April 15, 2026 ยท Fertilization & Weed Control · By Mike Harmon, Owner & Licensed Applicator
Most lawn care companies in Indiana apply pre-emergent one time each spring and call the crabgrass problem solved. We don't. On every yard we treat in Noblesville, Carmel, Fishers, Westfield, and surrounding Hamilton County, we apply pre-emergent twice: once in early spring and again in late spring.
It's not a sales tactic. It's what the research says produces a cleaner lawn. And the research on this comes from our own state university, Purdue.
The Purdue Study in Plain English
Researchers at Purdue's W.H. Daniel Turfgrass Research Center in West Lafayette ran a three-year controlled study comparing two pre-emergent strategies on Kentucky bluegrass plots with known crabgrass pressure:
- Strategy A: One full-rate pre-emergent application in early April
- Strategy B: Two half-rate applications, one in early April and one in early June
Same total amount of active ingredient. Same cost in product. The only difference was whether it all went down at once or in two passes.
Here are the percent crabgrass control numbers the study reported, averaged across three years:
Single Application (April only)
- August: 82% control
- September: 73% control
Split Application (April + June)
- August: 93% control
- September: 87% control
By August, the split strategy is 11 percentage points better. By September, when crabgrass goes to seed and prepares to re-infest the lawn next year, the split is 14 percentage points better. The study concluded that sequential applications "will more effectively and consistently control crabgrass than a single application."
Why One Application Fails by August
Pre-emergent herbicides like prodiamine, pendimethalin, and dithiopyr work by forming a chemical barrier in the top layer of soil that kills weed seedlings as they germinate. The problem is that barrier doesn't last forever. It breaks down over time from sunlight, watering, microbial activity, and rainfall.
A single April application typically provides reliable control for about 90 to 120 days. That sounds like enough, but crabgrass germination in Indiana doesn't stop in June. Warm-season weeds keep pushing up through July and August whenever soil conditions allow. By mid-summer, a single-application barrier has thinned out, and the late-germinating crabgrass walks right through it.
A second application in late May or early June essentially recharges the barrier right before that late-summer germination wave. That's why the August and September numbers diverge so dramatically in the Purdue data.
Why More Companies Don't Do This
Three reasons:
- It's more work. Two rounds means two separate visits, two separate applications, double the scheduling.
- It's harder to price. Most companies sell a simple 6-step program with one pre-emergent round so the invoice looks tidy.
- Most companies don't read the research. The industry default has been one application per spring for decades. Changing takes effort and most operators don't bother.
The Purdue researchers specifically called this out: "LCOs [lawn care operators] using multiple rounds can split their preemergence application from one into two and gain increased crabgrass control without additional costs." The product is already budgeted. It's just a matter of splitting it across two visits instead of dumping it all in one.
Does the Specific Product Matter?
The Purdue study also tested whether you had to use the same active ingredient in both applications, or whether you could switch. Older research from the 1990s said stick with the same product. The updated 2009 through 2011 data showed that prodiamine, pendimethalin, and dithiopyr can be mixed and matched across the two applications with no loss of control. All combinations in the study delivered 91% or better control in August.
This flexibility matters because weather and job scheduling sometimes dictate which product goes down when. Knowing the research supports switching means we can choose the best product for each application window rather than being locked into one.
What This Looks Like on a Sprout Lawn & Landscape Program
Our fertilization and weed control program is structured around the Purdue-backed split approach:
- Round 1 (early April): Pre-emergent plus early-season fertilizer, applied as soon as soil temperatures approach the 55-degree germination threshold. Timing is critical here. We track soil temps, not calendar dates.
- Round 2 (late May to early June): Second pre-emergent application plus post-emergent broadleaf herbicide for any perennials that popped up (dandelions, clover, violet). This is the recharge application the Purdue study identified.
- Rounds 3 through 6: Rotating fertilizer and targeted spot treatments through summer and fall, including a late-season winterizer to prepare roots for winter.
Does Split Pre-Emergent Still Matter If I Overseed?
Yes, but timing gets tricky. Pre-emergent doesn't distinguish between crabgrass seeds and grass seeds. It prevents both from germinating. If you're planning a fall aeration and overseeding, the second spring pre-emergent application needs to be scheduled carefully so it has time to break down before seed goes down. This is another reason professional program timing matters. We coordinate pre-emergent with overseeding schedules so homeowners don't accidentally sterilize their own lawn against new grass seed.
What About Perennial Weeds?
Pre-emergent only prevents annual weed seeds from germinating. It does nothing against perennials like dandelions, clover, ground ivy, and thistle, which grow back every year from established root systems. A complete weed control strategy for Indiana needs both pre-emergent (for annuals like crabgrass and foxtail) and targeted post-emergent treatments through the growing season (for perennials). Our program includes both.
The Bottom Line for Hamilton County Lawns
One pre-emergent application in April will give you decent crabgrass control through June. By August, you'll have crabgrass patches. By September, you'll have crabgrass going to seed and setting up an even bigger problem next year. Two pre-emergent applications, split the way Purdue's three-year study tested, closes that late-summer gap and keeps crabgrass pressure at 87 to 93% controlled through the entire season.
Same product. Same cost to you. Better results.
We run this program on lawns throughout Noblesville, Carmel, Fishers, Westfield, Fortville, McCordsville, Cicero, and the rest of Hamilton County. Get your fertilization and weed control price in under 60 seconds using our satellite measurement tool, or call (317) 900-7151.
Source: Patton, A., D. Weisenberger, and Z. Reicher. 2012. Sequential Applications of Preemergence Crabgrass Herbicides for Enhanced Control - Three Year Summary. Purdue University Turfgrass Science Program Annual Report, p. 63-65.