St Augustine grass lawns face particular weed pressures that require selective herbicides to protect the turf. Understanding the common weeds in st augustine grass is the first step toward a healthy, uniform lawn. You cannot simply spray any product, because St Augustine is sensitive to many chemicals.
This guide will walk you through the most common offenders, the best control methods, and a few simple tricks to keep your lawn thick and weed-free. We will focus on practical steps that work in real yards, not just theory.
Weeds In St Augustine Grass
Before you buy a spray, you need to know what you are fighting. The weeds in st augustine grass fall into three main categories: broadleaf weeds, grassy weeds, and sedges. Each one requires a different approach.
Broadleaf weeds are the easiest to spot. They have wide leaves and often show up in clusters. Grassy weeds look like your lawn but grow faster and stand out. Sedges look like grass but have a triangular stem and grow in wet areas.
Let us break down each type so you can identify them with confidence. You will also learn the exact herbicide families that are safe for your turf.
Common Broadleaf Weeds
Dandelions are the most common broadleaf weed in warm-season lawns. They have a deep taproot that makes them hard to pull by hand. If you see yellow flowers, the root is already established.
Chickweed is another frequent invader. It grows low to the ground and forms a dense mat. It thrives in thin areas of your lawn where the St Augustine is struggling.
Henbit and purple deadnettle are winter annuals. They appear in early spring and die off when temperatures rise. They are not hard to kill, but they spread quickly if ignored.
Clover is a sign of low nitrogen. It actually adds nitrogen to the soil, but most homeowners do not want it in a pure St Augustine stand. You can control it with a broadleaf herbicide.
- Dandelion: Deep taproot, yellow flowers
- Chickweed: Low growing, dense mat
- Henbit: Purple flowers, winter annual
- Clover: Three leaves, indicates low nitrogen
Grassy Weeds That Blend In
Crabgrass is the most notorious grassy weed. It germinates in spring and grows rapidly in summer heat. It stands out because it is lighter green and coarser than St Augustine.
Goosegrass looks similar to crabgrass but grows in compacted soil. You often see it along driveways and sidewalks where foot traffic is heavy.
Annual bluegrass, or poa annua, is a winter weed. It is light green and produces seed heads very quickly. It dies in late spring, leaving bare spots.
These grassy weeds are harder to control because most selective herbicides that kill them also damage St Augustine. You must use a pre-emergent in early spring and fall to stop them before they sprout.
Sedges And Rushes
Yellow nutsedge is the most common sedge in southern lawns. It grows faster than your grass and has a waxy leaf that repels many herbicides. It thrives in wet, poorly drained areas.
Purple nutsedge is more aggressive but less common. It spreads by underground tubers, making it very hard to eliminate. You may need multiple applications.
For sedges, you need a specific herbicide called a sedge killer. Products containing halosulfuron or sulfentrazone are effective. Do not use a general broadleaf killer on these.
Pre-Emergent Herbicides For Prevention
The best way to deal with weeds is to stop them from germinating. Pre-emergent herbicides create a barrier in the soil that kills seedlings as they sprout. You must apply them before the weeds appear.
Timing is everything. For crabgrass, apply the pre-emergent when soil temperatures reach 55 degrees Fahrenheit for three consecutive days. This usually happens in early spring.
For winter weeds like poa annua, apply in late summer or early fall. The exact timing depends on your region. In the deep south, this might be September. In the transition zone, it might be August.
You need to water the pre-emergent into the soil within 24 hours of application. If you do not water it in, the chemical barrier will not form properly.
- Wait for soil temperature to hit 55°F
- Apply the granular product evenly
- Water in with 0.5 inches of irrigation
- Reapply if the product label says so
Do not apply pre-emergent if you are planning to overseed. The barrier will prevent the new grass seed from germinating. St Augustine is usually not overseeded, but some homeowners do it in the fall.
Best Pre-Emergent Products
Prodiamine is a common active ingredient. It provides long-lasting control for up to eight months. It is safe for St Augustine when applied at the correct rate.
Dithiopyr is another option. It has the added benefit of controlling crabgrass that has already sprouted in the early stages. This gives you a little more flexibility.
Isoxaben targets broadleaf weeds specifically. It is great for preventing chickweed and henbit in the fall. You can combine it with a grassy weed pre-emergent.
Always read the label to confirm it is safe for St Augustine. Some products are only for Bermuda or Zoysia. Using the wrong one can damage your lawn.
