Maintaining a lush, green lawn is a goal for many homeowners, especially those in warmer climates where St. Augustine grass thrives. The desire for a pristine, weed-free turf often leads people to consider “weed and feed” products – a convenient, all-in-one solution promising to nourish the grass while simultaneously eradicating unwanted invaders. However, applying these combination products to St. Augustine grass requires a nuanced understanding, as this particular turf type is more sensitive to certain herbicides than others.

While the convenience of a weed and feed product is undeniable, a “one-size-fits-all” approach can sometimes do more harm than good. This article will delve into the specifics of St. Augustine grass, explain how weed and feed products work, outline crucial timing considerations, discuss the controversies surrounding certain chemicals like atrazine, and ultimately guide you toward maintaining a healthy, vibrant St. Augustine lawn effectively.

What Is St. Augustine Grass?

St. Augustine grass (Stenotaphrum secundatum) is a popular warm-season turfgrass widely grown in the Southern United States, including Florida, the Gulf Coast, and other humid, subtropical regions. It’s recognizable by its broad, coarse, blue-green blades and its vigorous, stoloniferous (above-ground running) growth habit.

Here are some key characteristics that make St. Augustine grass unique:

  • Warm-Season Grass: It actively grows when temperatures are consistently warm, typically from late spring through early fall. It goes dormant and may turn brown in cooler winter months.
  • Shade Tolerance: St. Augustine grass is considered one of the most shade-tolerant warm-season grasses, making it a good choice for lawns with partial shade (though it still needs at least 4-6 hours of direct sunlight per day).
  • Salt Tolerance: It exhibits good tolerance to salt spray and saline soils, making it suitable for coastal areas.
  • Spreading Habit: It spreads aggressively via thick, above-ground runners (stolons), forming a dense mat that helps suppress weeds. This also means it’s primarily established through sod, plugs, or sprigs, as viable seed production is poor.
  • Thatch Production: Due to its vigorous growth, St. Augustine can be prone to thatch buildup (a layer of organic matter between the soil surface and green blades), which can hinder water and nutrient penetration if not managed.
  • Sensitivity to Herbicides: This is a crucial characteristic when discussing “weed and feed.” St. Augustine grass is sensitive to certain herbicides, particularly those containing 2,4-D, dicamba, and MCPP (mecoprop) at higher concentrations. This means that a generic “weed and feed” product designed for common northern grasses might severely damage or kill St. Augustine.

Understanding these characteristics is fundamental to making informed decisions about its care, especially when it comes to chemical applications.

Weed and Feed Basics

“Weed and feed” products are designed to simplify lawn care by combining a broadleaf herbicide and a fertilizer into a single application. The idea is to simultaneously kill weeds while providing essential nutrients to encourage healthy grass growth.

How it Works:

  1. Herbicide Component: The herbicide typically targets broadleaf weeds (like dandelions, clover, dollarweed, etc.) by being absorbed through their leaves. The fertilizer component often provides a quick boost to weed growth, making them more susceptible to the herbicide. The selective herbicides in these products are formulated to kill broadleaf weeds without harming most common turfgrasses. However, the efficacy and safety depend heavily on the specific active ingredients and the turfgrass type.
  2. Fertilizer Component: The fertilizer supplies essential nutrients, primarily nitrogen (N) for lush green growth, along with phosphorus (P) for root development and potassium (K) for overall plant health and stress tolerance. This aims to strengthen the desirable grass, helping it outcompete any remaining weeds.

Application Method:

Most weed and feed products come in granular form, applied with a broadcast or drop spreader. Some are available as liquid concentrates that attach to a hose. For granular products, the herbicide adheres to the weed leaves, so it’s often recommended to apply when the grass is wet (from dew or light irrigation) to ensure the granules stick. Watering in the product immediately after application is usually required for the fertilizer component to dissolve and reach the soil.

When to Apply to St. Augustine Grass:

Timing is crucial for St. Augustine grass due to its specific growth cycle and sensitivity.

  • Ideal Time: The best time to apply a weed and feed product to St. Augustine grass is during its active growing season, typically in late spring (April-May) or early summer, after the grass has fully emerged from dormancy and has been mowed a couple of times. This ensures the grass is actively growing and can benefit from the fertilizer, and that weeds are also actively growing and susceptible to the herbicide.
  • Temperature Matters: Air temperatures should consistently be between 65°F and 85°F (18°C - 30°C). Applying in extreme heat can stress the grass and increase the risk of herbicide damage.
  • Moisture: Apply when weeds are actively growing, but the grass leaves are slightly damp. This helps granular herbicides stick to weed foliage.
  • Avoid Rain and Wind: Do not apply if heavy rain is expected within 24-48 hours, as it can wash away the product. Avoid windy days to prevent drift onto desirable plants.

