6 Best Pre Emergent Weed Killers For St. Augustine & When To Apply

St. Augustine lawns are typically found blanketing landscapes in temperate climate regions around the world. Coveted for its ability to thrive in full sun, shade, and different soil types, this broad-leafed grass is very forgiving. But, it’s not impervious. Especially, against weeds.

The most common weed varieties found in this turf are those that flourish in similar environments. Chickweed, Henbit, clover, and dollar weed have been the bane of many St. Augustine lawn connoisseurs for centuries. Followed by the need to get rid of them. To prevent you from having to sort through pages and pages of reviews and potentially buying something that doesn’t work, I’ve done all the work for you. Season after season of testing has narrowed down my list to six of the best pre emergent weed killers for St. Augustine grass.

Top Pre Emergent Weed Killers For St. Augustine

If you’re in a hurry then here are my recommendations for both granular and liquid pre-emergents. They have consistently performed well, and both are professional-grade products that will not let you down.

Best For Broadcasting

The Andersons Barricade

Professional grade granular pre-emergent. Ready-to-use with any broadcaster or spreader. Covers 5,800sq.ft

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Best Pre Emergent Spray

Quali-Pro Prodiamine

Very effective water-soluble pre-emergent, designed for use in herbicide sprayers. Covers 48,000 sq.ft

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Choosing Pre Emergent For St. Augustine Lawns

So, what does the best pre-emergent have in it? What does it do? Well, it depends on a few factors. Are you trying to keep these weeds from growing, kill existing ones, or both? How bad is the infestation? You either have just a few or they’re everywhere.

Some that I tested eliminate weeds at seed level, using different active ingredients. Others also break weeds down on a cellular level to inhibit growth once seeds have germinated. However, all of them are formulated to eliminate weeds without killing your grass.

Common Weeds In St. Augustine Grass

As mentioned, there are a few specific weed types that are typically seen dotting St. Augustine grass. Some have broad-leafed structures while others are more grassy, giving each type a distinct look. Yet, they all drop anchor the same way: Setting their seeds adrift in the wind toward your carefully curated lawn. Let’s take a look at some of them:

Broadleaf Weeds

Broadleaf weeds are relatively easy to identify as their structures are so different from that of healthy turf. Single or clustered flowers emerge in quick succession scattering hundreds of seeds at a time. The right pre-emergent will suppress these common varieties.

Chickweed

This winter annual grows in clumps with several flat, wide leaves growing along a single, central stem. In contrast to St. Augustine grass, this weed is noticeably light green in color. It also sends out tiny, white flowers between March and November that soon become troublesome seeds.

Clover

Each variety of this weed has a familiar three-petal design with low-growing flowers that bloom from early spring to late fall. Its root system is quite shallow making it easier to eliminate within the grass. For this reason, it’s also often grown in controlled settings as a ground cover.

Dollar Weed

Also known as pennywort, this prolific weed is considered a warm-season perennial and is easy to spot by its “silver dollar”-shaped leaves. Dollar weed thrives in soil with poor drainage, excess moisture, and lawns with shallow root systems.

Grassy Weeds

Like the broadleaf variety, these grassy annuals also form in clumps, but with singular blades that rise from a center crown. At the end of their life cycle, they usually send out either yellow or white multi-petaled flowers that produce an abundance of seeds.

Bermuda Grass

While often grown as landscape turf, Bermuda grass seeds can infest other types of lawns and quickly take over. While this isn’t necessarily a bad thing (Bermuda grass is very drought and heat tolerant), some prefer an unblemished spread of one type of grass and may require a herbicide for Bermuda grass.

Crabgrass

Crabgrass, on the other hand, is of no benefit to your lawn. Quickly multiplying clumps force your grass to fight for water and nutrients while giving your green landscape a blotchy appearance. Eliminating crabgrass as soon as it appears is your best strategy for preventing this.

Poa Annua

Poa Annua is the most persistent variety as it has adapted to grow in different climates and soils. Generally speaking, however, it prefers damp, shady spots. This short grass can survive being mowed to less than 1” so finding the best pre-emergent for St. Augustine grass is the way to go.

Can Preventative Herbicide Kill St. Augustine?

Pre-emergent weed killer will almost certainly kill newly re-seeded St. Augustine or freshly laid sod so only apply to established lawns. This is because most selective herbicides are designed to block seeds from growing and new root systems may be too weak to tolerate them, resulting in potentially fatal damage.

The ideal preventative weed killer for St. Augustine grass is one specially formulated to target the unique biology of weeds without any significant effect on established lawns.

Best Pre-Emergent Weed Killers For St. Augustine Reviewed

Now that I’ve covered the different weeds you might need to tackle, it’s time to choose what’s best for your lawn and take a look at some preventative products I’ve tested throughout the seasons.

For my selection criteria, I have included the products’ compounds & effectiveness, performance, value for the money, and the level of customer satisfaction. Read on for all you need to know about choosing the product that is best for your needs!

