Ask a Pro: “When Do You Put Down Pre-emergent? The Instructions Are So Confusing!”

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Register to join our Ask a Pro LIVE interactive event on Thursday, April 10 at 1:30 EDT: Dr. Shawn Askew will be taking your questions about all things weed eradication and control and offer his solutions for your unique weed issues! 

You’ll leave this event knowing:

• Mistakes you’re making when tackling weeds in the lawn and garden.
• Proven methods for getting rid of weeds once they take root.
• Top tactics for preventing weed growth.
• DIY landscaping projects that can help control weeds.

Welcome to Ask a Pro! We take your questions about home improvement, DIY, lawn and garden, and homeownership and answer them with the help of professionals. Our experts are vetted and verified—and they’re eager to share their knowledge.

Last month, BobVila.com readers submitted weed control questions on Facebook and Instagram and via our newsletter. We hand-picked our favorites, and tapped Shawn Askew, a turfgrass weed specialist at Virginia Tech to help answer them. In this installment of our Ask a Pro series, Shawn tackles your questions about getting rid of and preventing weeds in the lawn. He’ll also be joining us for our Ask a Pro LIVE webinar event about spring and summer weed control. Learn more here.

When do you put down pre-emergent? The instructions are confusing, so I end up putting down post-emergent instead. I’ve had to replant my St. Augustine grass because it was killed off in the places I sprayed. —Cty Chx. Tx via Facebook

Shawn’s advice: In a subtropical climate, where St. Augustine is typically grown, many weeds are considered “annuals” and they just don’t quit. That does not mean that pre-herbicides won’t help, but they may not get all of the weeds. Remember, they only work on weeds that have not yet sprouted. For summer weeds like crabgrass and goosegrass, it’s best to apply pre-herbicides (such as prodiamine or dithiopyr, which are safe for St. Augustine grass) before soil temperatures jump above 55 degrees. This will typically be between January 1 and mid-February. If you are going after winter weeds, apply pre-emergent granules around September 1 as soil temperatures drop below 70 degrees. As with any granular herbicide, water within a week of application if no rainfall occurs. This will safely address some, but not all, of your grassy weeds in a St. Augustine lawn. When you use post-herbicides, know that St. Augustine is one of the most sensitive grasses. Broadleaf products like 2, 4-D and grass products like atrazine can slam St. Augustine, especially during hot weather (above 85 degrees) or in spring when the grass is greening. Pre-emergent products are always safer. Remember, apply in spring before soil temperatures are consistently above 55 degrees and in fall before soil temps are consistently below 70 degrees. Try to use post-emergent products as “spot treatment” only, especially when the air temperature is above 85 degrees.

How often should I weed my yard? —Cassie U. via Instagram

Shawn’s advice: Ah, the favorite professor response: it depends! Depends on what? 1) What type of lawn grass do you have and what weeds are the problem? Most weeds become problematic in late winter or late summer. If you are hand-weeding, your window of opportunity to act is leading up to these times, which is when weeds mature. Time your hand-weeding when the soil is moist so the weeds will be easier to lift. If you have perennial weeds (wiregrass in tall fescue), you will need to weed those patches every 2 to 3 weeks for the season to prevent a small problem from getting bigger. 2) How dense is your lawn? A dense lawn kills weeds for you. The best way to improve lawn density is to improve the soil. Happy turf with good soil may need weeding less than once per year, assuming you are doing what you need to keep the lawn happy (fertilizing, irrigation, proper mowing). Patches of weeds are indicators that there is a problem in the lawn. Compacted soil, tree shade, and poor soil pH or mineral content are some examples of problems that can lead to patches of weeds. By identifying and correcting these underlying problems, the lawn grass will start working for you, and you’ll spend less time weeding. Be sure to add turf seed whenever you remove weeds to fill the gaps that allow the weeds to punch through the turf canopy.

How do you prevent crab grass? Are clover lawns a good idea? What are the best mulches that won’t attract termites? —@_amberger_helper via Instagram

Shawn’s advice: Crabgrass is best prevented by having a healthy lawn turf, but that’s not enough in some environments. In Virginia, where I live, the climate is where the warm humid areas to the South transition to the cool humid areas to the North. This makes weed control challenging. Regardless, weed pressure is lowest in a healthy lawn. Research shows that crabgrass can be reduced by 99 percent just by having a healthy lawn. To hedge your bets against climate and poor soil, use a pre-emergent herbicide. These are applied before soil temperatures consistently exceed 55 degrees. You can also follow other indicators: apply the product before dogwood buds turn white, before daffodil blooms are 100 percent wilted, and before forsythia drops 50 percent of its blooms. These indicators may vary regionally, so be sure to check local information. 

