Seeing weeds in St Augustine grass can be frustrating for any homeowner. Weeds invading St. Augustine grass take advantage of this turf’s specific growth habits and potential weaknesses. This article will help you identify common invaders and give you a clear plan to remove them and prevent their return.
St. Augustine is a popular warm-season grass known for its thick, carpet-like growth. However, its shallow root system and preference for shade can sometimes create openings for weeds. The key to a healthy lawn is understanding both the grass and its competitors.
Weeds In St Augustine Grass
To effectively combat weeds, you first need to know what you’re dealing with. Weeds in your lawn generally fall into three main categories: broadleaf weeds, grassy weeds, and sedges. Each type requires a slightly different approach for control.
Common Broadleaf Weeds
Broadleaf weeds are usually easy to spot because their leaf structure looks different from your grass blades. They have wide leaves with net-like veins.
- Dandelion: Recognizable by its bright yellow flower and puffball seed head. It has a deep taproot that makes it hard to pull completely.
- Clover: Often has three-part leaves and small white or pink flowers. It can indicate low nitrogen levels in your soil.
- Dollarweed: A common invader in moist areas. Its round, penny-sized leaves look like lily pads and it spreads quickly.
- Spurge: This low-growing weed forms a dense mat. It has small, oval leaves arranged opposite each other on a reddish stem.
- Chickweed: A cool-season weed with small, star-shaped white flowers. It thrives in thin, shady areas of the lawn.
Troublesome Grassy Weeds
Grassy weeds are trickier because they look similar to St. Augustine grass, especially when young. They blend in until they mature and start to stand out.
- Crabgrass: A summer annual that germinates in spring. It forms a crab-like shape with wide, light green blades and spreads low to the ground.
- Goosegrass: Often found in compacted soil. It has a white, silver, or translucent center at the base of the stem and forms a flat rosette.
- Dallisgrass: A perennial grassy weed that grows in unsightly clumps. It grows faster than St. Augustine and has coarse, light green blades.
- Annual Bluegrass (Poa Annua): A cool-season grass that appears as light green patches in the winter or early spring when St. Augustine is dormant or slow-growing.
Sedges And Other Problem Plants
Sedges are often mistaken for grasses, but they have a distinct triangular stem. They are a sign of overly wet soil conditions.
- Yellow Nutsedge: Grows faster than turfgrass and has a yellow-green color. It has a waxy, upright growth habit and a triangular stem you can feel.
- Purple Nutsedge: Similar to yellow nutsedge but with a reddish-purple seed head. It’s particularly aggressive in warm climates.
- Wild Violet: While a broadleaf weed, it’s worth mentioning seperately due to its waxy leaves that resist herbicides. It has heart-shaped leaves and purple flowers.
Why Weeds Invade St Augustine Lawns
Weeds don’t just appear randomly. They exploit specific weaknesses in your lawn’s health or environment. Understanding these causes is the first step toward prevention.
Shallow Root System And Thatch
St. Augustine grass spreads via above-ground runners called stolons. This creates a dense sod but can lead to a shallow root system. A thick layer of thatch—the mat of dead grass between the soil and green blades—can prevent water and nutrients from reaching the roots, weakening the grass and giving weeds a foothold.
Improper Mowing Practices
Mowing too short is a major cause of weed invasion. Scalping your St. Augustine stresses the grass and allows sunlight to reach weed seeds in the soil. This turf prefers a mowing height of 3.5 to 4 inches. Keeping it taller shades the soil and helps it outcompete weeds.
Watering And Drainage Issues
Both overwatering and underwatering create opportunities for weeds. Overwatering promotes diseases and favors weeds like dollarweed and nutsedge. Underwatering stresses the St. Augustine, causing thin patches where weeds can establish. Poor drainage creates chronically wet areas that weeds love.
Soil Compaction And Low Fertility
Compacted soil prevents air, water, and nutrients from reaching grass roots. Weeds like goosegrass and dallisgrass thrive in compacted ground. Similarly, soil with low fertility or an improper pH (St. Augustine prefers a pH of 6.0 to 7.5) will result in weak, thin turf.
Incorrect Herbicide Use
Using the wrong herbicide can damage your St. Augustine grass, creating bare spots. Furthermore, overusing a single type of herbicide can lead to resistant weed populations that are even harder to control.
Effective Weed Removal Strategies
Once you’ve identified the weed and understood why it’s there, you can choose the best removal method. Your strategy should combine immediate action with long-term lawn health improvements.
Manual Removal Techniques
For isolated weeds or small infestations, hand-pulling can be very effective. The key is to remove the entire root system to prevent regrowth.
- Water the area the day before to soften the soil.
- Use a weeding tool or trowel to loosen the soil around the weed.
- Grip the weed at its base and pull gently but firmly, ensuring you get the root.
- Dispose of the weed in yard waste, not your compost, to avoid spreading seeds.
Post-Emergent Herbicide Application
For larger infestations, a targeted herbicide may be necessary. It is crucial to select a product labeled as safe for St. Augustine grass.
Choosing The Right Herbicide
- For Broadleaf Weeds: Look for herbicides containing 2,4-D, dicamba, or MCPP. These are often found in “weed and feed” products or three-way herbicide mixes.
- For Grassy Weeds: Control is more difficult. Selective herbicides like sethoxydim (found in some grass killers) can target grassy weeds without harming St. Augustine. Always check the label for St. Augustine safety.
- For Nutsedge: Use herbicides with active ingredients like halosulfuron-methyl or sulfentrazone, which are labeled for nutsedge control in St. Augustine lawns.
