3/27/2018 - By Harrell's

Have you ever wondered where all of those weeds came from in your lawn and/or landscape around Halloween?  Especially, after working so diligently through the summer to keep weeds out and then it seems that nothing but weeds are present.

This is very common given that summer annual weeds are quite different than winter annual weeds.  During the summer, we worry about controlling crabgrass, goosegrass, nutsedge, and spurge.  These are all summer weeds that thrive in warm weather.  Winter annual weeds include annual bluegrass (Poa annua), dandelion, henbit, and wild garlic and onion.  Of course, there are many others but these are the most common.

So, what is the best approach for control of these winter weeds?

Preventative weed control is always much better and economical approach compared to curative control after the weeds are present.  The most effective weed control is a healthy turfgrass.   Weeds are often the result of a weakened and thin turfgrass not the cause of it. 

Preemergence (PRE) applied herbicides are one of the most effective approaches to controlling these winter annual weed pests.   Preemergence herbicide selection depends on the type of turfgrass that you are considering treating.   Many of these PRE herbicides can be applied to both cool-season and warm-season turfgrasses; however, other PRE herbicides will cause significant injury if applied to intolerant turfgrasses. 

Many of the standard PRE herbicides that are used for crabgrass control in the spring are great choices for winter annual weed control.  Herbicides such as Barricade (prodiamine), Dimension (dithiopyr), and Pendulum (pendimethalin) provide excellent control of annual bluegrass and many small seeded winter annual broadleaf weeds.  Unfortunately, these PRE herbicides do not provide adequate control of many of the broadleaf weeds. 

If winter annual grassy weeds and winter annual broadleaf weeds are a problem, then a combination of one of the PRE herbicides mentioned above should be tank mixed with a preventative broadleaf herbicide such as Gallery (isoxaben).  These tank mix combinations will provide very effective broad-spectrum weed control of annual winter weeds. 

Caution must be given if winter annual seeding of desirable turfgrasses are being done such as improving tall fescue or Kentucky bluegrass stands with some fresh fall seeding or overseeding of perennial ryegrass in bermudagrass for winter color.  These PRE herbicides will affect successful germination of these newly seeded turfgrasses. 

If no preventative weed control is applied and you are going to take a “wait and see” approach, then you may have to apply postemergence (POST) herbicides to selectively remove these winter annual weeds. 

Ideally, these POST herbicides such as Trimec, Three-Way Broadleaf, Escalade II, Cool Power, etc are applied as liquid sprays.  Many times, these POST herbicides are applied as granular Weed and Feed products.  Much superior weed control will be obtained from liquid POST herbicide applications versus granular Weed and Feed.  You may ask why?

Granular Weed and Feed products are basically the same herbicides that are applied as liquid sprays, but they are simply “stuck” onto the outside of a granular carrier such as corn cob, limestone, clay, or fertilizer.  The POST herbicide must then be removed from the granular carrier onto the weeds that you are trying to control.  This is why label use recommendations for these Weed and Feed products will tell you to irrigate or apply these products when the turfgrass is wet or contains a dew.  This is by design so some of the granular POST herbicide will stick to the leaves of the weeds.  Guess what, they stick to the leaves of the turfgrass as well.  Not only are these Weed and Feed products not very effective as it is almost impossible to get very much of the herbicide to stick to the leaves of the weeds, but they also have injury potential to the turfgrass.  Many of these Weed and Feed formulations are on granular fertilizers, which have burn potential as well.  The fertilizer burn can provide better weed control than the herbicides!! 

Where turfgrasses go completely dormant in the winter, it has become common practice to apply RoundUp (glyphosate), Reward (diquat), or Finale (glufosinate) as POST herbicides for control of established winter annual weeds.  These herbicides are “non-selective”, but selectivity is achieved by the desired turfgrass being dormant.  CAUTION should be taken when applied these non-selective herbicides to make certain that the turfgrass is for sure dormant and to apply these herbicides to the entire turfgrass area.  The reason for applying these herbicides to the entire turfgrass area is that they will delay the spring greenup of the desirable turfgrass in the spring, so if areas are left untreated, they will greenup much faster than areas that have been treated. 

Another herbicide option for dormant turfgrasses in the winter is to apply either Atrazine or Princep (simazine) as a fall PRE herbicide.  Please consult your state regulatory agencies to see if these herbicides are registered for this application and use in your area.  Normally, two applications are required in the winter with the first application being made in November followed by a second application in late January or early February.  These triazine herbicides can be very effective and economical, but resistance to these herbicides has been documented with annual bluegrass and some annual broadleaf weed species. 

Weed control in landscape beds is not very different than weed control in lawns as the herbicides used and application timing are very similar.  There are some granular combination herbicides that offer very good PRE weed control in landscapes such as Snapshot and Freehand.  If applied in a timely manner before winter annual weed seed germinate, these granular herbicides provide very effective weed control. 

Good luck with your winter annual weed control as weeds left untreated will only contribute to the soil seed bank and make weed control in the future more difficult.