Crabgrass is an aggressive, warm-season annual weed that competes directly with the cool-season grasses common in Minnesota lawns. It germinates from seed each spring and grows vigorously during the summer, spreading seeds before dying with the first frost. The most effective defense is a pre-emergent herbicide, which creates a chemical barrier in the soil to prevent seeds from sprouting. Success relies on applying the product at a precise time, just before germination begins in the spring.
Understanding the Critical Timing Window
The timing for applying crabgrass preventer in Minnesota is governed by soil temperature, not a specific calendar date. Crabgrass seeds germinate only after the soil temperature at a depth of one to two inches consistently reaches 55 degrees Fahrenheit for several consecutive days. Applying the pre-emergent just before this threshold is reached is necessary for a successful application.
Applying the product too early, when soil temperatures are still in the 40s, allows the herbicide to degrade before the seeds sprout. This breakdown leaves the lawn unprotected later when the main flush of germination occurs. Conversely, waiting too long means the seeds have already sprouted, making the pre-emergent ineffective.
A reliable visual indicator for homeowners is the blooming of certain local plants, a concept known as phenology. When forsythia shrubs display yellow flowers or lilacs start to bloom, the soil temperature is typically approaching 55 degrees. This natural sign signals the narrow window for effective application. You can also track local soil temperature readings online through university extension sites to pinpoint the exact moment.
Preparing the Lawn and Selecting the Preventer
Before spreading any product, the lawn must be prepared to ensure the herbicide makes direct contact with the soil surface. Clear away debris, such as leaves or dead grass clumps, that could interfere with barrier formation. Mow the lawn at its normal spring height, but avoid “scalping” the grass, as this can disturb the soil surface and break the chemical barrier.
Homeowners typically choose between granular or liquid pre-emergent herbicides. Granular products are spread with a broadcast or drop spreader and are generally easier to apply consistently. Liquid herbicides, applied with a sprayer, offer more uniform coverage and faster activation into the soil.
Many products combine crabgrass preventer with fertilizer, known as a “weed-and-feed.” While convenient, this combination can be problematic in the Minnesota spring. The ideal window for pre-emergent application is often too early for the first fertilizer application on cool-season grasses. Fertilizing too soon promotes excessive shoot growth at the expense of root development. If using a combination product, choose one with a slow-release nitrogen source to mitigate the risk of over-fertilization.
Step-by-Step Application Guide
Accurate application requires proper equipment use, starting with calibrating your spreader according to the manufacturer’s instructions. An uncalibrated spreader results in uneven coverage, leading to visible streaks of crabgrass later in the summer. Use a consistent walking speed, as variations in speed drastically change the application rate.
For uniform coverage, use a technique called “halving the rate.” Set the spreader to half the recommended rate and cover the entire lawn. Apply the second half of the product by walking a perpendicular pattern to the first pass. This cross-hatching method ensures full coverage and prevents the streaking often seen with a single pass.
Following the application of a granular pre-emergent, the product must be activated by watering it into the soil. This step dissolves the herbicide and moves the active ingredient into the top half-inch of soil, forming the preventative barrier. Aim to apply about one-quarter to one-half inch of water via irrigation or rainfall within 24 hours. Applying the product right before a light rain is ideal, but avoid application if heavy rainfall or high winds are forecasted, which could wash away or unevenly distribute the chemical layer.
Handling Late Applications and Post-Emergent Control
If the ideal pre-emergent window is missed, a corrective approach using a post-emergent herbicide becomes necessary once the crabgrass has sprouted. Post-emergent products, such as those containing Quinclorac or Mesotrione, are designed to kill the actively growing plant. These treatments are most effective when the crabgrass is still small, ideally before it develops more than four tillers, which are secondary shoots from the main plant.
Larger crabgrass plants require more aggressive treatment, often needing multiple applications spaced two to three weeks apart for complete control. Quinclorac is particularly effective on established plants, while Mesotrione works well on young crabgrass and can be used when seeding new turf.
For lawns with a history of severe infestation, a split pre-emergent application can provide extended protection. This involves applying half the maximum seasonal rate during the initial 55-degree soil temperature window and following up with the second half six to nine weeks later. The secondary application reinforces the chemical barrier, ensuring season-long control through the peak summer heat when germination can continue.