Post-Emergent Herbicides For Active Weeds
If weeds are already growing, you need a post-emergent herbicide. These are sprayed directly on the weed foliage. The chemical is absorbed and moves to the roots.
For broadleaf weeds, look for products with 2,4-D, dicamba, or MCPP. These are safe for St Augustine when used correctly. They are often sold as a three-way mix.
Be careful with the temperature. Do not spray when it is above 90 degrees Fahrenheit. The herbicide can volatilize and drift onto nearby plants. It can also stress your St Augustine.
For grassy weeds, your options are limited. Quinclorac is one of the few products that controls crabgrass without killing St Augustine. It is not effective on all grassy weeds.
For sedges, use a dedicated sedge killer. Halosulfuron is the most common. You may need to apply it twice, with a gap of four to six weeks between applications.
How To Apply Post-Emergent Herbicides
Spray on a calm day with no wind. This prevents drift onto your flower beds or vegetable garden. Early morning or late evening is the best time.
Do not mow for two to three days before spraying. You want the weed leaves to be large and healthy so they absorb the chemical. Mowing after spraying is fine, but wait at least 24 hours.
Use a surfactant. This is a wetting agent that helps the herbicide stick to the waxy leaves of weeds. Many weeds have a natural wax coating that repels water.
- Mix the herbicide in a pump sprayer
- Add a surfactant at the recommended rate
- Spray the weeds until they are wet but not dripping
- Wait 48 hours before watering
You may see results in a few days. Broadleaf weeds will curl and yellow. Sedges take longer. Do not pull them until they are completely dead, or you may leave the tubers behind.
Cultural Practices To Prevent Weeds
Herbicides are only part of the solution. A healthy, thick St Augustine lawn will naturally crowd out weeds. If your grass is thin, weeds will always find a way in.
Mowing height is critical. St Augustine should be mowed at 3.5 to 4 inches. This is higher than most people think. Taller grass shades the soil, preventing weed seeds from germinating.
Do not scalp your lawn. Cutting it too short exposes the soil to sunlight and encourages weed growth. It also stresses the St Augustine, making it more susceptible to disease.
Water deeply but infrequently. This encourages deep root growth. Shallow watering creates weak roots and promotes weed germination at the surface.
Fertilize at the right time. St Augustine needs nitrogen in spring and summer. Do not fertilize in late fall, as this can encourage winter weeds.
Soil Health And Aeration
Compacted soil is a major cause of weed problems. It prevents water and air from reaching the roots. It also creates the perfect environment for goosegrass and other weeds.
Aerate your lawn in late spring or early summer. This removes small plugs of soil, allowing water and nutrients to penetrate. It also reduces thatch buildup.
Thatch is a layer of dead grass and roots that sits on top of the soil. If it is thicker than half an inch, it can harbor pests and diseases. It also prevents herbicides from reaching the soil.
You can reduce thatch by mowing regularly and not letting grass clippings pile up. You can also use a core aerator to break it up.
Proper Fertilization Schedule
St Augustine grass is a heavy feeder. It needs about 2 to 4 pounds of nitrogen per 1,000 square feet per year. The exact amount depends on your soil type.
Apply a slow-release nitrogen fertilizer in spring. This provides a steady supply of nutrients without causing a growth surge. A growth surge can make your lawn more susceptible to pests.
In summer, apply a balanced fertilizer with potassium. Potassium helps the grass tolerate heat and drought. It also strengthens the cell walls, making it harder for weeds to invade.
Do not fertilize in winter. The grass is dormant and will not use the nutrients. Instead, the fertilizer will feed the winter weeds.
Organic And Natural Control Methods
If you prefer to avoid synthetic chemicals, there are organic options. They are not as effective, but they can work for small infestations.
Corn gluten meal is a natural pre-emergent. It prevents weed seeds from germinating. It also contains nitrogen, which feeds your lawn. Apply it in early spring and fall.
Vinegar is a contact herbicide. It will kill the top growth of weeds, but it does not affect the roots. This means the weeds will regrow. It is best for spot treatment on young weeds.
Boiling water is another option. It will kill any plant it touches, so use it carefully. It is best for weeds growing in cracks in the sidewalk or driveway.
Hand pulling is the most labor-intensive but safest method. It works well for broadleaf weeds with shallow roots. For dandelions, use a weeder to get the entire taproot.
