Pre-Emergent vs Post-Emergent Weed and Feed
Pre-emergent weed and feed contains a herbicide that prevents weed seed germination by disrupting cell division in the seed as it tries to sprout. The product creates a chemical barrier in the upper layer of the soil. When a weed seed activates and begins germinating, the growing shoot encounters this barrier and cannot develop further.
Pre-emergent herbicide does nothing to weeds that have already germinated and are growing above the soil surface. Once a weed is visible, a pre-emergent product is ineffective against it.
Common pre-emergent active ingredients found in combination weed and feed products include:
- Pendimethalin: Widely used for crabgrass and annual grass weed prevention
- Prodiamine: A longer-lasting pre-emergent used in professional and consumer products
- Dithiopyr: Provides some early post-emergent activity on very young crabgrass seedlings in addition to pre-emergent action
The fertilizer component of pre-emergent weed and feed delivers the same NPK nutrition as any other weed and feed product. The timing of the application, before soil temperatures trigger weed seed germination, is what makes it pre-emergent rather than the fertilizer formulation itself.
How Post-Emergent Weed and Feed Works
Post-emergent weed and feed contains a selective broadleaf herbicide that kills weeds already growing in the lawn. The herbicide is absorbed through the weed’s foliage and travels systemically to the root system, killing the plant from the inside out. Because the herbicide needs living, actively growing foliage to absorb into, the weed must be present and growing at the time of application.
Post-emergent formulas are effective against broadleaf weeds such as dandelions, clover, plantain, chickweed, and henbit. Most do not control grassy weeds such as crabgrass or goosegrass unless the product label specifically lists those species.
The active ingredients most common in post-emergent weed and feed products are 2,4-D, dicamba, and MCPP, which are described in detail in the guide to what weed and feed is and how it works.
The Critical Timing Difference
The key distinction between the two types is timing relative to weed emergence, not timing on the calendar. However, weed emergence follows predictable seasonal patterns that make the calendar a useful guide.
| Pre-Emergent | Post-Emergent | |
|---|---|---|
| Target | Weed seeds before germination | Actively growing weeds |
| Mechanism | Soil barrier preventing germination | Foliar absorption, systemic kill |
| Application window | Before soil temp triggers germination | While weeds are actively growing |
| Common target weeds | Crabgrass, annual bluegrass, goosegrass | Dandelions, clover, plantain, chickweed |
| Works on visible weeds? | No | Yes |
| Works on seeds? | Yes | No |
When to Apply Pre-Emergent Weed and Feed
Pre-emergent weed and feed for crabgrass control must be applied before soil temperatures at the 2-inch depth reach 55 degrees Fahrenheit. Crabgrass seeds begin germinating at this temperature threshold. Once temperatures exceed this level and germination has started, the pre-emergent window has closed.
The traditional timing guidance is to apply when forsythia is in full bloom or when soil temperatures consistently read between 50 and 55 degrees Fahrenheit. In most of the United States, this means:
- Southern regions (USDA Zones 7-9): Late February to early March
- Transition zone (Zones 6-7): Mid-March to early April
- Northern regions (Zones 4-6): Early to mid-April
The fertilizer component of a spring pre-emergent weed and feed supports cool-season grasses entering their spring growth flush. For warm-season grasses still approaching dormancy break, delay application until the turf is actively growing.
Pre-Emergent in Fall
A fall pre-emergent application controls winter annual weeds such as annual bluegrass (Poa annua) and hairy bittercress, which germinate in late summer and fall when soil temperatures drop back below 70 degrees Fahrenheit. If these weeds are a recurring problem on your lawn, a fall pre-emergent application in late August to September addresses them at the source.
When to Apply Post-Emergent Weed and Feed
Post-emergent weed and feed should be applied when target weeds are actively growing and have enough leaf area to absorb the herbicide. In practice, this means applying in mid-spring once broadleaf weeds have established several leaves, or in early fall when perennial broadleaf weeds are in active growth and moving nutrients toward their roots.
Spring post-emergent applications are most effective when weeds are young and still in the seedling-to-juvenile growth stage. Mature weeds with deep root systems are harder to fully kill with a single broadcast application and may require a targeted spot-spray of a concentrated herbicide for complete control.
Fall is often the better post-emergent window for perennial broadleaf weeds. In fall, plants translocate energy and nutrients downward toward their roots in preparation for winter. Systemic herbicides applied at this stage travel more efficiently to the root system, killing more of the plant than a spring application targeting young foliage alone.
Do Any Products Combine Both?
Some weed and feed products contain both a pre-emergent and a post-emergent herbicide in a single formula. These combination products address germinating weed seeds and existing weeds at the same time. Scotts Turf Builder with Halts Crabgrass Preventer is one example, where Halts (pendimethalin) provides pre-emergent activity and a broadleaf herbicide provides post-emergent control.
These products must still be applied before crabgrass germination if pre-emergent action is the goal. The timing rule for the pre-emergent component is the binding constraint.
Which Type Do You Actually Need?
Work through this simple decision:
- Do you see weeds growing right now? If yes, post-emergent is what you need. A pre-emergent applied to visible weeds will not kill them.
- Are crabgrass or annual grassy weeds a recurring problem? If yes, and it is still early spring before germination temperatures are reached, a pre-emergent is the right choice.
- Do you have both problems at the same time? A combination pre-emergent and post-emergent product addresses both, or apply a post-emergent first and follow up with a pre-emergent the following spring.
For readers focused specifically on standalone herbicide strategies, the pre-emergent herbicide guide in the weed control hub covers application rates, product selection, and timing in greater depth without the fertilizer component.
Back to the Hub
For the complete overview of weed and feed options by grass type, see best weed and feed products for home lawns. For seasonal timing guidance, see when to apply weed and feed to your lawn.