Late-summer seedlings of cool-season grasses should avoid peak germination periods of weed populations. Proper seedbed preparations put weeds at a disadvantage by speeding the germination of turfgrass seed, enabling stronger competition against weed seedlings. Newer herbicides offer unique strategies in weed control.

Like it or not, we sometimes must seed cool-season grasses just before or during periods of maximum weed growth. Crabgrass (Digitaria species), goosegrass (Eleusine indica) and other warm-season annual grasses can be troublesome competition during establishment of seedlings from late fall, winter, spring or summer sowings. Winter annual weeds can be tough on dormant seedings or seedings done in the late fall before a relatively mild winter. Annual bluegrass (Poa annua) can cause problems in new seedings following almost any seeding date.

Fortunately, a number of cultural tools and a few herbicides can reduce grassy weed competition and improve stand establishment.

Seeding date
Late summer is the best time to seed cool-season grasses. At this time of year turf will develop most quickly, and weed pressure is at its lowest. Depending on the area of the country, mid- to late August is considered optimal. Seeding very much earlier may allow crabgrass and goosegrass to become problems. On the other hand, waiting until after Labor Day to seed often sets up a germination race that annual bluegrass (Poa annua) typically wins.

If you expect crabgrass or goosegrass to be your most vigorous weeds, mid-August would be the ideal time to seed. However, if you suspect annual bluegrass to be your most aggressive weed, consider seeding even earlier. Seeding in July or August will give the desired cool-season turf the greatest amount of time to establish before maximum annual bluegrass germination, which normally occurs in late August and September (1). Obviously, closing areas of the course for seeding during the peak playing period of the year may be easier said than done.

Seedbed preparation
Soil preparation is a primary factor determining the success of stand establishment, which in turn influences the amount of weed pressure. Good seed-to-soil contact will minimize the time to emergence and decrease the time needed for a young seedling to develop. A light rolling or "dimpling-in" with a sand bunker rake is a very effective way to maximize seed-to-soil contact on a newly tilled and seeded area.

When renovating turf, it is effective to scalp with low mowing, then aggressively aerify with large, hollow tines and shatter the cores. Though it might take more time to decrease hole spacing or to make multiple passes with the aerifier, the additional seed-to-soil contact is well worth the effort.

Regardless of how it is done, maximized seed-to-soil contact is a must to maximize fill-in and minimize weed pressure. To further help seedling development, a starter fertilizer in the seedbed applied at 1 to 1.5 pounds P2O5 per 1,000 square feet will provide much-needed phosphorus when the young roots cannot explore sufficient soil volume to find it. A second application of starter fertilizer within four weeks of germination will also help, as will frequent fertilization with low rates of nitrogen every two to four weeks thereafter. Your soil type and soil test recommendations will determine the exact fertilization needs.

Start mowing as soon as a few seedlings reach the desired height. Mowing encourages lateral growth and increased density, so mow early and often after seeding. Though mowing is necessary, use caution to reduce potential damage. Smooth rollers are preferred over grooved rollers on greens and even fairway units; walking units are preferred over riders. Be especially careful when turning the mower and on clean-up passes.

Weed control
Some products can be used effectively before seeding to suppress annual weedy grasses such as crabgrass and annual bluegrass. By far the most effective method is fumigation with methyl bromide, which kills weed seeds in the soil (as well as insects, nematodes and disease propagules). However, methyl bromide application requires specialized equipment that only a few companies nationwide offer, and it can be expensive and time-consuming in terms of area preparation. But for high-dollar areas where you cannot risk any weed problems, methyl bromide fumigation is most effective.

Basamid is a soil fumigant that is gaining popularity in turf as the availability of methyl bromide decreases. Basamid is distributed by TopPro and comes in a light granular, almost powdery formulation. Early research in 2000 at Purdue, Penn State University and University of Illinois reveals that this product can be very effective when applied under optimal conditions (2). It has also been used successfully in a number of golf course renovation projects in recent years. This product holds tremendous potential, but research and on-course experience are still limited. Therefore, if you choose to use Basamid, prepare many months in advance by gathering information from company representatives, other superintendents and researchers. At Purdue, we became more comfortable and accurate the more we used this product, so plan on experimenting on small areas before using Basamid on a large scale.

Pre-emergence weed control
Most commercially available pre-emergence herbicides cannot be used immediately before or after seeding. Most labels limit application to at least 90 or 120 days before seeding cool-season grasses to avoid phytotoxicity to the sown seed. The one exception is siduron (Tupersan), which can be used safely just before seeding cool-season grasses to control warm-season weedy grasses. Siduron does not control annual bluegrass, however. The residual control from siduron is limited, so multiple applications are needed for extended control.

A multi-chemical approach might call for one or two applications of siduron to provide weed control shortly after seeding and emergence, followed by a selective post-emergence product or a long-lasting pre-emergence herbicide after seedlings mature.

Modern weed control
Our studies at Purdue have looked at two relatively new products: Drive (quinclorac) from BASF and TopPro, and Dimension (dithiopyr) from Rohm and Haas. These products exhibit both pre- and post-emergence control of annual grasses and other weeds. Dimension provides excellent pre-emergence control and slightly less post-emergence control. On the other hand, Drive provides much better control with a post-emergence application than with a pre-emergence application.

