange capacity and pH levels, and take steps to modify if necessary. If it’s a new seeding, you can consider the addition of organic matter to improve the chemical and physical properties of the soil.

Purchase high-quality seed from a reputable dealer and apply the seed at the recommended rate for the species. The temptation is to apply much more than necessary. But on newly seeded areas not only does this waste money, but also the excess competition from having too many seedlings will actually delay establishment, which might increase weed pressure. After seeding, be sure to irrigate and mow according to the recommendations of your state’s extension service for the species you’re establishing.

Pre-emergence herbicide options when seeding/overseeding

One strategy is to seed in early spring and then, after the seedling turf has established, apply an herbicide with pre- and early post-emergence activity, such as dithiopyr. This strategy requires very careful timing. And, on most athletic surfaces, overseeding is not a once-per-year operation. Once the application of dithiopyr is made, as is the case with most pre-emergence herbicides, future overseeding operations must be delayed according to the label. In fact, on areas that you plan on seeding or overseeding in late spring or summer, hopefully you did not apply a pre-emergence herbicide. If you did, then be aware that almost all of the pre-emergence herbicides on the market are very effective at controlling not only weed seedlings, but also the seedlings of our desired turfgrasses.

Use Table 1 to determine the recommended reseeding interval for the active ingredients used as pre-emergence herbicides in turfgrass. Note that most of the intervals are long enough that, were they to be applied in March or April, you would not be able to safely overseed until summer.

Three pre-emergence herbicides are labeled for use at seeding time: siduron, mesotrione and topramazone. Siduron has been available for use in turf for many years. It is safe for use on seedling turf. Follow the label directions carefully. When used properly, siduron will reduce crabgrass, goosegrass, foxtail and many summer annual broadleaf weeds by about 80 percent.

Figure 2. Control of annual weeds following application of mesotrione or SquareOne™ herbicide. Perennial ryegrass was seeded into the area, lightly incorporated and then mesotrione was sprayed over the top. Application of SquareOne™ was made 7 days after ryegrass seedling emergence. Seeding was conducted in mid May. Photographs were taken 60 days after seeding.

Mesotrione is in a unique class of chemistry, and this product has a very diverse label, including pre- and post-emergence control of both broadleaf weeds and annual grasses. It also controls sedges pre-emergence and certain perennial weedy grasses post-emergence. One of its key uses is the pre-emergence control of annual grassy and broadleaf weeds in newly seeded turfgrass (Figure 1).

When used as directed, Tenacity herbicide will result in nearly complete control of crabgrass, goosegrass, foxtail and many summer annual broadleaf weeds. But, it will not affect the growth and development of the seedling turf. Most effective use of this product is to apply it to the soil surface right after the seeds have been raked in but before mulch is applied. You then can begin to irrigate as you normally would to establish seedling turfgrass. Meostrione is very safe to seedling turf. But some phytotoxicity has been reported if it is applied to young turfgrass seedlings. If you are using multiple applications of mesotrione as part of a program to control stubborn weeds, such as creeping bentgrass, then you want to avoid overseeding or reseeding the area until you are making your last mesotrione application. In other words, it is better to wait and reseed with the second or third mesotrione application, then to seed when the first round of Tenacity is being applied.

Topramazone is a more recent introduction to the turfgrass market. It is similar to mesotrione in its weed control spectrum and its safety to seedling turfgrass.

Turf Tolerances to Preemergence Herbicides Table 1. Do not apply a preemergence herbicide to areas that you also plan on overseeding or reseeding unless stated in the table. Otherwise, it will be necessary to wait per the label before seeding.

Post-emergence herbicide options when seeding/overseeding

Most post-emergence herbicides for broadleaf weed control have language on the label stating that, following seeding, the turf needs to be sufficiently established so that it has been mowed three times before the product can be safely used. But there are three active ingredients/products that have label language that allows their use on turfgrass seedlings. These are SquareOne (carfentrazone + quinclorac), carfentrazone and pyraflufen-ethyl.

SquareOne is a more recent introduction that combines carfentrazone with quinclorac. According to the label, SquareOne can be applied as soon as seven days following the emergence of Kentucky bluegrass, perennial ryegrass, tall fescue and fine fescue.

In side-by-side field trials with mesotrione, it was observed that neither mesotrione nor SquareOne inhibited the germination and establishment of the desired turfgrass. However, both products were quite effective at reducing weed germination (Figure 2). One observation of note was that Tenacity was perhaps slightly better at controlling germinating grassy weeds, while SquareOne was slightly better at controlling germinating broadleaf weeds, which makes sense, based on the herbicides in each product.

All of the herbicides mentioned in this article are good products and can be quite effective. You can help to improve your chances of success by avoiding the two-to-four-week period each year that is the peak of germination for the particular weed species that dominate your fields. For example, each of these products is quite effective at reducing weed establishment when seeding or overseeeding perennial ryegrass in July (when weed competition begins to drop off – see Figure 1). But, each of these products can produce less than complete weed control if used in mid- to late-May (the peak of germination for crabgrass and other warm season annual weeds in the Midwest) – see Figure 2. This is more likely to be a problem if the May timing is in conjunction with seeding a slower-to-germinate species.

By simply waiting a couple of weeks (or seeding a couple of weeks earlier), weed seed competition may be greatly reduced, which further increases your chances of success when seeding or overseeding.

Check out more overseeding considerations should you keep in mind.