Pre emergent herbicides are designed to prevent weed seeds from germinating and establishing in your lawn. But many homeowners wonder if using a pre emergent will also negatively impact grass seed that is spread at the same time. Understanding more about how pre emergents work can help you determine if using a pre emergent herbicide will kill grass seed.
How Pre Emergents Work
Pre emergent herbicides such as prodiamine, dithiopyr, and pendimethalin work by forming a barrier just below the surface of the soil that prevents weed seeds from being able to germinate and establish roots. The chemicals in pre emergents inhibit cell division and root growth in young seedlings by targeting specific enzymes and proteins. Without the ability to form an established root system, weed seeds will not be able to grow into mature plants.
Pre emergent herbicides are selective, meaning they target broadleaf or grassy weeds while having little to no effect on turfgrass. The ideal time to apply pre emergents is early spring before soil temperatures rise and before weed seeds start to germinate. The herbicide barrier typically lasts 60-90 days depending on environmental factors.
Impact on Grass Seed Germination
Because pre emergents form a barrier that inhibits root development, there is a risk that applying a pre emergent herbicide at the same time as grass seed could prevent the seed from being able to take root and grow. The grass seed needs to be able to germinate and establish its root system uninhibited in order to grow into a mature plant.
Most lawn care experts recommend applying pre emergent herbicides 4-6 weeks before overseeding or laying new grass seed. This allows time for the herbicide to break down enough that it will not prevent grass seed germination. If you must seed and apply an herbicide at the same time, it's recommended to use siduron (Tupersan) which interferes less with grass seed germination.
Timing is Key
Proper timing is the key factor in using pre emergents effectively without impacting new grass seed. Here are some tips on timing:
- Apply pre emergent 4-6 weeks before laying grass seed. This allows the product to take effect on weeds while breaking down before seed is applied
- If re-seeding in the fall, apply pre emergent in early spring, seed in late summer, and apply pre emergent again after new grass has established.
- If laying grass seed in the spring, hold off on pre emergent application until late summer to avoid interfering with seeding.
- When planting grass and applying pre emergent together, use siduron which has less impact on germination.
Follow these best practices when using pre emergents around the time of seeding to avoid inhibiting grass seed germination:
- Always read and follow label instructions for the specific pre emergent product. Never exceed recommended application rates.
- Apply pre emergents at the proper time - early spring before soil temperatures reach 55-65°F.
- Allow at least 4-6 weeks between pre emergent application and seeding or overseeding.
- When planting seed, apply grass seed first before spraying pre emergent.
- Use siduron for spring seeding to minimize impact on germination.
- Communicate with your lawn care company to coordinate timing if they apply pre emergents.
- Overseed instead of full re-seeding if possible to avoid disrupting pre emergent applications.
- Adjust your pre emergent schedule the following year if seeding was disrupted.
How long does it take for pre emergents to break down enough to seed?
Most pre emergent herbicides will break down enough within 4-6 weeks to allow grass seed to germinate and establish. Cooler soil temperatures slow breakdown while warm soils speed it up.
Can you put down pre emergent and grass seed together?
It's best to avoid applying pre emergent and grass seed together. The chemicals will likely inhibit germination and development of grass seed. If you must do both, use siduron which has less impact on seeding.
Can I apply pre emergent in the spring and seed in the fall?
Yes, you can apply pre emergent in early spring and still seed grass in the late summer/early fall since the herbicide will have broken down sufficiently by that point. Just be sure to apply pre emergent again after fall seeding.
How soon can I apply pre emergent after laying new grass seed?
It's best to wait at least 4 weeks after laying new grass seed before applying pre emergent. This allows the new grass to fully germinate and establish roots before applying herbicide.
While pre emergent herbicides are very effective at preventing weed growth, they can inhibit germination and establishment of grass seed if not applied properly. Following the recommended guidelines on timing and product selection will allow you to enjoy the benefits of pre emergents while still successfully growing lush, thick grass from seed. With some planning and coordination of applications, you can have both a weed-free yard and a thriving new lawn.