Our initial investigations were targeted at crabgrass control in spring seedings of Kentucky bluegrass (Poa pratensis) and perennial ryegrass (Lolium perenne). Drive 75DF at 0.75 pound active ingredient per acre and Dimension 1EC at 0.5 pound active ingredient per acre were applied separately at various times before seeding and after emergence of Kentucky bluegrass and perennial ryegrass (3).

As expected, we found that even though Drive produced some pre-emergence control of crabgrass, it was far more effective as a selective post-emergence product. We also found that Drive did not inhibit seedling development of either Kentucky bluegrass or perennial ryegrass, regardless of application timing. Drive's label states that it can be used before seeding or 28 days after emergence of perennial ryegrass or Kentucky bluegrass. We did not use a methylated seed-oil additive with Drive in our study. Research shows that although the methylated seed oil increases efficacy of Drive on mature crabgrass, it may also increase burn potential on young plants of our desired species. Applied post-emergence, Drive will not control annual bluegrass or goosegrass, but it effectively controls a wide variety of weeds including crabgrass, clover and dandelion.

In this same study, we found Dimension to be more damaging on young Kentucky bluegrass and perennial ryegrass. Dimension prevented turf germination when applied before seeding and also caused significant thinning when applied within 14 days after turf emergence. Thus, Dimension's current label states that it should not be applied earlier than 28 days after turf emergence. Applied after turf emergence, Dimension will only control younger crabgrass before it tillers.

However, pre-emergence applications of Dimension will control a wide variety of weeds, including such golf course nemeses as annual bluegrass, crabgrass and goosegrass. Therefore, we are continuing research on the exact safety margin for Dimension over new seedlings, trying to shorten the delay of application after emergence. If we can safely apply Dimension shortly after emergence of our desired species -- but before germination of annual bluegrass, crabgrass or goosegrass -- we should be able to control these weeds without damaging the desired turf. We may be willing to risk minor herbicide damage to our desired seedlings in order to give them a competitive advantage over the weeds.

Creeping bentgrass seedings
Creeping bentgrass (Agrostis palustris) seedlings tend to be more susceptible to herbicide damage than either perennial ryegrass or Kentucky bluegrass. Therefore, much of our research is focused on herbicide efficacy and safety on fairway-height creeping bentgrass. In cooperation with North Carolina State and Iowa State universities in 2000, we looked at the safety of Drive on a number of cultivars of creeping bentgrass (4). Treatments included Drive 75DF at 0.75 pounds active ingredient per acre applied seven days before seeding, on the day of seeding, and 14 or 28 days after emergence. Results varied depending on the location. Drive applications did not affect establishment of Providence, L-93 or Putter in Indiana or L-93, Pennlinks or Penncross in Iowa.

Conversely, all applications of Drive reduced cover of L-93, Pennlinks and Penncross in North Carolina. We're not sure of all the reasons for the different results, but air temperature or soil organic matter levels could have affected the outcomes. Others have reported Drive to be safe on cool-season grasses in the cooler climates, but less safe in warmer temperatures. Air temperatures in Indiana and Iowa were rarely above 85 F during the study, whereas temperatures exceeded 85 F on 22 of 49 days in North Carolina. Additionally, others have reported Drive to be more damaging on soils with low organic matter. Organic matter in the soils of Indiana and Iowa was 7.3 percent and 4.1 percent respectively, whereas organic matter content of the North Carolina soil was 1.8 percent. Regardless of the reasons for the damage from Drive in North Carolina compared with Indiana and Iowa, conservative recommendations are to use caution when applying Drive to seedlings of creeping bentgrass and be sure to follow label instructions.

Seeded zoysia and bermudagrasses
Weed control is important when establishing the newly available seeded zoysiagrasses (Zoysia species) and bermudagrasses (Cynodon hybrids). Zoysiagrass in particular is very slow to establish by seed, allowing weeds to populate the stand quickly. It is too early to report any definite results, although herbicide efficacy and safety are being evaluated on these new grasses at Purdue University, University of Arkansas and other institutions.

The bottom line
Choosing the proper seeding date will go a long way toward limiting weed pressure in new seedings. Ensuring good seed-to-soil contact as well as good post-seeding care will increase the speed of establishment, thereby helping minimize weed pressure. If herbicides are needed, a number of options are available, depending on the target weed and the situation. Once a herbicide is chosen, refer to the label for specific use recommendations.

Literature cited

1. Kind, M. 2000. Seed bentgrass in June? Sounds crazy, but . . . Golf Course Management 68(1):48.
2. Reicher, Z. 2001. New strategies for bentgrass fairway renovation. Proceedings of the 71st Annual Michigan Turfgrass Conference 30:130.
3. Reicher, Z.J., D.V. Weisenberger, and C.S. Throssell. 1999. Effect of dithiopyr and quinclorac on establishment of Kentucky bluegrass and perennial ryegrass. Weed Technology 13:253-256.
4. Reicher, Z.J, G.A. Hardebeck, F.F. Yelverton, N.E. Christians, B. Bingaman and J. Turner. 2001. Tolerance to quinclorac by seedling creeping bentgrass (Agrostis palustris Huds.) HortScience. In press.

Zac Reicher is an assistant professor and turfgrass Extension specialist at Purdue University.