Pre-emergent and post-emergent weed killers should not be applied at the same time. Plan to apply pre-emergent herbicide early in spring before weeds have begun to sprout. Then, apply post-emergent weed killer in late spring to attack weeds that have appeared or greened up. The reasons you should not apply pre-emergent and post-emergent at the same time are:
- Pre-emergent needs to be watered into the soil directly after application to be effective.
- Watering post-emergent after application can wash it away, reducing effectiveness.
- Pre-emergent only kills germinating weed seeds belowground. It should be applied in early spring before weeds are visible.
- Post-emergent only kills weeds that have sprouted above the surface. It should be applied in late spring when weeds have appeared above ground.
These two types of herbicides work best when used to combat different problems. Use post-emergents to reclaim weedy yards. Use pre-emergents to maintain a weed-free look once you’ve killed the weeds growing in your yard.
Why Pre-Emergent and Post-Emergent Don’t Mix
Pre-emergent herbicide only attacks germinating weed seeds underground, while post-emergent only attacks weeds that have sprouted. Because they work in different ways, there are rarely times when both are effective. Basically, if weeds are already present in your yard, you’ve missed your pre-emergent window and it’s time to use post-emergent.
- If you are dealing with a yard overrun by weeds, use post-emergent only.
- Pre-emergent is best used as a preventative measure after you’ve killed existing weeds with post-emergent.
- To be effective, pre-emergent must be applied at the right time. Applying outside this window will not deliver results.
Applying pre-emergent and post-emergent at the same time means additional labor and cost for little to no benefit. Once you’ve wiped out those pest grasses and broadleaf weeds with post-emergent, it’s time to transition to using herbicides on a more structured schedule.
Pre-Emergent and Post-Emergent Weed Killer Schedule
The best way to combat weeds and save money is to follow a schedule that prioritizes pre-emergent weed control to stop all types of weeds. Then, small amounts of post-emergent can be applied to your lawn as necessary to kill any stubborn weeds. The following schedule will yield the best results:
- Apply pre-emergent in spring, when soil temperatures rise to 55℉ (13℃) for 2–3 consecutive days.
- Pre-emergent timing can be between mid-February and mid-April, depending on the region.
- Wait 4 weeks after pre-emergent application.
- If there is an abundance of weeds in your lawn, use this post-emergent to kill both broadleaf and grassy weeds at the same time.
- If your pre-emergent effectively stops weeds from sprouting, there’s no need for post-emergent.
By applying pre-emergent and waiting for results, you can evaluate if you need to use any post-emergent at all. If your spring application is effective, you’ll have a weed-free yard without having to spend any money on post-emergent herbicide.
Can You Apply Pre-Emergent at the Same Time as Post-Emergent?
Never apply pre-emergent and post-emergent herbicide at the same time. Pre-emergents work by being watered into the soil to kill weeds before they sprout. Post-emergents work by being left to dry on weeds that have already sprouted. If it’s the right time to use one type of herbicide, it’s not the optimal time to use the other.
Clean up weed-choked yards with a strong post-emergent first. Once you’ve won the battle against the weeds, use a pre-emergent to keep them from returning. This will save you time and money. Once you reclaim your yard from weeds, you can keep it weed-free with pre-emergent applications alone.