Application Best Practices
- Apply on a calm, cool day when temperatures are between 60-85°F to avoid drift and turf stress.
- Do not mow for a few days before or after application.
- Water your lawn deeply a day or two before applying, unless the label says otherwise.
- Mix and apply the herbicide exactly according to the label instructions—more is not better.
- Spot-treat weeds whenever possible instead of blanketing the entire lawn.
Pre-Emergent Herbicide Timing
Pre-emergent herbicides create a barrier in the soil that prevents weed seeds from germinating. Timing is absolutely critical for these products to work.
- For Summer Weeds (Crabgrass, Spurge): Apply in early spring, when soil temperatures consistently reach 55-60°F for 3-4 consecutive days.
- For Winter Weeds (Annual Bluegrass, Chickweed): Apply in early fall, when soil temperatures drop below 70°F.
- Water the product into the soil after application as directed on the label to activate the barrier.
- Remember, pre-emergents will also prevent new grass seed from germinating, so do not overseed after application.
Building A Weed-Resistant Lawn
The most sustainable way to beat weeds is to grow such a thick, healthy St. Augustine lawn that weeds simply can’t get started. This is a cultural approach that focuses on optimal lawn care.
Optimal Mowing For Health
Mowing correctly is your first defense. Keep your mower blades sharp to make clean cuts that heal quickly. Never remove more than one-third of the grass blade height in a single mowing. For example, if you want a 4-inch lawn, mow when it reaches about 6 inches tall.
Smart Watering Schedules
St. Augustine grass needs about 1 inch of water per week, including rainfall. Water deeply and infrequently to encourage deep root growth. It’s better to water for 30-45 minutes once or twice a week than to water for 10 minutes every day. Early morning is the best time to water, reducing evaporation and fungal disease risk.
Fertilization Based On Soil Tests
Don’t guess about your soil’s needs. A soil test from your local extension office will tell you exactly what nutrients are lacking and the pH level. St. Augustine typically benefits from a balanced fertilizer applied 2-4 times during the growing season (spring through early fall). Avoid fertilizing too early in spring or too late in fall.
Aeration And Dethatching
If your soil is compacted or you have over half an inch of thatch, these practices are essential.
- Core Aeration: Performed in the growing season, this process removes small plugs of soil to relieve compaction, improve air and water movement, and encourage root growth.
- Dethatching: Use a vertical mower or dethatching rake to remove the thick layer of dead organic matter. This is best done in late spring or early summer when the grass can recover quickly.
Overseeding And Sunlight Management
St. Augustine cannot be overseeded with more St. Augustine seed (it’s typically established from sod or plugs). However, you can fill in thin areas with new sod or plugs. Also, manage sunlight by trimming tree branches to allow more light into shady areas where St. Augustine struggles.
Seasonal Weed Control Calendar
A proactive, seasonal approach keeps you ahead of weed problems. Here’s a general guide for a St. Augustine lawn in a warm-season climate.
Spring (March-May)
- Apply pre-emergent herbicide for summer weeds.
- Begin mowing regularly as growth starts.
- Apply first round of fertilizer based on soil test results.
- Spot-treat any existing winter weeds with post-emergent herbicide.
- Core aerate if soil is compacted.
Summer (June-August)
- Maintain consistent mowing and watering.
- Monitor for and spot-treat summer weeds like crabgrass and dollarweed.
- Apply a second, lighter summer fertilizer if needed (avoid high nitrogen during peak heat).
- Dethatch if necessary.
Fall (September-November)
- Apply pre-emergent herbicide for winter weeds.
- Apply a potassium-rich “winterizer” fertilizer to help with cold tolerance.
- Rake leaves promptly to prevent smothering the grass.
- Overseed with new plugs to repair summer damage.
Winter (December-February)
- Mow only if needed, as growth slows or stops.
- Spot-treat winter weeds like annual bluegrass on a mild day.
- Plan your lawn care strategy for the upcoming year.
- Service your mower and other lawn equipment.
Frequently Asked Questions
What Is The Best Killer For Weeds In St Augustine Grass?
There is no single “best” killer, as it depends on the weed type. For broadleaf weeds, a three-way herbicide with 2,4-D, dicamba, and MCPP is often effective and safe for St. Augustine. For grassy weeds, you need a selective grass herbicide like sethoxydim. Always read the label to confirm it is safe for your turf.
How Do I Permanently Get Rid Of Weeds In My Lawn?
Permanent control is achieved through consistent lawn health, not just one-time herbicide use. Combine correct mowing, watering, and fertilization with timely pre-emergent applications. A thick, healthy St. Augustine lawn is the best long-term weed prevention, as it leaves no room for weeds to grow.
Can Vinegar Kill Weeds In St Augustine?
Household vinegar is a non-selective desiccant, meaning it will burn the foliage of any plant it touches, including your grass. While it may kill small annual weed leaves, it usually does not kill the root system, leading to regrowth. It also poses a high risk of damaging your St. Augustine, so it is not recommended for use on the lawn.
Why Is My St Augustine Grass So Thin And Weedy?
Thin, weedy St. Augustine is usually a symptom of an underlying problem. Common causes include mowing too short, soil compaction, poor drainage, inadequate fertilizer, or pest/disease issues. Address these cultural practices first to thicken your turf, which will naturally supress future weed growth.
When Should I Apply Pre Emergent In Florida?
In Florida, timing is regional. For summer weeds (like crabgrass), apply pre-emergent in late January or early February in South Florida, and in February or early March in North Florida. For winter weeds (like annual bluegrass), apply in late September or early October. Monitoring soil temperature is the most accurate method.