When To Avoid Weed and Feed Products

While convenient, there are specific situations and times when applying weed and feed to St. Augustine grass should be avoided:

  1. Newly Established Lawns:

    • New Sod: Do not apply weed and feed to newly laid sod until it has been firmly rooted for at least 4-6 weeks (you can’t easily lift it). The young roots are highly susceptible to chemical burn.
    • New Plugs/Sprigs: Wait even longer, typically until plugs or sprigs have significantly spread and the lawn is well-established (usually 2-3 months or after several mowings). The exposed soil and tender new growth are very vulnerable.
    • Reseeding/Overseeding: St. Augustine grass is not commonly seeded, but if you have overseeded with a cool-season grass for winter color, wait until those grasses are well-established, as the herbicide component will prevent grass seed germination.

  2. Dormant or Stressed Grass:

    • Fall/Winter: Do not apply weed and feed in late fall or winter when St. Augustine grass is dormant or beginning to enter dormancy. The fertilizer will stimulate growth that can be damaged by cold temperatures, and the herbicide will be ineffective as weeds are not actively growing.
    • Drought/Heat Stress: Never apply weed and feed to a lawn that is under stress from drought, extreme heat, or disease. The chemicals can cause severe damage to already weakened grass. Ensure your lawn is well-watered and healthy before application.

  3. Specific Weed Types:

    • Weed and feed products primarily target broadleaf weeds. If your lawn is infested with grassy weeds (like crabgrass, dallisgrass, or torpedo grass), a weed and feed product designed for broadleaf weeds will be ineffective and you’ll need a specialized herbicide.
    • For pre-emergent control (preventing weed seeds from germinating), a separate pre-emergent herbicide application is usually more effective than a weed and feed product alone, and should be timed differently.

  4. Before or After Mowing:

    • Before: It’s generally recommended to mow your lawn a few days before applying granular weed and feed to ensure the weed leaves are present to absorb the herbicide.
    • After: Avoid mowing for at least 2-3 days (and ideally 7 days) after application. This allows the herbicide to be fully absorbed by the weeds. Mowing too soon can remove the herbicide before it has a chance to work.

Problems With Atrazine (and What To Use Instead)

Atrazine is a herbicide that has historically been very common in weed and feed products specifically for St. Augustine and Centipedegrass. It works as both a pre-emergent (preventing some seeds from germinating) and a post-emergent (killing existing weeds) herbicide. While effective on many broadleaf and some grassy weeds, atrazine has faced increasing scrutiny and is often recommended with caution due to several factors:

Problems with Atrazine:

  • Environmental Concerns: Atrazine is known to be persistent in the environment and can leach into groundwater and runoff into surface waters, posing risks to aquatic ecosystems and potentially human health. Many municipalities and states have restrictions or outright bans on its use.
  • Resistance: Overuse of atrazine has led to the development of herbicide-resistant weed populations, making it less effective over time.
  • Narrow Window of Safety: While generally considered safe for St. Augustine at recommended rates, improper application (e.g., too much, or in hot/stressed conditions) can still cause temporary yellowing or thinning of the turf.
  • Limited Spectrum: While it controls many weeds, it’s not a universal solution and can lead to selection pressure for weeds it doesn’t control.

What to Use Instead (Alternatives to Atrazine for St. Augustine Grass):

Given the concerns with atrazine, many homeowners and lawn care professionals are looking for safer and more environmentally friendly alternatives. The key is to find products labeled specifically for St. Augustine grass, as many broadleaf herbicides can damage it.

  1. For Broadleaf Weeds (Post-Emergent):

    • Sulphur-containing Herbicides: Products containing active ingredients like Thiencarbazone-methyl + Iodosulfuron-methyl + Foramsulfuron (e.g., Celsius WG) or Trimec Southern formulations (containing 2,4-D, MCPP, and dicamba, but specifically formulated at lower concentrations safe for St. Augustine) are often recommended. Always ensure the product label explicitly states it is safe for St. Augustine grass.
    • Metsulfuron-methyl: Effective on a range of broadleaf weeds and sedges, and generally safe for St. Augustine.
    • Spectracide Weed Stop for Lawns Plus Crabgrass Killer for Southern Lawns: This product contains 2,4-D, MCPP-p, and Dicamba, and is formulated for Southern grasses like St. Augustine and Centipedegrass. Always follow the label rates carefully.
    • Ortho WeedClear Lawn Weed Killer for Southern Lawns: Similar to Spectracide, this product is designed for St. Augustine and Centipedegrass and contains a blend of broadleaf herbicides.

  2. For Grassy Weeds (Post-Emergent):

    • Controlling grassy weeds in St. Augustine is very challenging, as many effective grassy weed killers will also harm St. Augustine.
    • For specific grassy weeds like torpedo grass, some products containing Sethoxydim might offer suppression, but complete eradication without harming St. Augustine is difficult and often requires spot treatment with a non-selective herbicide (like glyphosate) followed by replanting.
    • For crabgrass, a pre-emergent herbicide is the most effective strategy.

