Pre-Emergent Weed Control: When and How to Use It in 2024

What Is Pre-Emergent Weed Control?

Pre-emergent weed control is a proactive approach that involves applying a chemical barrier to the soil before the weed seeds start to germinate.

This barrier, often containing specialized herbicides like isoxaben or prodiamine, creates an environment that’s hostile to weed growth without harming existing greenery.

In contrast, post-emergent weed control targets weeds after they’ve already emerged using herbicides like glyphosate or 2,4-D.

So while post-emergent control is a reactive solution, pre-emergent control is considered a preventative measure.

Pre-emergent herbicides are particularly effective against annual grasses like crabgrass, as well as some small-seeded broadleaf weeds.

They’re generally ineffective against perennial weeds that regrow from established root systems rather than seeds though.

How Do Pre-Emergent Herbicides Work?

When you apply a pre-emergent herbicide to your lawn, it creates a thin protective layer within the top inch or so of the soil.

As soon as the weeds germinate and their roots or shoots come into contact with this layer, the herbicide goes to work.

The active ingredients in pre-emergent herbicides interfere with cell division in the newly germinating seeds.

This disruption prevents the seedlings from developing proper root systems.

As a result, the weeds die before they even break the soil surface.

It’s important to note that pre-emergent herbicides don’t affect the seeds themselves. Instead, they target the crucial moment when the seeds begin to sprout and grow.

This selective action is what makes this type of herbicide relatively safe for established plants.

Once applied, these products remain active in the soil for 8-12 weeks on average, depending on the specific herbicide and environmental conditions.

So you’re getting ongoing protection against wed germination throughout the entire growing season.

That being said, pre-emergent herbicides aren’t a one-and-done solution. Over time, soil microbes gradually break down the herbicide.

This degradation—combined with factors like soil disturbance and rainfall—eventually reduces the concentration of the herbicide below the critical level needed for weed control.

When Is the Best Time to Apply Pre-Emergents?

Apply too early, and the herbicide may break down before weed seeds begin to germinate.

Apply too late, and you’ll miss the critical window, allowing weeds to establish themselves in your lawn.

So when exactly should you be reaching for a pre-emergent herbicide?

The golden rule here is to get the product down before weeds start to germinate.

For most regions, this means focusing on two key application periods: early spring and early fall.

Application Period	Timing	Temperature	Key Considerations
Early Spring	March or April (varies by region)	65-70°F soil temperature for 4	Apply before weed seeds germinate. Ideal for summer annual weeds like crabgrass.
Early Fall	September or October	Below 70°F	Optional but helpful against winter annual weeds like annual bluegrass and henbit.
Reapplication	6-8 weeks after initial application	N/A	Reinforces the barrier as the herbicide breaks down. Check product label for specific reapplication guidelines.
Watering	Within a few days of application	N/A	Water with about a half-inch to activate the herbicide in the soil for effective protection.

Spring Application

As the soil begins to warm up in early spring, you’ll want to be ready with your pre-emergent.

The ideal time is when soil temperatures consistently reach between 65 and 70 degrees Fahrenheit for at least four consecutive days.

For many areas, including the Midwest, this spring application window falls sometime in March or April, but it can vary depending on your specific climate and the types of weeds common in your region.

If you’re unsure about timing in your area, consider reaching out to a local garden center or university cooperative extension for region-specific advice.

Fall Application

While not always necessary, a fall application of pre-emergent herbicide can provide additional protection against winter annual weeds.

You’ll want to wait until temperatures drop below 70 degrees Fahrenheit, which often occurs in September or October.

Common targets of fall pre-emergent treatments include annual bluegrass (Poa annua) and henbit.

Temperature Considerations

Soil temperature is a more reliable indicator for timing your pre-emergent application than calendar dates.

You can use a soil thermometer to monitor your soil’s temperature or check with local agricultural extension offices.

It’s generally better to apply pre-emergent herbicides a bit early rather than late.

If you wait until you see weeds sprouting, you’ve already missed the optimal window for pre-emergent control.

Reapplication

You may want to consider a second application about 6-8 weeks after your initial treatment to reinforce the chemical barrier as it starts to break down.

Different products have different reapplication recommendations though, so always check the product label beforehand.

Watering After Application

Watering your lawn within a few days of applying pre-emergent herbicides is important because it helps move the herbicide into the soil where it can form its protective layer.

Most products we’ve come across require about a half-inch of water to properly activate.

What Are the Most Effective Pre-Emergent Herbicides?

Dinitroanilines are considered the most popular and reliable class of herbicides for pre-emergent control.

They’re excellent for controlling both summer and winter annual grass weeds, in addition to some broadleafs.

Examples of dinitroaniline herbicides include:

• Surflan (Oryzalin): Perfect for woody landscape beds. It’s particularly effective against ragweed, evening primrose, and morning glory.
• Barricade (Prodiamine): Safe for many woody ornaments but may injure herbaceous plants. Best used against spurge.
• Pendulum (Pendimethalin): Not as powerful as Surflan or Barricade, but still offers reliable protection against horseweed.

