Weed Control Before Seeding

Plant Turf Seed or Control Weeds First? A common question this time of year is whether to plant turf seed now and control weeds later OR to instead control the weeds now and then seed later. Here Spray weeds or get seeding? Perennial grasses are best hit with an early glyphosate application. Grasses are very sensitive to glyphosate, and produce leaf area more quickly and earlier in the

Plant Turf Seed or Control Weeds First?

A common question this time of year is whether to plant turf seed now and control weeds later OR to instead control the weeds now and then seed later. Here are my thoughts.

Cool-season turf seed mixture.

Option 1 for areas with mainly broadleaf weeds and a few annual grasses: Plant now and control weeds later.
The optimum time to control perennial broadleaves and germinating winter annual broadleaves is in October. As such, seeding in late August and early September should allow enough time for seeds to germinate, grow, and be mown twice (assuming you irrigate and fertilize these newly seeded areas) prior to an October herbicide application. You can delay the herbicide application until late October and early November if you get a late start on seeding or if your seedlings are slow to establish (like Kentucky bluegrass). Most broadleaf herbicide labels suggests delaying application until newly seeded areas have been mown twice. Some herbicide labels allow a shorter interval between seeding and an application. These shorter interval herbicides include Quicksilver, SquareOne, and Drive (shorter interval tall fescue compared to perennial ryegrass and Kentucky bluegrass).

Typical language about postemergence herbicide application delays after seeding turf.
Drive XLR8 label instructions regarding application timing and seed and seedling emergence.

Option 2 for areas where you have lots of weed competition: Spray weeds now and seed tomorrow.
When there is a lot of crabgrass or broadleaf weed cover, then you would likely benefit from spraying the weeds now and seeding afterwards. Tenacity, Pylex, and SquareOne, and Drive are among the herbicides that can be applied today and allow seeding as soon as tomorrow (after the herbicide has had a chance to translocate in the target weed). Once the turf germinates, then follow the guidelines in option 1 above to provide follow-up weed control if needed.

SquareOne application instructions allowing seeding one day following the herbicide application.
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Option 3 for areas perennial grassy weed competition: Spray weeds now and seed later.
When there are a lot of perennial grassy weeds such as quackgrass, fountain grass, etc., then your best bet to remove these perennial grasses before you seed as there are few herbicide options afterwards.
Best results with a non-selective herbicide like glyphosate are seen on perennial grassy weeds when multiple applications are made. Ideally, you would have started this process several weeks ago. Why? It is not uncommon for the weed to regrow from stolons or rhizomes (if a spreading grass) a few weeks after the first glyphosate application. One must allow the weed to regrow (wait at least three weeks) before the next application. At least two applications are recommended, but three or more may be needed. One must realize that the area will be dead and unsightly for a number of weeks or months if optimum control is desired.

Starting this process now, means that you won’t be able to seed until after the optimum seeding window of August 15 to September 15. If you are still committed to renovating this year, another seeding option is to seed in December through March as a dormant seeding after you have finished killing your perennial grassy weeds. Learn more about dormant seeding by clicking here.

If there are only a small number of patches, spot applications can be made with glyphosate. Reseeding can take place a day or two following final herbicide application. Once the area has been infested with a large number of patches, killing the entire area will be most effective with multiple applications of glyphosate. Renovation can begin five to seven days following final glyphosate application. Refer to AY-13, “Lawn Improvement Programs” for information on reestablishment.

One caveat to the above, Option 3 recommendation, is that we have a few selective herbicides for perennial grassy weed control that might be options for use during turf renovation.

Table 1. Non-selective and selective herbicide options for controlling perennial grassy weeds in cool-season turf.

