Can You Put Grass Seed Down After Pre Emergent? - Plant Care Guide

No, you generally cannot put grass seed down after applying pre-emergent herbicide, as this is a common gardening mistake that will prevent your new grass seed from germinating. Pre-emergent herbicides work by forming a chemical barrier in the soil that inhibits the cell division process in germinating seeds, effectively stopping both weed seeds and new grass seeds from sprouting. Therefore, applying them before seeding will lead to very poor or no grass seed growth.

What is a Pre-Emergent Herbicide?

A pre-emergent herbicide is a type of weed control product designed to prevent weeds from growing in your lawn or garden before they emerge from the soil. Unlike post-emergent herbicides, which kill existing weeds, pre-emergents create a protective barrier that stops weed seeds from successfully germinating and establishing themselves.

Here’s a detailed look at what a pre-emergent herbicide is:

• Mechanism of Action:

  • Barrier Formation: When applied, a pre-emergent herbicide forms a thin chemical barrier on the surface of the soil or in the top layer of the soil (typically 1-2 inches deep).
  • Inhibits Cell Division: This chemical barrier works by inhibiting cell division in young plant roots as they attempt to sprout from the seed. When a seed germinates and its root radical emerges and contacts this barrier, the herbicide disrupts its ability to form new cells and grow, causing the seedling to die before it ever breaks the soil surface.
  • No Effect on Established Plants: Crucially, pre-emergents have no effect on established plants with developed root systems, which is why they can be safely used on existing lawns.

• Target Weeds:

  • Grassy Weeds: They are most effective at controlling annual grassy weeds like crabgrass, foxtail, and goosegrass. These weeds are notoriously difficult to control once they are established.
  • Some Broadleaf Weeds: Some formulations can also control certain annual broadleaf weeds like spurge and oxalis, but they are generally less effective on a wide range of broadleaf weeds compared to grassy weeds.
  • Not for Perennial Weeds: Pre-emergents do not control perennial weeds (like dandelions or thistle) that grow from existing root systems or tubers, as these are already established plants.

• Application Timing:

  • Preventative: The timing of application is critical. Pre-emergent herbicides must be applied before the target weed seeds begin to germinate.
  • Spring Application: For most grassy weeds like crabgrass, this means applying in early spring (often when soil temperatures consistently reach 50-55°F or 10-13°C) before these weeds sprout.
  • Fall Application: A second application in late summer or early fall can control cool-season annual weeds.
  • Activation: Most pre-emergents need to be watered in after application to activate the chemical barrier.

• Active Ingredients:

  • Common active ingredients include pendimethalin, prodiamine, dithiopyr, and benefin. Different active ingredients have varying lengths of residual activity (how long they remain effective in the soil). You can find crabgrass preventer products with these ingredients.

• Limitations:

  • No Post-Emergent Control: They do not kill existing weeds. If weeds have already emerged, a pre-emergent will be ineffective against them.
  • Inhibits ALL Seeds: The critical limitation for this discussion is that pre-emergents are non-selective to seeds. They cannot distinguish between weed seeds and desired grass seeds. This is why you cannot overseed or reseed with grass seed immediately after applying a pre-emergent.

Understanding what a pre-emergent herbicide is highlights its effectiveness as a preventative weed control measure, but also underscores the precise timing and planning required for its safe and proper use in conjunction with lawn seeding.

Why Can't You Seed After Pre-Emergent?

You cannot seed after applying pre-emergent herbicide because the very mechanism by which pre-emergents prevent weed seeds from germinating will also prevent your desired grass seeds from sprouting. This is a crucial point that many new gardeners overlook, leading to wasted effort and resources.

Here’s a detailed explanation of why you can't seed after pre-emergent:

• Non-Selective to Seeds:

  • Pre-emergent herbicides do not distinguish between desirable grass seeds and undesirable weed seeds. Their chemical action is designed to stop any seed from successfully germinating by disrupting the cell division necessary for root growth.
  • When your grass seed absorbs water and begins to sprout its first root (the radical), it will encounter the pre-emergent barrier in the soil. The herbicide will then prevent that root from developing further, effectively killing the young sprout before it can emerge from the ground.

• The Protective Barrier:

  • Pre-emergents form a chemical "shield" in the top layer of the soil. This shield remains active for a certain period, typically ranging from 2 to 4 months, depending on the specific product's active ingredient, application rate, and environmental factors like soil type, moisture, and temperature.
  • During this active period, any new seed that attempts to germinate will be affected by the herbicide.