Other Herbicide Classes

Along with dinitroanilines, the following herbicides are also effective for pre-emergent control:

• Pennant Magnum: Primarily used for pre-emergent nutsedge and annual grass control. Be cautious with the EC (emulsifiable concentrate) formulation as it can burn new tender growth.
• Devrinol: Safe on both herbaceous and woody plants. Though weaker on many broadleaf weeds, it’s a good option for sensitive plantings.
• Tower: Takes care of yellow nutsedge and controls annual grasses and spurge. Like PM, the EC formulation can be too harsh on new tender growth.
• Ronstar: Effective on annual broadleaf weeds, but can injure herbaceous ornamentals, particularly spring bulbs like Star of Bethlehem.

Combination Herbicides

Combination products like Snapshot TG (isoxaben and trifluralin) and Freehand (Tower and pendimethalin) tend to offer broad-spectrum weed control of both broadleaf and grassy weeds.

They’re typically safe on most woody ornamentals and common perennials.

How Do You Choose the Right Pre-Emergent?

Pre-emergent herbicides aren’t one-size-fits-all.

Their effect can vary based on factors such as formulation and the types of weeds you’re dealing with.

So before buying a pre-emergent, be sure to consider the following factors:

Types of Weeds

Not all pre-emergents target the same types of weeds.

So what you need to do is take note of the weed species that typically appear in your lawn year after year.

For instance, if crabgrass is your primary concern, products containing active ingredients like prodiamine or dithiopyr should be your go-to.

For broadleaf weed control, look for herbicides with isoxaben or other broad-leaf-specific ingredients.

Lawn Grass Type

Similarly, you need to take note of your type of grass as some herbicide products may harm certain grass species, especially if applied incorrectly.

The information on the product label should indicate whether or not it’s compatible with your specific type of grass.

Climate and Region

Your local climate can influence the effectiveness of different pre-emergent herbicides.

In areas with heavy rain, for example, a liquid pre-emergent might work faster and stay in the soil longer than a granular pre-emergent herbicide.

This segues into…

Formulation

Pre-emergent herbicides come in both liquid and granular forms, and each form has its advantages:

Advantages of Liquid Herbicides:

• More uniform coverage
• Quick action in the soil
• Only need a sprayer for application
• Cost-effective for large areas

Advantages of Granular Herbicides:

• Relatively easier to apply
• Can be applied with a standard fertilizer spreader
• May be preferable in windy conditions
• Often combined with fertilizers for a “weed and feed” approach

Control Duration

The duration of lawn weed control differs from one product to another.

Some may last an average of 8-12 weeks, while others provide season-long control.

With that in mind, consider how often you’re willing to reapply when choosing your product.

What Are Common Mistakes in Pre-Emergent Weed Control?

It’s easy to make mistakes with pre-emergent weed control, which is why it helps to know the most common pitfalls you might encounter:

• Incorrect Timing: Applying too late renders pre-emergents ineffective, whereas applying too early can lead to the herbicide breaking down before peak weed germination periods.
• Uneven Application: Skipping areas or overlapping too much can lead to patchy weed control. Be sure to calibrate your spreaders/sprayers properly.
• Improper Application Rate: Overuse can easily harm desirable plants and potentially contaminate your entire lawn, while underuse may not provide adequate weed control.
• Disturbing the Herbicide Barrier: Aerating, dethatching, or heavy raking after application can disrupt the protective layer.
• Ignoring Label Instructions: Each product has specific guidelines for use, reapplication intervals, and safety precautions. Failing to follow these instructions can lead to poor results.
• Failing to Water in Time: Not watering after application prevents the herbicide from forming an effective barrier in the soil.
• Expecting Immediate Results: Pre-emergents work by preventing weed growth, not killing existing weeds. So don’t mistake the lack of immediate visible effects for product failure.

Should You Hire a Professional or DIY Pre-Emergent Control?

Can’t decide between tackling pre-emergent weed control yourself or hiring a professional lawn care service? Consider the pros and cons:

DIY Weed Control

Pros	Cons
– Cost-effective for smaller areas- Personal satisfaction from maintaining your own lawn- Flexibility in timing and product selection- Opportunity to learn about lawn care and weed management	– Requires time and effort to research and apply correctly- Need to purchase and store equipment and products- Potential for mistakes in application or product selection- May lack access to professional-grade products

Professional Weed Control

Pros	Cons
– Expertise in timing and product selection- Consistent, reliable applications- Access to professional-grade equipment and products- Time-saving for the homeowner- Often part of a comprehensive lawn care program	– More expensive than DIY weed control for smaller properties- Reliance on the service provider’s schedule

Not Interested in DIY Weed Control? SunCo Has Got You Covered

While many homeowners enjoy the hands-on approach of DIY lawn care, others prefer to leave it to the professionals.

If you’re in the latter group, SunCo offers expert pre-emergent weed control services that can help you have the lush, weed-free lawn you desire without the hassle of doing it yourself.

Not only can we treat your lawn quickly and effectively but also address other lawn issues you might not be aware of.

We also integrate other lawn care services for overall health and provide regular, scheduled treatments to ensure ongoing protection.

Don’t let weeds take over your lawn; contact SunCo today to schedule a consultation and take the first step towards a weed-free lawn!