When To Use Organic Methods
Organic methods work best for prevention and small infestations. If you have a severe weed problem, you will need a synthetic herbicide to get it under control.
You can also combine organic and synthetic methods. Use corn gluten meal in the fall and a synthetic pre-emergent in the spring. This gives you double protection.
Remember that organic does not mean harmless. Vinegar can burn your skin and eyes. Always wear gloves and protective eyewear when handling any herbicide.
Seasonal Weed Calendar
Weeds follow a predictable schedule. If you know what to expect each season, you can stay ahead of the problem. Here is a simple calendar for St Augustine lawns.
Spring is the time for pre-emergents. Apply them when soil temperatures warm up. This targets summer annuals like crabgrass and goosegrass.
Summer is the time for post-emergents. Broadleaf weeds are actively growing. Spray them on a cool morning. Also, watch for sedges in wet areas.
Fall is the time for a second pre-emergent application. This targets winter annuals like poa annua and henbit. It also helps control any late-season crabgrass.
Winter is the time for spot treatment. Most weeds are dormant, but you may see some winter annuals. Pull them by hand or spray them on a warm day.
Regional Differences
St Augustine grass is grown in the southern United States. The specific weeds vary by region. In Florida, you may see more sedges. In Texas, you may see more broadleaf weeds.
Check with your local extension office for a list of common weeds in your area. They can also tell you the best timing for pre-emergent applications.
Your local climate also affects the timing. In the deep south, spring comes earlier. In the transition zone, you may have a shorter growing season.
Frequently Asked Questions
Here are some common questions about managing weeds in St Augustine grass. These answers are short and practical.
What kills weeds but not St Augustine grass?
Products containing 2,4-D, dicamba, or MCPP are safe for St Augustine. For grassy weeds, use quinclorac. For sedges, use halosulfuron. Always read the label to confirm it is safe for your turf.
When is the best time to apply pre-emergent to St Augustine?
Apply in early spring when soil temperatures reach 55 degrees Fahrenheit. Apply again in late summer or early fall for winter weeds. Water the product in within 24 hours.
Can I use vinegar to kill weeds in St Augustine?
Vinegar will kill the top growth of weeds, but it will not kill the roots. It is not selective and can damage your grass. Use it only for spot treatment on young weeds in cracks or driveways.
Why do I have so many weeds in my St Augustine lawn?
Weeds thrive in thin, stressed lawns. Poor mowing height, overwatering, and compacted soil are common causes. Focus on cultural practices like proper mowing and fertilization to thicken your lawn.
Is it better to pull weeds or spray them?
Pulling is better for small infestations and broadleaf weeds with shallow roots. Spraying is better for large infestations and weeds with deep taproots or underground tubers. For sedges, always spray.
Final Tips For A Weed-Free Lawn
Consistency is the key to weed control. You cannot spray once and expect perfect results. You need a year-round program that combines prevention, treatment, and good lawn care.
Keep a record of when you apply products. This helps you track what works and what does not. It also helps you plan next year’s schedule.
Inspect your lawn weekly. Walk around and look for new weeds. Early detection makes treatment much easier. A single weed can produce thousands of seeds.
Do not overwater. St Augustine needs about 1 to 1.5 inches of water per week. Overwatering creates the perfect environment for sedges and other moisture-loving weeds.
Finally, be patient. It can take a full season to get a severe weed problem under control. Stick with your program, and your lawn will improve.
If you are unsure about a product, ask a professional at your local garden center. They can recommend the right herbicide for your specific weeds and your region. They can also help you with application rates and timing.
Remember that a dense, healthy St Augustine lawn is your best defense. Focus on the basics: mow high, water deeply, and fertilize on schedule. The weeds will have a hard time competing.
With a little effort and the right products, you can enjoy a thick, green lawn all year long. The weeds will be a thing of the past, and your neighbors will be asking for your secret.
Start with a soil test to know exactly what your lawn needs. This takes the guesswork out of fertilization. Your local extension office can provide a test kit for a small fee.
Once you know your soil pH and nutrient levels, you can tailor your fertilizer program. This is the most effective way to grow a dense turf that naturally resists weeds.
Do not forget to sharpen your mower blades. Dull blades tear the grass, leaving ragged edges that are more susceptible to disease and weed invasion. Sharpen them at least twice a season.
By following these guidelines, you will have a lawn that is the envy of the neighborhood. It takes work, but the results are worth it. Your St Augustine grass will be thick, green, and virtually weed-free.