• Wasted Seed and Effort:

  • If you apply grass seed immediately after a pre-emergent, you will likely see very little to no germination. This means you've wasted your grass seed, your time, and the fertilizer you may have applied with it.
  • Your lawn will remain bare in the seeded areas, or new weeds will emerge once the pre-emergent eventually breaks down, leaving you back at square one.

• Timing is Everything:

  • The purpose of a pre-emergent is to target specific weed germination windows (e.g., crabgrass in early spring, cool-season weeds in fall).
  • The purpose of seeding is to establish new grass, also requiring specific germination windows.
  • These two windows almost always conflict because the active period of the pre-emergent will overlap with the germination period of new grass seed.

To successfully establish a new lawn or overseed an existing one, you must ensure that there is no active pre-emergent herbicide in the soil. This requires careful planning and adherence to the product's label instructions regarding the waiting period before seeding. Skipping this waiting period will invariably lead to poor or failed grass seed germination. For successful lawn care, choosing the right type of grass seed for your region is also vital.

How Long Do You Have to Wait to Seed After Pre-Emergent?

The waiting period to seed after applying a pre-emergent herbicide is crucial and depends primarily on the specific active ingredient in the product you used. It's not a one-size-fits-all answer, and checking the product label is paramount. Ignoring this waiting period will lead to failed grass seed germination.

Here's a general guide for how long you typically have to wait:

• Read the Product Label:

  • This is the most important step. Every pre-emergent herbicide label will clearly state the recommended waiting period before you can safely sow new grass seed. Always follow these instructions. The manufacturer knows their product best and has conducted tests to determine its residual activity.

• Common Active Ingredients and Waiting Periods:

  • Prodiamine (e.g., Barricade): This is one of the longest-lasting pre-emergents. The waiting period for seeding can range from 4 to 6 months. Some labels might even state up to 8 months for certain grass types or application rates.
  • Dithiopyr (e.g., Dimension): Generally has a shorter residual activity than prodiamine. The waiting period is often around 8 to 12 weeks (2 to 3 months).
  • Pendimethalin (e.g., Pre-M): Similar to dithiopyr, the waiting period is typically 8 to 12 weeks (2 to 3 months).
  • Benefin: Usually has a waiting period of around 6 to 10 weeks (1.5 to 2.5 months).
  • Corn Gluten Meal: This is an organic pre-emergent. While it prevents germination, its residual effects are generally shorter, and it's considered safe for seeding after 4-6 weeks or less, though effectiveness can vary.

• Factors Affecting Breakdown:

  • Application Rate: Higher application rates generally lead to longer residual activity.
  • Soil Type: Breakdown can be slower in heavier clay soils and faster in lighter, sandy soils.
  • Environmental Conditions: Temperature, moisture, and microbial activity in the soil influence how quickly the herbicide breaks down. Warmer, moister conditions with active microbes generally lead to faster breakdown.

• "Seed-Safe" or "Starter Fertilizer with Weed Preventer" Products:

  • Some products are specifically formulated to prevent weeds while simultaneously allowing new grass seed to grow. These typically use unique active ingredients like tenacity (mesotrione) or are designed for specific timing.
  • Always read the label carefully on these products, as they are exceptions to the general rule. They are usually marketed explicitly as "seed-safe" or "for new lawns."

Consequences of Not Waiting:

• Failed Germination: The grass seed will fail to sprout, leading to bare spots and wasted time/money.
• Weak Seedlings: Even if some seeds manage to germinate, the seedlings may be weak and stressed due to residual herbicide, making them susceptible to disease and death.

Planning your lawn care schedule around these waiting periods is essential. If you want to seed in the fall, you must choose a spring pre-emergent with a shorter residual period or forgo the spring pre-emergent altogether. Similarly, if you want to overseed in the spring, a fall pre-emergent would be fine, provided its residual effect wears off by spring. Having a calendar and a lawn care planner can help you keep track.

What Are My Options if I Applied Pre-Emergent But Need to Seed?

If you've already applied a pre-emergent herbicide but now realize you need to sow grass seed, you're in a bit of a tricky situation. Your options are limited and depend on how long ago the pre-emergent was applied and which product was used. The most important thing is to avoid wasting more time and resources by trying to seed prematurely.

Here are your options if you applied pre-emergent but need to seed:

1. Wait Out the Residual Period (Most Common Option):

   • The Best Approach: The safest and most recommended option is to simply wait until the active ingredient in the pre-emergent has broken down sufficiently to allow new seeds to germinate safely.
   • Check the Label: Refer to the product label of the specific pre-emergent herbicide you used. It will specify the waiting period before seeding for various grass types. This could be anywhere from 2 to 6 months, or even longer for some products like Prodiamine.
   • Plan Ahead: Use this time to prepare your soil, address any other lawn issues, and plan for your next optimal seeding window (usually fall for cool-season grasses, late spring/early summer for warm-season grasses).

