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When to Apply Spring Pre-Emergent

There’s a lot of false data and misinformation surrounding pre-emergent use on lawns. Pre-emergents, when used properly, can be one of the most effective tools for keeping your yard healthy and weed free. It’s important to understand how they work and what their limitations are before you can truly harness the power of this tool.

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Sod University features a guest writer, Nick Radford of Todd Valley Farms Inc., who discusses the effective use of a pre-emergent as the season for spring pre-emergent applications grows closer.

What is a spring pre-emergent?

What is the difference between a pre-emergent and a post-emergent?

First, it’s important to note the difference between pre-emergent and post-emergent herbicides. A pre-emergent, as its name suggests, prevents the weed from emerging in the first place—this is useful if you are expecting the same kind of weed to appear annually or to control weeds before they appear so that you can reduce labor spent outside hand-pulling them.

A post-emergent herbicide kills weeds after they have already appeared—you’ve already spotted them growing in your lawn. A pre-emergent applied before weeds germinate as temperatures rise after a long winter are used to prevent spring and summer weeds.

“Contrary to popular myth, pre-emergents do not prevent seeds from germinating,” explains Radford. “They attack seeds during the germination process. Pre-emergents work by blocking the plant from producing the protein that allows plant cells to divide during mitosis. This cell division is what we see as growth.

“The initial cell division happens inside the seed coat. If the seed matures past this stage, the root development will have been damaged, causing the plant to die lacking the ability to properly find or process nutrition. The pre-emergent then also acts as a root pruner. This causes young vulnerable roots to turn inward and stunts their growth.”

Photo Credit: www.rkcpest.com

“The other myth that I often hear is that if you aerate your lawn after applying a pre-emergent, it will make the pre-emergent ineffective. This is also not 100 percent true. It’s not something that you should do as it does not help the effectiveness.”

“However, if you must aerate after an application, it will not completely ruin the effects of the pre-emergent. Pre-emergents stay in your soil and create a barrier layer that seeds in this zone are affected by. The soil removed during the aeration process will break apart and fill back into the holes over time or you discard the dirt plugs letting the holes fill back in with remaining soil. In the case of the latter, you have wasted money on the pre-emergent that remained in the soil plugs you discarded.”

Photo Credit: www.horizononline.com

What weeds do spring pre-emergents control?

Pre-emergents typically control a wide array of various broadleaf and grassy weeds. The best way to control a certain type of weed you may see year after year is to check the product label and see if the weed you experience is listed. No single pre-emergent works for all weed types.

In the spring or summer, some of the most common weeds homeowners and turfgrass professionals alike experience are clover, crabgrass, prostrate knotweed, spurge, goosegrass, lespedeza, dallisgrass, nutsedge, dandelion, chickweed, doveweed and dollarweed.

Pictured above: Row 1 – Clover, crabgrass, knotweed and spurge; Row 2 – Goosegrass, lespedeza, dallisgrass and nutsedge; Row 3 – Dandelion, chickweed, doveweed and dollarweed.

If you aren’t sure of which weeds you usually encounter, our blog on Identifying Common Lawn Weeds goes into tips for identifying many different types of weeds.

When should spring pre-emergents be applied?

The exact window for spring pre-emergent applications can vary depending on where you’re geographically located. In the southern most parts of the country, you can begin applying pre-emergents as soon as January 1st.

Other areas can apply pre-emergents between March 15th–May 5th while the most northern regions of the country should apply pre-emergents after May 25th and into the month of June. The best rule to go by is apply your first pre-emergent application of the spring when soil temperatures reach 55 degrees Fahrenheit.

If you aren’t sure how to determine your soil’s temperature, use a soil thermometer like the one listed below. Pre-emergents should be applied again in the fall to prevent fall and winter weeds.

What is the difference between a spring pre-emergent and a fall pre-emergent?

Pre-emergents should be applied at least twice a year. As previously mentioned, they should be applied in the spring before the ground temperature reaches 55 degrees Fahrenheit. Then it should be applied again in the early fall after temperatures begin to come down. Not much changes besides the timing from spring and fall and the types of weeds you’ll prevent. Read How to Use a Pre-Emergent Herbicide in the Fall for more details.

Nick Radford says, “A good tip is that it is better to apply a pre-emergent early rather than late. As previously stated, pre-emergents do not kill existing weeds—putting it down after weed seeds have germinated is not effective. Unless it is washed away by heavy rains, pre-emergents will remain dormant in your soil until the soil temperatures rise. So, applications made in the late fall may still be effective the following spring.”

Which spring pre-emergent should I use?

There are many types of pre-emergents and many pre-emergents are the same with a different name. The two most common are Prodiamine and Dithiopyr (Dimension 2EW).  These are both good products and they both are easy to find and apply. It is important to know that no single pre-emergent works for all weed types.

Prodiamine is less expensive and stays put in your soil longer. Dithiopyr is a little more expensive and can wash through your soil more easily, but it also has post-emergent properties that others do not have. It will kill young weeds up to the three-leaf stage of growth. This makes it a better choice if you are slightly late with your application.