As for clover lawns being a good idea or not, it depends on what you want from your lawn. If you’re using your lawn to play games with your pets and kids, clover lawns may not be the best choice in that particular area—lawn grasses will be more resilient. Plus, some clovers create blooms that attract pollinators like bees. This is great for the bees, but may be less ideal for spaces for your kids or pets. The point is that the use of a particular part of your lawn is your main consideration. We can feed bees and play badminton, just not in the same place. A benefit of a clover lawn is that the clover will feed the grass with nitrogen. A downside, even for microclovers that don’t produce as many flowers, is that the uniformity of the lawn will not match that of pure turfgrass.

Unfortunately, I’m a turfgrass weed specialist and don’t know much about the interaction of mulches and termites. Note from the editor: Check out this article for more information about whether or not mulch attracts termites.

How do you kill a weed that is low-growing with little nodules throughout the length of the vine, with small leaves with a white flower? It’s very invasive and drowns out the grass. I live in South Mississippi. —Teresa via the newsletter

Shawn’s advice: I love riddles and “weed” riddles even more. In contemplating your description, I came up with two possible weeds. 

1) Florida betony will cover the lawn with creeping roots that have chains of tubers that look like nodules. It is often called “rattlesnake weed” as these chains of root nodules are shaped like the snake’s rattles. They are white and often look like large grubs, but are clearly attached to the roots. The plant also has small heart-shaped leaves and white blooms. Use 2,4-D and dicamba in early spring and repeat the treatment every 3 to 4 weeks; be cautious at greenup with sensitive grasses like centipede grass or St. Augustine grass. Spot treat only. Use isoxaben (Gallery) pre-emergent in late summer to keep new weeds from emerging in the fall. 

2) Chamberbitter or long-stalked phyllanthus are plants that have compound leaves that consist of tiny leaflets aligned in rows (like mimosa). These leaves have a row of round seed capsules along the bottom of the midrib that look like nodules. The weed can cover large areas of the lawn quickly as they produce up to 1000 seeds per plant. The flowers are tiny and white to green, and are often overlooked. Hand pull before seed shatter (before mid-May). Spray over the top with 2,4-D and dicamba in early spring (March to April). Apply atrazine pre-emergent between November and January to prevent new weed emergence next spring.

How do I get rid of the viney grass that grows in my yard choking all my new grass? —Deborah via the newsletter

Shawn’s advice: “The viney grass” in this case must be the bane of all cool-season lawns: wiregrass or bermudagrass. This is one of the most troublesome problems in my area of Virginia due to our transition-zone climate. Both warm and cool-season grasses grow in Virginia, and the bermudagrass (wiregrass) often invades the tall fescue or Kentucky bluegrass. Hand pulling is difficult but not unwarranted. The grass makes both rhizomes (underground stems) and stolons (above-ground stems) and creeps up all over the place. Just pulling the stolons does not kill all of the plant, but personally, I take great joy in knowing that it took the wiregrass a week to grow that 12-inch stem that I snatched up in just two seconds. On a day when soil moisture is high, take the time to pull, pull, pull. Be sure to discard the stems carefully to avoid infesting other areas. I collect and bag them to be sure they don’t blow to new areas and sprout. 

The first step in any wiregrass control program is to raise the mowing height of the lawn to at least 4 inches. Wiregrass is always favored over tall fescue when the mowing height is lower. 

Herbicides can be used in tall fescue or Kentucky bluegrass lawns. Fenoxaprop (Acclaim Extra) and triclopyr (Hi-Yield, Turflon) can be mixed at 3 and 4 teaspoons per gallon of water, respectively. Add 2 teaspoons nonionic surfactant or dish washing liquid per gallon of water as a spreader. This solution can be applied once or twice in early summer and three times in fall. The fall treatments kill wiregrass best, the early summer treatments slow the summer expansion. Repeat the treatments at 3 to 4 week intervals. If you intend to do all five treatments in one year, reduce the early summer treatments to 2 teaspoons of Acclaim Extra so that you can use the full 3-teaspoon rate for the 3 treatments in the fall and not exceed EPA limits. Don’t use this mixture on Kentucky bluegrass less than 3 months old, and be very cautious on young Kentucky bluegrass. It’s better for tall fescue lawns. Don’t use this mixture with broadleaf herbicides, or the Acclaim will not work well. As a side note, Acclaim Extra can also be used to control wiregrass, crabgrass, goosegrass, foxtails, and most other grasses in your ornamental beds or garden. It only hurts grasses. Most other plants will not even know you have sprayed. Check the label for a full list of ornamental plants for which Acclaim Extra is safe. Don’t use the Turflon near any ornamentals, but it can be used anywhere in your lawn grass to kill broadleaf plants.

An alternate organic approach is to “shade it out.” Place a piece of plywood over each patch of wiregrass and weigh it down with a brick. Leave in place for 2 months (July and August), then remove the plywood and seed new tall fescue in September. This only works if you cover the entire patch. Hand weed, especially mid-summer, but don’t hand weed right before an herbicide treatment.

 

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