2. Use a "Seed-Safe" Weed Preventer (If Not Already Applied):

   • Not an Option If Already Applied: This option is only for situations where you haven't applied a traditional pre-emergent yet but need both weed prevention and seeding.
   • Specific Products: Some products use different active ingredients, such as mesotrione (often found in Tenacity Herbicide), which prevents weed seeds from germinating but allows most new grass seeds (especially cool-season grasses like fescues and perennial ryegrass) to sprout.
   • Caution: Always read the label meticulously, as "seed-safe" claims can have specific restrictions on grass types, application rates, or timing. These are typically more expensive than traditional pre-emergents. You can find Tenacity herbicide for this purpose.

3. Hydro-seeding with Activated Carbon (Experimental and Risky):

   • Carbon to Bind Herbicide: This is a more advanced and experimental technique, primarily used by professionals, where activated carbon (charcoal) is mixed with water and sprayed over the treated area. The activated carbon can bind to and neutralize some residual herbicides in the soil.
   • Followed by Seeding: After the activated carbon application, grass seed can then be applied.
   • High Risk/Cost: This method is costly, requires specialized equipment (hydroseeder), and the effectiveness can vary greatly depending on the specific pre-emergent, soil type, and proper application of the charcoal. It's not generally recommended for homeowners.

4. Accept the Loss and Focus on Established Grass:

   • Sometimes, the most practical solution is to accept that seeding is not an option for the current season.
   • Focus on Existing Lawn: Instead, focus on nurturing your existing lawn. Ensure it's well-fertilized, watered, and mowed to encourage it to fill in naturally. Address existing weeds with post-emergent herbicides or manual removal.
   • Plan for Next Season: Use the time to formulate a solid lawn care plan for the next optimal seeding window, ensuring that pre-emergent and seeding applications do not conflict.

In most homeowner scenarios, the best course of action after accidentally applying pre-emergent before needing to seed is patience. Mark your calendar according to the product label, and use the waiting period to improve your soil and prepare for a successful seeding attempt later.

What is the Best Timing for Applying Pre-Emergent and Grass Seed?

Mastering the best timing for applying pre-emergent and grass seed is a cornerstone of effective lawn care. These two critical tasks have conflicting requirements, so careful planning is essential to achieve a healthy, weed-free lawn without inhibiting new growth. The optimal timing largely depends on your grass type and local climate.

Here's the best timing for applying pre-emergent and grass seed:

For Cool-Season Grasses (Kentucky Bluegrass, Fescues, Perennial Ryegrass):

Cool-season grasses thrive in spring and fall, but fall is generally the best time for seeding.

• Spring Pre-Emergent Application:

  • Timing: Apply in early spring (typically March to early April in many regions), when soil temperatures consistently reach 50-55°F (10-13°C) for several days. This is just before crabgrass and other summer annual weeds begin to germinate.
  • Purpose: To prevent spring-germinating weeds (like crabgrass) from taking over your lawn during the summer.
  • Impact on Seeding: Applying pre-emergent in spring means you cannot sow new grass seed until the product's residual period has expired (often 2-4 months or more, depending on the chemical). This effectively rules out spring seeding.

• Fall Seeding (Best Time):

  • Timing: The optimal time to sow cool-season grass seed is late summer to early fall (typically late August to mid-October, depending on your climate).
  • Why Fall is Best:
    • Warm Soil: Soil is still warm from summer, promoting quick germination.
    • Cooler Air: Cooler air temperatures are ideal for new seedling growth, reducing heat stress.
    • Reduced Weed Competition: Many aggressive summer annual weeds have died back or are no longer germinating.
    • Ample Moisture: Often, there is more reliable rainfall.
    • Long Growth Period: New grass has several months to establish strong roots before the stresses of winter and the heat of the next summer.
  • Pre-Emergent Consideration: If you plan to seed in the fall, you must either skip a spring pre-emergent or use one with a short residual period that will break down by late summer. You can apply a pre-emergent in late fall (after seeding has established) to target cool-season weeds.

For Warm-Season Grasses (Bermudagrass, Zoysiagrass, St. Augustinegrass):

Warm-season grasses thrive in hot weather.

• Late Winter/Early Spring Pre-Emergent Application:

  • Timing: Apply in late winter or early spring (typically February to March in warmer regions), before spring green-up and before warm-season annual weeds (like crabgrass) begin to germinate.
  • Purpose: To prevent weeds during the warm growing season.

