Commonly Asked Questions About Pre-Emergent Herbicide

Pre-emergent herbicides keep weeds and undesirable grasses like crabgrass from invading your turf. When you apply a pre-emergent, it is absorbed by germinating seedlings in the soil and prevents cell division, which results in their death before emergence.

There are two main pre-emergent herbicides: dithiopyr (also known as Dimension 2EW Herbicide) and prodiamine. When choosing a pre-emergent herbicide, it’s important to get those commonly asked questions answered before you apply one to your turf. Read on to learn pre-emergent dos and don’ts and get the information you need to make a decision.

When and How To Apply Pre-Emergent Herbicide

What month should I apply pre-emergent?

There is no set month that is best to apply pre-emergent, but generally speaking, applying in the early spring or early fall is most common, before the target weeds germinate. For a more comprehensive read, check out our Pre-Emergent 101 blog post.

Is April too late for pre-emergent?

There’s no set date or time when it’s “too late” to apply pre-emergent, but it’s best to get it down before soil temperatures consistently reach 55 degrees F.  While April may be a later-than-normal time to apply pre-emergent, weather depending, you should still apply it. Remember that when it comes to applying pre-emergent, it’s better late than never.

Although some emergence may have already occurred, crabgrass and other types of grasses and weeds don’t all emerge at the same time, meaning you can curb future growth by applying pre-emergent, and then treat any existing growth with post-emergent, if necessary.

Can you put down pre-emergent too early?

Yes. Avoid putting pre-emergent down before the soil has thawed. Apply pre-emergents when the soil temperature is between 50°F and 55°F for best results.

Can you put out too much pre-emergent? What issues can overapplication cause?

Yes, you can apply too much pre-emergent. Applying too much can stunt the growth of your turf, resulting in a later green up. Be sure to consult the product’s label for application rates and instructions.

Tip: Learn more about the differences between prodiamine and Dimension and their recommended use rates.

Can you use prodiamine and dithiopyr?

Yes, you can do split applications with both products. Since dithiopyr offers some post-emergent control, it may make sense to some professionals to apply prodiamine early in the spring, and then apply dithiopyr at a later time. Using one type of product is typically enough, so while using both products is not a recommendation, it’s also not something that can be problematic, either.

Can I mix Dimension (dithiopyr) and prodiamine?

Yes, you can mix the two together so long as you do not exceed the annual application limit for both products. Using both products at full strength may not be the best option, however, and split applications may provide better results.

Before You Apply

Will prodiamine prevent grass seed from germinating?

Yes, prodiamine will prevent grass seed from germinating. If you’re using prodiamine—or any pre-emergent herbicide, for that matter—in the spring, you won’t be able to seed until the fall.

Do I need surfactant with prodiamine?

Prodiamine has pre-emergent activity only, so you do not need to use a surfactant. 

Can you put down pre-emergent and grass seed at the same time?

No. The pre-emergent will kill the grass seedlings before they emerge from the soil.

Can you put mulch over pre-emergent?

Yes. Some herbicides that are affected by photodegradation are best suited to be applied before putting mulch down. Others can be applied on top of mulch.

Is pre-emergent safe around trees?

Pre-emergents only penetrate the top few inches of soil, so they do not pose a threat to most trees when used correctly. Some pre-emergents are not meant for use around fruit trees, so be sure to check your specific product’s SDS for more information.

Timelines

How long does dithiopyr last in the soil?

Dimension (dithiopyr) lasts in soil for approximately 12 weeks (3 months) with two applications according to research conducted by North Carolina State Univiersity. 

How long does dithiopyr last?

We discussed above how long dithiopyr lasts in the soil, but if you are looking to store dithiopyr, it should last at least one year unopened and may last for several years.

Watering

What happens if it rains after applying pre-emergent?

The pre-emergent’s efficacy may be hindered by excessive rainfall. Prodiamine doesn’t readily leach, whereas Dimension (dithiopyr) is more mobile in the soil. If consistent, heavy rainfall is a hallmark of spring in your region, prodiamine may likely be the better option.

An ATS representative found that when pre-emergent applications were made in a wet, northeastern region of Ohio in March, prodiamine outperformed Dimension.

Does dithiopyr need to be watered in?

Yes, Dimension (dithiopyr) must be watered in. Apply ½ inch of water over the treated area within seven days via irrigation or rainfall.

How long can I wait to water in prodiamine?

You should not wait more than two weeks to water in prodiamine with ½ inch of water, whether it be through rainfall or irrigation.

What happens if you don’t water in pre-emergent?

Without water, your pre-emergent will simply sit above the soil and will be ineffective. Water is required for pre-emergent to work.

Efficacy

What weeds does dithiopyr prevent?

Dithiopyr prevents crabgrass and myriad other grasses and weeds. In fact, Dimension is approved for use on over a dozen annual grasses and 27 types of broadleaf weeds.

Does dithiopyr prevent dandelions?

Yes, dithiopyr can prevent dandelions. Any dandelions that have already emerged from the soil will require a post-emergent.

Which is better? Dithiopyr vs. prodiamine

Prodiamine and Dimension (dithiopyr) are two excellent products that essentially do the same thing. Depending on your region and rainfall amounts, one product may suit your needs better than another. 

A prodiamine-based product like ArmorTech Kade 4L provides selective control of over 30 grassy and broadleaf weeds all season long, making it a good all-around option for lawns, sports turf, and golf courses.

Dithiopyr-based Dimension 2EW is a great option for crabgrass, offering both pre- and post-emergent control; a claim to fame that many other herbicides simply can’t compete with.

There are plenty of other options, and chatting with your local ATS representative can give you a better idea of what product is best for your turfgrass.