Pre-Emergent vs Post-Emergent Herbicides
Two different tools for two different jobs. Understanding when to use each one is the key to effective weed control.
Overview
What This Comparison Covers
Pre-emergent and post-emergent herbicides are not competing options — they are two halves of a complete weed control strategy. Pre-emergents prevent weed seeds from germinating. Post-emergents kill weeds that are already growing. Most failed DIY weed control programs either skip the pre-emergent entirely or apply it at the wrong time. This guide explains how both work, when to use each in Middle Georgia, and why you need both.
Head to head
Side-by-Side Comparison
Pre-Emergent Herbicides
Pros
Prevents weeds before they ever appear — the most effective long-term strategy
Creates a chemical barrier in the top layer of soil
Drastically reduces the number of weeds that need post-emergent treatment
Effective against crabgrass, goosegrass, and annual weeds that spread by seed
Applied just twice per year for season-long coverage
Cons
- Must be applied within a narrow timing window tied to soil temperature
- Will not kill weeds that are already growing
- Needs activation by water (rain or irrigation) within a few days of application
- Can prevent desirable grass seed from establishing if applied before overseeding
Best for:
The foundation of any weed control program. Prevents the majority of annual grassy weeds from ever establishing in your lawn.
Post-Emergent Herbicides
Pros
Kills weeds that are already visible and actively growing
Selective formulas target specific weed types without harming your grass
Essential for perennial weeds that pre-emergent cannot prevent
Can be applied as blanket spray or targeted spot treatment
Works on tough weeds like nutsedge, dallisgrass, and clover
Cons
- Only works on weeds that have already germinated — reactive, not preventive
- Some products can stress or damage sensitive grass types (especially centipede)
- Multiple applications often needed for established perennial weeds
- Temperature restrictions — many products should not be applied above 90 degrees
- Does nothing to prevent new weeds from germinating afterward
Best for:
Treating visible weeds that have broken through the pre-emergent barrier or perennial weeds that cannot be prevented with pre-emergent alone.
Factor by factor
Detailed Comparison
Factor
Pre-Emergent Herbicides
Post-Emergent Herbicides
How It Works
Pre-Emergent Herbicides
Creates a barrier that stops seed germination
Post-Emergent Herbicides
Absorbed by the weed and kills it from within
How It Works
Creates a barrier that stops seed germination
Absorbed by the weed and kills it from within
Timing in Middle Georgia
Pre-Emergent Herbicides
Late Feb-early March (spring), September (fall)
Post-Emergent Herbicides
When weeds are actively growing (spring through fall)
Timing in Middle Georgia
Late Feb-early March (spring), September (fall)
When weeds are actively growing (spring through fall)
Target Weeds
Pre-Emergent Herbicides
Annual grassy weeds (crabgrass, goosegrass, poa)
Post-Emergent Herbicides
Visible broadleaf and grassy weeds, perennials
Target Weeds
Annual grassy weeds (crabgrass, goosegrass, poa)
Visible broadleaf and grassy weeds, perennials
Applications Per Year
Pre-Emergent Herbicides
2 (spring and fall)
Post-Emergent Herbicides
3-6 depending on weed pressure
Applications Per Year
2 (spring and fall)
3-6 depending on weed pressure
Speed of Results
Pre-Emergent Herbicides
Invisible — prevents weeds you never see
Post-Emergent Herbicides
Visible wilting in 7-14 days, die-off in 2-3 weeks
Speed of Results
Invisible — prevents weeds you never see
Visible wilting in 7-14 days, die-off in 2-3 weeks
Risk to Your Lawn
Pre-Emergent Herbicides
Very low when applied at correct rate
Post-Emergent Herbicides
Moderate — wrong product or rate can damage turf
Risk to Your Lawn
Very low when applied at correct rate
Moderate — wrong product or rate can damage turf
Cost per Application
Pre-Emergent Herbicides
Higher (but only 2 per year)
Post-Emergent Herbicides
Lower per treatment (but more treatments needed)
Cost per Application
Higher (but only 2 per year)
Lower per treatment (but more treatments needed)
The verdict
Bottom Line Verdict
You need both. Pre-emergent is the foundation — it prevents 70-80% of annual weeds from ever appearing. Post-emergent is the cleanup crew for the weeds that break through and the perennial weeds that pre-emergent cannot stop. In Middle Georgia, the spring pre-emergent window is late February through early March when soil temperatures hit 55 degrees at 4 inches. Miss that window, and you will be relying entirely on post-emergent all season — a much more expensive and less effective approach.
Why Attaboy
What Attaboy Does Differently
Common questions
Frequently Asked Questions
Can I apply pre-emergent and post-emergent at the same time?
In some cases, yes. Combination products exist, and professional applicators sometimes tank-mix pre-emergent with post-emergent for efficiency. However, the timing priorities are different — pre-emergent is driven by soil temperature, while post-emergent is driven by active weed growth. A professional program coordinates both for optimal results.
What happens if I miss the pre-emergent window?
If crabgrass and other annual weeds have already germinated, pre-emergent will not help for that season. You will need to rely on post-emergent herbicides to control weeds that have already emerged. This is more expensive and less effective than preventing them in the first place, which is why timing the pre-emergent correctly is so important.
Does pre-emergent kill existing weeds?
No. Pre-emergent only prevents seeds from germinating. It has no effect on weeds that are already growing. If you see weeds in your lawn, you need a post-emergent herbicide to kill them. Pre-emergent prevents future weeds from joining them.
How do I know which post-emergent to use?
The correct post-emergent depends on two things: the weed species and your grass type. Using the wrong product on centipede grass, for example, can cause serious damage. Professional applicators identify the weed species on-site and select the appropriate selective herbicide at the correct rate for your turf.
When is the fall pre-emergent window in Middle Georgia?
The fall pre-emergent application targets winter annual weeds like annual bluegrass (poa annua) and henbit. In Middle Georgia, this application should go down in September before soil temperatures drop below 70 degrees. This prevents cool-season weeds from establishing during fall and winter.
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