Weed in cool-season turf Non-selective herbicide option Selective herbicide option
Bermudagrass glyphosate Pylex (topramezone), Acclaim Extra (fenoxaprop)
Creeping bentgrass glyphosate Tenacity (mesotrione)
Dallisgrass glyphosate No effective selective control
Field paspalum glyphosate No effective selective control
Fountain grass glyphosate Drive (quinclorac) and other quinclorac containing herbicides
Nimblewill glyphosate Tenacity (mesotrione), Pylex (topramezone)
Orchardgrass glyphosate No effective selective control
Quackgrass glyphosate No effective selective control
Rough bluegrass glyphosate Velocity (bispyribac-sodium) for golf courses and sod farms.
Tall fescue glyphosate No effective selective control, formerly Corsair (chlorsulfuron)
Windmillgrass glyphosate Tenacity (mesotrione)
Zoysiagrass glyphosate Pylex (topramezone)(based on preliminary results following labeled instructions for bermudagrass control)
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With any of these options, it is especially important to read the herbicide label and understand the application restrictions both before and after seeding and other application instructions (such as adjuvant and timing recommendations) so that you can maximize weed control and maximize turfgrass establishment.

For more information on weed control, search this blog and check out our Turfgrass Weed Control for Professionals Publication.

Spray weeds or get seeding?

Perennial grasses are best hit with an early glyphosate application. Grasses are very sensitive to glyphosate, and produce leaf area more quickly and earlier in the season than broadleaf perennials. Grass weeds in this canola field (above and below) in 2011 should have been hit pre-seed.

Seeding is a priority for many growers this week, but fields with a large population of weeds, especially advancing winter annuals, should get a pre-seed burnoff. For annuals and winter annuals, glyphosate needs only 24 hours to get to the growing point and set the control process in motion. (It may be quicker than that for some specialty glyphosates in good growing conditions.) After a day, the crop can be seeded. For perennial weeds, the recommended delay ranges from 3 to 5 days depending on weather conditions. If sunny and warm, translocation will take place fairly quickly so 3 days should be enough. If weather is cloudy and/or cool when applied, 5 days is recommended before seeding. When deciding how long to wait to seed, remember that the conditions on the day the glyphosate is applied are the most important to efficacy, making them relatively more important than conditions in the days that follow.

In weedy fields, the economic return will likely be higher for a pre-seed burnoff than if the grower seeded over these weeds and then took a chance on getting them sprayed before crop emergence or in-crop.

In addition, weeds present when the crop emerges will have a substantially greater negative impact on the yield of the crop if the pre-emergent application is missed altogether.

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University of Saskatchewan research shows that where weed competition is significant the weed control provided by an early burnoff application can have a greater influence on crop yield than seeding date. The study looked at seeding dates in early May vs. late May with burnoff treatments applied the day previous to seeding. The third treatment was a burnoff conducted with the early seeding date but not seeded until the late seeding date. The late burnoff treatment yielded significantly lower than either of the early spraying treatments, regardless of when they were eventually seeded. The study was done on wheat, but lead researcher Ken Sapsford says results would be similar for canola. One of the benefits of early weed control is that it stops weeds from taking up moisture and nutrients. Those nutrients will be returned to the soil eventually after the weeds decompose, but not in time to be useful to this year’s crop.

Post-seeding/pre-emergence spray window. The cleaner you have the field as canola emerges from the ground, the better. One option if growers miss the pre-seed burnoff, is to apply glyphosate post seeding and pre-emergence. But since shallow seeded canola can emerge within 5 days under warm and generally moist soil conditions, and since growers need to leave weeds for a couple days to start growing again after the seeding operation, the window for post-seed/pre-emergence spraying is very narrow. With all the wind these days, that opportunity may never come. (The time lag between seeding and spraying is necessary to allow weeds buried by the seeding operation to re-emerge and to allow weeds uprooted to overcome the stress placed on them.)

In crop sprays and glyphosate resistance. A 2010 predictive model by Hugh Beckie, research scientist with AAFC in Saskatoon, ranked kochia first among weeds at risk of developing glyphosate resistance on the Prairies. The prediction was accurate. The next most likely weeds to develop resistance, in order, are wild oats, green foxtail and cleavers. While these weeds are unlikely to develop glyphosate resistance when glyphosate is applied in crop only one year in four, tight rotations of Roundup Ready canola or seasonal pre-seed applications timed to target these weeds increase that risk substantially. Read the article on page 11 of the March 2011 Canola Digest.