1. The Andersons Barricade Pre-Emergent Weed Control

• Ingredients: Prodiamine
• Weeds: Crabgrass, poa annua, goosegrass, and henbit
• Application: Spreader
• Formulation: Ready to Use
• Action time: After Watering
• Rainproof: Yes
• Coverage: 5,800 sq ft
• Use: Lawns, ornamental beds

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Pros      

• Effective on Crabgrass, poa annua, goosegrass, and henbit
• Beneficial weed killer for lawns and garden borders
• No significant impact when used around border plantings

Cons

• Must be activated with water

A nice plus with Anderson’s Barricade is that it eliminates weeds in your prized St. Augustine lawn as well as in your garden borders without harming most ornamentals. And that’s exactly where I chose to test this — after reading the ‘protected plant list’ on the bag, of course.

You do need to water this one in because only weeds that come into contact with the granules and water will be eliminated. But that’s fine since I’d be watering anyway.

Following spring’s last frost, I spread the Anderson’s granules around my beds, wearing protective clothing to prevent skin irritation. Prodiamine is what fuels this selective herbicide, formulated to target the biology of weeds. Within a week, I was relieved to see unsightly weeds shriveling away, leaving my perennials untarnished. Click for Anderson’s Barricade Pre-Emergent Weed Control.

The only drawback was the stains left behind by granules I should’ve swept into my borders prior to watering. It’s also important to note that your lawn will be safe for people and pets once these granules are watered in and allowed to dry.

2. Quali-Pro Prodiamine Pre-Emergent Herbicide

• Ingredients: Prodiamine
• Weeds: Barnyardgrass, Annual Bluegrass, Chickweed, Crabgrass, Henbit and others
• Application: Sprayer
• Formulation: Water soluble
• Action time: After watering
• Rainproof: Yes
• Coverage: Up to 44,000 sq. ft.
• Use: turfgrass, lawns and borders

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Pros      

• 5lb bottle good for up to 44,000 sq ft or multiple applications
• Great value for money
• Prevents seed germination in a broad range of weeds

Cons

• Not for use on established weeds

This option can also be applied to lawns and ornamental beds and needs to be watered in. I liked the long list of weeds it covers and, because it’s applied by sprayer, one 5lb bottle covers up to 44,000 sq ft. This means multiple applications for the same price if you have a smaller space to treat. View Quali-Pro Pre Emergent prices here.

Bear in mind, this one is formulated to prevent seed germination. It has no effect on sprouted seedlings. I tested it on some perennial borders and a small lawn in spring and fall. This product keeps working for up to 6 months, so it was a while before I could document its full effect. I did observe far fewer weed seedlings over time, though.

While initial skin exposure can result in irritation or rash, your lawns and garden areas will be safe for all to use after watering the product in and allowing it to dry.

3. Bonide DuraTurf Crab-Grass Pre-Emergent Control For Lawn

• Ingredients: Dithiopyr
• Weeds: Crabgrass, Barnyardgrass, Black Medic, Chickweed, Henbit and others
• Application: Spreader
• Formulation: Ready to Use
• Action time: After watering
• Rainproof: Yes
• Coverage area: Up to 5,000 sq. ft.
• Use: Lawns and Borders

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Pros      

• Blocks root growth in a broad range of weeds
• One easy application needed for the whole season
• Doesn’t stain sidewalks or paving stones

Cons

• More than one bag may be needed for larger projects

This weed eliminator from Bonide also covers a long list of weeds in both lawns and garden beds. It stops both seedlings and established weeds from growing at the root level, using the active ingredient Dithiopyr. While it doesn’t cover as large of a surface area as other products, the simple method of application was a definite plus.

Covered in protective gear at the recommended time (early spring), I spread this on my front lawn and flower beds. Once weed seeds germinate and form roots, the Dithiopyr prevents those roots from developing any further by blocking cell division and growth. One application can last for up to four months. Click here for Bonide Crabgrass Pre-Emergent from Amazon.com

All this, plus it doesn’t stain pavement or border stones. The only downside is that, depending on the severity of your weed infestation, more than one bag may be needed. And due to the dust that arises during the application, it’s recommended to wait a day or so before spending time in the targeted areas.

Bonide Crabgrass Preventer

4. Southern Ag Atrazine St. Augustine Grass Weed Killer

• Ingredients: Atrazine
• Weeds: Poa annua, Chickweed, Crabgrass, Henbit, Knotweed and others
• Application:  Sprayer
• Formulation:  Water soluble
• Action time: 15- 20 days
• Rainproof: Yes
• Coverage: 3,700 sq. ft.
• Use: St. Augustine Grass and Centipede Grass

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Pros      

• Covers an extensive list of weeds
• Eliminates established weeds
• Blocks seed germination

Cons

• More than one bottle may be needed for larger areas

This highly effective pre-emergent herbicide is specifically formulated for St. Augustine grass. It’s not a selective product, so using in garden borders isn’t recommended. But it does cover the second-widest array of weeds on my list — everything from Poa Annua and chickweed to woodsorrel and knotweed.

The active ingredient, atrazine, works differently. It gets absorbed by established weed roots, spreads through stems and leaves, then blocks photosynthesis (a process that is necessary for growth and seed production). It also blocking the germination of seeds.