  3. For Pre-Emergent Weed Control (Preventing Weed Seeds):

    • Prodiamine, Dithiopyr, or Pendimethalin: These are common active ingredients in pre-emergent herbicides that create a barrier to prevent weed seeds (both grassy and broadleaf) from germinating. They are generally safe for established St. Augustine grass. Apply in early spring before soil temperatures reach 55°F for summer weeds like crabgrass, and in fall for winter weeds like annual bluegrass. Do not use on newly laid sod or areas you plan to seed (though St. Augustine is rarely seeded).

When choosing any herbicide, whether standalone or in a weed and feed product, the most critical step is to read the product label carefully. Ensure it explicitly states that it is safe for St. Augustine grass and follow all application rates and instructions precisely. Misapplication can lead to severe turf damage.

How To Maintain a Healthy Lawn

A healthy, dense St. Augustine lawn is your best defense against weeds, reducing your reliance on chemical weed and feed products. Proper cultural practices can significantly improve your lawn’s vigor and natural weed suppression.

  1. Proper Mowing:

    • Height: Mow St. Augustine grass relatively high, typically between 2.5 to 4 inches. Taller grass shades the soil, suppressing weed seed germination and promoting deeper root growth, making the lawn more resilient.
    • Frequency: Mow often enough so that you never remove more than one-third of the blade height in a single mowing. For St. Augustine, this might mean weekly or bi-weekly mowing during peak growing season.
    • Sharp Blades: Always use a sharp mower blade for a clean cut, which reduces stress on the grass and prevents tearing, making it less susceptible to disease.
    • Mulch Clippings: Leave grass clippings on the lawn (unless there’s disease present). They return valuable nutrients and organic matter to the soil.

  2. Appropriate Watering:

    • Deep and Infrequent: Water deeply and infrequently to encourage deep root development. Aim for 1 inch of water per week, applied in one or two sessions, rather than frequent, shallow sprinklings.
    • Timing: Water early in the morning to allow the grass blades to dry before nightfall, reducing the risk of fungal diseases.
    • Monitor: Watch for signs of wilting (blue-gray color, folded leaves) to know when your lawn needs water.

  3. Strategic Fertilization:

    • Soil Test: Conduct a soil test every 2-3 years to determine your lawn’s specific nutrient needs and pH levels. This will guide your fertilizer choices and application rates.
    • Nitrogen Focus: St. Augustine grass is a heavy nitrogen feeder for its lush growth. Apply a slow-release nitrogen fertilizer during its active growing season (spring and summer). Avoid excessive nitrogen, as it can lead to thatch buildup and increased pest problems.
    • Iron Supplements: St. Augustine can sometimes suffer from iron chlorosis (yellowing) in alkaline soils. Apply liquid or granular iron supplements if needed.

  4. Thatch Management:

    • Due to its vigorous growth, St. Augustine is prone to thatch buildup. If the thatch layer exceeds 0.5-0.75 inches, it can impede water and nutrient movement and harbor pests.
    • Consider dethatching (verticutting) in late spring or early summer when the grass is actively growing and can recover quickly.

  5. Aeration:

    • Aerating compacted soils improves air, water, and nutrient penetration to the roots. Do this during the active growing season (late spring/early summer).

  6. Integrated Pest Management (IPM):

    • Regularly scout your lawn for pests (like chinch bugs, grubs) and diseases (like brown patch). Address issues promptly, starting with cultural controls and resorting to targeted chemical applications only when necessary.

By prioritizing these cultural practices, you create a robust St. Augustine lawn that naturally outcompetes weeds and is less reliant on chemical interventions.

Final Thoughts on Weed and Feed Products

While the allure of a “weed and feed” product is strong for its convenience, especially for busy homeowners, it’s essential to approach its use on St. Augustine grass with caution and knowledge. St. Augustine’s unique physiology and sensitivity to certain herbicides mean that not all weed and feed products are suitable.

Key takeaways for homeowners with St. Augustine grass:

  • Choose Wisely: Always select a weed and feed product specifically labeled as safe for St. Augustine grass. Generic formulas may contain herbicides that can severely damage your lawn.
  • Timing is Everything: Apply only during the active growing season (late spring to early summer) when temperatures are moderate and the grass is healthy. Avoid application on new sod/plugs, dormant grass, or stressed lawns.
  • Understand Atrazine: Be aware of the environmental concerns and potential for resistance with atrazine. Consider alternatives that are safer for the environment and still effective for St. Augustine.
  • Prioritize Cultural Practices: The most sustainable and effective long-term strategy for a beautiful, weed-free St. Augustine lawn is consistent, proper cultural care: correct mowing height, deep and infrequent watering, and appropriate fertilization based on soil tests. A healthy, dense lawn is naturally the best weed suppressor.
  • Targeted Approach: For severe weed infestations or specific weed types, consider using separate, targeted herbicides and fertilizers. This allows for more precise application and reduces the amount of chemicals applied unnecessarily.

Ultimately, a beautiful St. Augustine lawn is a result of consistent care and informed decisions. While weed and feed products can be a part of your lawn care regimen, they should be used judiciously and in conjunction with best practices to ensure the long-term health and vibrancy of your turf.