• Late Spring/Early Summer Seeding (Best Time):

  • Timing: The optimal time to sow warm-season grass seed is late spring to early summer (typically April to July, depending on your climate).
  • Why This Time is Best:
    • Warm Soil: Soil temperatures are consistently warm, ideal for germination and growth.
    • Active Growth: This aligns with the active growing season for warm-season grasses.
  • Pre-Emergent Consideration: If you apply a pre-emergent in late winter/early spring, you'll need to wait for its residual period to expire before seeding. Therefore, if you plan to seed, you must often skip the first pre-emergent application of the year or use a "seed-safe" product (if available for warm-season grasses).

Key Takeaways for Timing:

• Prioritize: Decide if your priority is weed prevention or new grass growth. You generally cannot do both effectively in the same season with traditional pre-emergents.
• Read Labels: Always, always, always read the product label for specific waiting periods and grass type compatibilities.
• Plan Ahead: Mark your calendar and schedule your applications appropriately based on your region's climate and your lawn goals.

By carefully timing your pre-emergent and grass seed applications, you can successfully manage weeds and cultivate a dense, healthy lawn. A lawn calendar can be a valuable tool.

What Are Alternatives to Using Pre-Emergent Before Seeding?

If you need to sow grass seed but also want to manage weeds, and using a traditional pre-emergent herbicide isn't an option due to its inhibitory effect on new seed germination, you still have several alternatives. These methods focus on preparing the soil and managing weeds through different strategies without harming your new grass.

Here are alternatives to using pre-emergent before seeding:

1. "Seed-Safe" Pre-Emergent Products (Mesotrione-based):

   • Mechanism: These are specific formulations that contain active ingredients like mesotrione (often branded as Tenacity Herbicide). Mesotrione works by inhibiting photosynthesis in many common weeds (both germinating seeds and some young existing weeds) but does not typically harm most newly sown cool-season grass seeds. Some can also be used for warm-season grasses.
   • Application: These products can be applied at the same time or shortly after seeding, making them ideal for overseeding or establishing a new lawn where weed control is also needed.
   • Caution: Always read the product label carefully for specific instructions on grass types it's safe for, application rates, and target weeds. They are often more expensive than traditional pre-emergents. You can find mesotrione herbicide for purchase.

2. Corn Gluten Meal (Organic Pre-Emergent):

   • Mechanism: This is a natural, organic product that also acts as a pre-emergent. It releases peptides that inhibit root growth in germinating seeds. It also contains nitrogen, acting as a natural fertilizer.
   • Timing: While it acts as a pre-emergent, its effect on new grass seed is generally less severe than synthetic chemicals, and new grass can often be seeded 4-6 weeks after application, or even sooner for some grasses.
   • Effectiveness: It's generally less potent and effective than synthetic pre-emergents and requires consistent application over several seasons to build up its weed-fighting benefits.
   • Pros: Safe for pets and children immediately after application. Also acts as a fertilizer.
   • Cons: Higher cost per square foot than synthetic options, less aggressive weed control, and needs careful timing.

3. Cultural Practices for Weed Suppression:

   • Soil Preparation: Thoroughly prepare your soil before seeding. Remove existing weeds manually or with a non-selective herbicide (like glyphosate) several weeks before seeding. Ensure proper grading and remove debris.
   • Thick Seeding: Sow grass seed at the recommended rate or even slightly higher (for overseeding). A dense stand of new grass will naturally outcompete many weeds by blocking sunlight and occupying space.
   • Topdressing with Compost: Applying a thin layer of compost (1/4 to 1/2 inch) after seeding can help suppress some weed seeds, improve soil health, and aid grass seed germination.
   • Proper Watering: Consistent, shallow watering for new grass seed discourages deep-rooted perennial weeds.
   • Appropriate Mowing: Once new grass is established enough to mow, maintain a slightly higher mowing height. Taller grass shades the soil, making it less hospitable for weed seeds to germinate.

4. Post-Emergent Weed Control (After Grass is Established):

   • Patience is Key: Allow your new grass to germinate and become well-established before applying any post-emergent weed killers. This usually means waiting until the new grass has been mowed at least 2-3 times.
   • Targeted Treatment: Once the grass is established, you can use selective post-emergent herbicides to control any weeds that manage to sprout. Spot treatment can minimize chemical use.
   • Pros: Allows you to get grass established without chemical interference.
   • Cons: Requires manual weed removal in the interim and patience before applying chemicals. A selective broadleaf weed killer can be used once grass is mature.

By choosing one of these alternatives, you can address weed concerns while still successfully establishing new grass, avoiding the common pitfalls of traditional pre-emergent use before seeding.