I tested this on my back lawn and, within 2 weeks, all those ugly, light green patches began to disappear. By the end of the season, my St. Augustine grass had grown into those areas, giving it a more consistent appearance. Check Southern Ag | Atrazine St. Augustine Grass Weed Killer prices here.

The one downside is that it needs to be applied with a pump sprayer. Applying it to large areas may require a larger sprayer and more than one bottle. But, given its performance, it’s worth it!

5. Hi-Yield Fertilome Turf & Ornamental Weed And Grass Stopper

• Ingredients: Dithiopyr
• Weeds:  Poa Annua, crabgrass, oxalis,lespedeza, henbit, chickweed and others
• Application: Spreader
• Formulation: Ready to Use
• Action time:  Immediately after watering in
• Rainproof: Yes
• Coverage: Up to 5,000 sq. ft.
• Use: Lawns, trees, borders

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Pros      

• Easy to apply
• Prevents roots from growing and spreading
• Prevents seeds from germinating

Cons

• Must be applied at specific times for full effectiveness

My next pick is from Fertilome. Since it’s not necessarily formulated for St. Augustine grass, I wanted to test this using a very small amount per sq ft to see how effective it was. I applied it on a small patch of a weed-infested lawn but also in my perennial beds. Tilling the soil first will loosen any embedded weed seeds and bring them to the surface. Again, you’ll want to read the ‘protected plant’ list on the bag.

This needs to be watered in to activate. And I also found that since this should be applied in spring and fall, the larger bag is actually more economical. View Fertilome Weed & Grass Stopper here.

The active ingredient, Dithiopyr, prevent roots from growing as they emerge from the seed. This is also why you’ll want to wear protective clothing when applying with your spreader. After a few days, I did see some browning patches on the lawn but overall, not too bad. My perennial beds were clean as a whistle!

6. Spectracide Weed Stop Plus Crabgrass Preventer Granules

• Ingredients: Dimethylamine salt, Quinclorac and Dicamba
• Weeds: 200+ different weed types
• Application: Pump sprayer
• Formulation: Water soluble
• Action time: 5 hours
• Rainproof: Within 3 hours
• Coverage: 3000 sq ft.
• Use: Lawns

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Pros      

• Covers 200+ weed types
• Fast results
• Keeps lawns clear of weeds when applied in spring and fall

Cons

• May adversely affect non-targeted plants and water sources

My final pick is from Spectracide. This powerful herbicide and crabgrass preventer for St. Augustine grass employs three active ingredients to eliminate over 200 hundred different weed types — the largest range on my list. Because of its vigorous method of weed removal, this should not be used on anything but grass. Or near any kind of water source.

Donning a lot of protective gear (and washing my hands after), I tested this on a fairly large swathe of lawn that was pretty infested with all kinds of weeds. In both spring and fall when temps were around the 13C mark, I applied an even coat using a backpack sprayer. Click here for Spectracide Weed Stop Granules.

The bag says to apply it to wet grass, but I tried it dry and went a little heavier due to the number of weeds. The effective mix of active ingredients produced results very quickly. By day’s end, I saw weeds start to brown and wither. 

When To Use Pre Emergent On St. Augustine

The best time to apply your pre-emergent will somewhat depend on the active ingredient in the product you choose and how it works. But early spring and fall are typically the optimum application times. Success means staying ahead of the game and this is when weeds are either actively producing seeds or when those seeds are starting to germinate.

How To Apply Pre Emergent To St. Augustine

Pre-emergents are most effective when the outside temperature is around 13C/55F. Any lower and the active ingredients may go dormant. Any higher and those ingredients may burn your lawn.

How to apply will vary depending on your choice of product.  When covering a large plot of lawn, a granular one may work better using a push spreader. Water-soluble ones are easier to apply using either a backpack or handheld sprayer.

Can You Get Rid Of Weeds Without Killing St. Augustine?

To ensure the health of your lawn, carefully read the label of your chosen herbicide to make sure that it’s the best pre-emergent for St. Augustine grass. All the options I tested work well for this grass type. So, to make it easy on yourself, feel free to follow the links for a safe purchase.  

Verdict: Best Pre Emergent Weed Killers For St. Augustine

After a number of in-depth trials, the verdict is in! To varying degrees, all six of my recommendations have proven effective in eliminating weeds from St. Augustine grass. Various active ingredients and application methods are used to approach this from different angles. This gives you the flexibility to choose one that fits your individual needs and end goals. 

So now it’s up to you to choose. Will it be The Anderson’s Pre-Emergent Weed Control, a granular formula that not only stops weeds from invading your prized lawn but can also be used in borders?

Or maybe you prefer the ease of use of a water-soluble option from Quali-Pro. This too offers weed prevention for both your lawn and your borders but is administered via a sprayer.

Alternatively, is Bonide | DuraTurf Crab-Grass Pre-Emergent Control for Lawn what you are looking for? This is not only effective as a preventative treatment in southern lawns such as St Augustine but can also be used in northern lawns and is particularly effective for long-term removal of crabgrass.

FAQ Pre Emergent Weed Killers For St. Augustine