Spring Preventive Weed Control Strategy for Florida Lawns

Weed Control

By Floridist

What Is Pre-Emergent Herbicide?

Applying pre-emergent herbicides is a proactive way to keep weeds out of warm-season lawns like St. Augustine grass, which dominates South Florida landscapes.

Pre-emergent herbicides work by targeting weeds before they sprout through the soil surface. They create a chemical barrier in the topsoil that kills weed seedlings as they germinate. Key points to understand:

  • Preventive Action: Pre-emergents do not kill existing weeds – they only affect germinating seeds. Any weeds already growing in the lawn must be removed or treated with post-emergent herbicides.
  • Weed Types Controlled: They are particularly effective on annual weeds (weeds that sprout from seed each year). In lawns, this often includes summer annual grasses like crabgrass and goosegrass, as well as some broadleaf weeds.
  • Warm-Season Lawns: St. Augustine grass (the most common lawn grass in Florida) is susceptible to invasion by annual weeds if not protected. Pre-emergents are especially valuable because post-emergent options for grassy weeds in St. Augustine are limited.

Recommended Synthetic Pre-Emergent Herbicides (and Annual Rate Limits)

Several synthetic pre-emergent herbicides are effective and labeled safe for use on warm-season grasses like St. Augustine. The most commonly recommended products for spring weed prevention in South Florida include:

  1. Prodiamine (e.g., Barricade):
    • Excellent control of annual grasses (like crabgrass) and some broadleaf weeds.
    • Long residual in the soil; one application can last several months.
    • Typical rates: about 0.5 to 1.0 lb of active ingredient (a.i.) per acre in a single application (depending on desired length of control).
    • Annual Limit: Labels often cap total yearly use around 1.0 to 2.3 lbs of product per acre. Exceeding this can harm turf and is illegal. Always follow the specific product label.
  2. Dithiopyr (e.g., Dimension):
    • Provides pre-emergent control of annual grasses and some broadleaf weeds, plus early post-emergent control of crabgrass seedlings.
    • Typical rates: about 0.25 to 0.5 lb a.i. per acre for a single application (3–4 months of control).
    • Annual Limit: ~1.5 lbs a.i. per acre per year. Exceeding this can injure turf.
  3. Pendimethalin (e.g., Scott’s Halts/WeedEx):
    • Another dinitroaniline that controls annual grasses and some broadleaf weeds.
    • Slightly shorter residual (6–8 weeks) compared to prodiamine.
    • Annual Limit: Often around 4 lbs a.i. per acre per year. Check the label for specific restrictions.
  4. Atrazine:
    • Provides both pre-emergent and early post-emergent control of broadleaf weeds and some annual grasses.
    • Safe on St. Augustinegrass, but do not use on Bermudagrass or Bahiagrass (it can damage them).
    • Annual Limit: Typically two applications per year. Avoid using during very hot (>85°F) weather to minimize turf stress.
  5. Isoxaben (e.g., Gallery):
    • Targets broadleaf weeds only. Often tank-mixed with a grassy pre-emergent for broader coverage. More commonly used in ornamental beds, but can be used on lawns if labeled.
  6. Indaziflam (e.g., Specticle):
    • Newer pre-emergent with long residual control of both grasses and broadleaf weeds.
    • Often restricted to professional use and can be costly.

Important: Always read the label for exact rates and annual limits. Overusing a single product can stunt root growth or harm the environment.

Organic Pre-Emergent Herbicide Options

For those seeking organic or natural weed prevention, corn gluten meal (CGM) is the main option:

  • Mode of Action: CGM can inhibit germination of some weed seeds if conditions are ideal.
  • Effectiveness: Research results vary; it may reduce weed seed germination by ~50–60%, but it does not kill established weeds.
  • High Application Rate: Often ~20–30 lbs per 1,000 sq ft, which also provides a large nitrogen input (10% N by weight).
  • Practical Use: Apply around the same time as synthetic pre-emergents, water lightly, then keep relatively dry for a few days. Efficacy can be inconsistent compared to synthetics.

Besides corn gluten meal, there are few proven organic pre-emergent products for turf. A thick, well-maintained lawn is often the best organic defense against weeds.

Application Techniques: Broadcast vs. Spot Treatment

Broadcast/Blanket Application: This is the standard method for pre-emergent herbicides because you want an unbroken soil barrier everywhere in the lawn.

  • Granular: Spread evenly, then water in to dissolve the granules into the soil.
  • Liquid: Spray uniformly and follow label instructions about post-application irrigation.
  • Advantages: Ensures uniform protection, critical in lawns with widespread weed pressure.

Spot Treatment: Applying pre-emergent only to known “problem areas” is less common, because weed seeds are often scattered widely. If you opt for spot treatments, ensure correct calibration for small areas.