Hi-Yield Atrazine and Mesotrione 4 SC Select are also post-emergent herbicides that treat weeds after germination. They can function as both pre- and post-emergent herbicides depending on the time of application and the application rate.

Hi-Yield Atrazine is the safest pre- and post-emergent to use on St. Augustine and centipede lawns as both grass types can be sensitive and easily damaged by other active ingredients found in weed control products.

“Most premixed over the counter pre-emergents have two or more varieties mixed to cover a broader spectrum of weeds. I personally like to use a combination of Prodiamine, Dithiopyr and a small percentage of Isoxaben (linked below). I have found this to be the most effective for the large variety of weeds I encounter,” says Radford.

As with any chemical product, be sure to read product labels thoroughly and follow application instructions.

How do I apply a pre-emergent in the spring?

Pre-emergents are available as granules or liquid products. Granular pre-emergents should be applied with a broadcast or drop spreader in accordance to the product label.

Liquid pre-emergents are typically tank mixed and applied with a backpack sprayer. Although this takes more time and effort, you usually get a lot more product. For example, a 50 lb. bag of granular Crabgrass Control Plus 0-0-7 covers 12,500 sq. ft. Whereas a half gallon bottle of liquid Dimension covers 87,000–228,000 sq. ft. Refer to the product label for mixing instructions and follow all safety precautions when mixing/applying chemicals.

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Should I use a pre-emergent upon sod installation, seeding or plugging?

Can I use a pre-emergent on new sod?

“There is no reason to use pre-emergent on newly laid sod. If you have taken all the proper steps prior to putting the sod down and thoroughly sprayed out all the pre-existing weeds, there should be few weeds that survive. Those that do survive are easily pulled by hand or controlled with a post-emergent later,” Radford explains.

“It’s important to know that the root pruning effect of the pre-emergent is damaging to sod. Some varieties of turfgrass are less susceptible to major damage, but it is not healthy for any variety. When you are seeding a lawn on the other hand, your common pre-emergents will kill the seed that you put down. It is never recommended to use Prodiamine, Dithiopyr, Isoxaben or any derivative of these when seeding. It will destroy your lawn 100 percent of the time.”

Can I use a pre-emergent on new grass plugs?

“When you are plugging a lawn, it is highly recommended to use a pre-emergent. After spending a lot of time on experimenting with different pre-emergents on different types of plugs, I have not found one yet that damages the plugs. In fact, when you plug your lawn without pre-emergents, it takes two to three times longer for the yard to fill in!” Check out the images below to see two plots of land: one with a pre-emergent applied and one without a pre-emergent applied.

Pictured above: Untreated plot of land on June 24th (left) and the same untreated plot of land vs. a treated plot of land on September 16th (right).

“I have read many, many university studies on pre-emergent herbicides and researched how pre-emergents work. Based on the studies and the data I found, this should not be the case. The root pruning properties and the effect it has on mitosis should prevent the plug from spreading or maybe living at all. I have a few theories for why this is not true for plugs.”

“The best reason I can give is this: the stolons the plugs spread out obtain enough nutrients from the mother plant to sustain the root development through the layer of pre-emergent. The original plug is mature enough to not be affected by the pre-emergent. Whatever the reason, I have seen no damage to plugs using pre-emergent and the reduction of competition from weeds allows the plugs to fill in at an accelerated rate.”

“Also, warm season grasses can have negative effects when post-emergent weed killers are applied in temperatures over 90 degrees Fahrenheit. For this reason as well, it is better to control the weeds before they become an issue.”

“When you use it while planting grass plugs, you should put pre-emergent down immediately following the planting of the plugs. I recommend applying it to newly planted plugs every three months until the area has filled in completely.”

Can I use a pre-emergent on new seed?

“It should not be used if you have overseeded or intend to overseed within 6–8 weeks (or sometimes longer depending on the pre-emergent used).”

Although fertilizer isn’t going to help kill your weeds, a good fertilizer program will help your lawn stay healthy and thick so that it chokes out weeds. Be sure to check out our recommended Lawnifi® Spring Fertilizer Box and Lawnifi’s granular product, Foundation, to help give your lawn the nutrition it needs when it needs it. Lawnifi is a fertilizer program powered by Catalyst Technology™ that matches seasonal changes with combined fertilizer boxes for spring, summer and fall. The Spring Fertilizer Box contains one bottle of Boost and two bottles of Maintain, which work together to help your lawn come out of winter dormancy and prepare for the heat of summer. Visit Why Use Lawnifi Fertilizer? to learn more.

For more information on pre-emergents, be sure to take a look at the following related articles: Weed Control in Your Lawn & GardenIdentifying Common Lawn Weeds and How to Use a Pre-Emergent Herbicide in the Fall. Lastly, check out the variety of herbicides, including pre-emergents, Sod Solutions offers here.

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