Best Timing for Spring Application in South Florida

Timing is crucial for pre-emergent herbicides to be effective. If you apply too late, weeds may have already germinated; too early, and the barrier might degrade before peak germination. In South Florida:

  • Early Application: UF/IFAS generally recommends applying around late January or early February (around Feb 1) to prevent summer annual weeds like crabgrass and goosegrass.
  • Soil Temperature: Aim to apply before soil temperatures consistently reach ~65–70°F.
  • Split Applications: Some products (prodiamine, dithiopyr) can be split: half-rate in late Jan/early Feb, then a second half-rate 6–8 weeks later for extended control.
  • Don’t Delay: Once weeds are visible, a pre-emergent is no longer effective; you’d need post-emergent treatments.

Proper Application and Best Practices

  • Watering In: Most pre-emergent herbicides require immediate or prompt watering (about 0.5 inches) to move the chemical into the topsoil. Avoid excessive irrigation or runoff.
  • Avoid Soil Disturbance: Aerating or dethatching after application can break the chemical barrier. If you must aerate, do it before applying or be prepared to reapply (respecting label rate limits).
  • New Lawns or Repair Areas: Do not apply full-rate pre-emergent to recently sodded or seeded turf; it can inhibit root establishment. Wait 30–60 days for strong root growth.
  • Weather Conditions: Choose a low-wind day and avoid application before heavy rain. Light irrigation after application is usually recommended.
  • Calibrate Equipment: Whether using spreaders or sprayers, proper calibration ensures you apply the correct amount. Over-application can injure your lawn and the environment.
  • Weed & Feed Products: If using a fertilizer combined with pre-emergent, ensure your lawn actually needs fertilization at that time. Follow label directions meticulously.

Safety Considerations for Lawns, People, and Environment

Turf Safety

Make sure the herbicide is labeled for St. Augustine (or your grass type). Using the wrong herbicide on the wrong turf can cause yellowing, thinning, or bare patches. Never exceed label rate limits.

Human & Pet Safety

  • Wear protective gear (gloves, long sleeves, pants, eye protection).
  • Keep children and pets off treated areas until the product is watered in and dried.
  • Wash hands and clothing after handling herbicides.

Environmental Safety

  • Prevent Runoff: Do not apply on waterlogged soil or before torrential rain. Sweep any granules off driveways or sidewalks into the lawn.
  • Buffer Zones: Keep a safe distance from ponds, canals, or drains as specified by product labels.
  • Storage & Disposal: Follow label directions for storing and disposing of containers. Never pour excess product into drains or gutters.

Legal Note: Always read and follow the herbicide label. Using a product in any manner inconsistent with its label is illegal and potentially harmful.

Suggested Application Schedule for South Florida

Below is a general schedule tailored for South Florida (Zone 10-ish) focusing on warm-season turf (especially St. Augustine). Adjust to your lawn’s specific needs and product labels:


  1. Late January to Early February – Spring Pre-Emergent (Round 1)

    Target Weeds: Summer annuals (crabgrass, goosegrass, spurge, oxalis).

    Herbicide: Apply prodiamine, dithiopyr, or pendimethalin as a blanket treatment. Ensure it’s down by around Feb 1. Water in promptly.

  2. Late March to Early April – Spring Pre-Emergent (Round 2, if needed)

    Target Weeds: Late-germinating weeds, extend control into summer.

    Herbicide: If using split applications, this is the booster round ~6–8 weeks after the first. Or spot-treat any breakthrough weeds with a post-emergent.

  3. Summer (June–August) – Monitor & Spot Treat

    Pre-emergent is rarely applied in mid-summer. Address any break-through weeds individually or with a suitable post-emergent product.

  4. Late September to Early October – Fall Pre-Emergent

    Target Weeds: Winter annuals (annual bluegrass, chickweed, henbit).

    Herbicide: Another round of prodiamine, dithiopyr, etc. Atrazine can also be used on St. Augustine to address winter broadleaf weeds.

Note: Many lawns do well with two pre-emergent treatments per year (spring and fall). Ensure you do not exceed annual label limits for any product. Healthy turf maintenance (proper mowing, irrigation, fertilization) is equally critical to prevent weed invasions.

Final Thoughts

By following this comprehensive approach—choosing the right herbicide at the proper time, adhering to label instructions, and maintaining healthy turf—you’ll set up your St. Augustine or other warm-season lawn for a thriving, weed-free spring. Prevention is far easier (and often cheaper) than dealing with entrenched weed problems later. Apply your pre-emergent early, water it correctly, and enjoy the benefits of a clean, healthy lawn in South Florida’s warm climate!

References & Resources

  • University of Florida IFAS Extension – St. Augustinegrass for Florida Lawns
  • UF/IFAS – Weed Management Guide for Florida Lawns
  • Manufacturer Labels: Prodiamine (Barricade), Dithiopyr (Dimension), Pendimethalin, Atrazine, etc.
  • EPA & UF/IFAS Pesticide Safety Recommendations