A crabgrass preventer, technically known as a pre-emergent herbicide, is a chemical product designed to stop crabgrass seeds from sprouting in your lawn. It works by forming a thin, protective barrier on the soil surface that kills the weed seedling as it begins to germinate. Applying this product is a proactive measure meant to halt the weed problem before it ever becomes visible. For homeowners in New Jersey, successful control depends entirely on precise timing, relying on environmental cues rather than a fixed date.
The Critical Role of Soil Temperature
The trigger for crabgrass germination is soil temperature, specifically at a depth of one to two inches. Crabgrass seeds begin to sprout when the soil consistently reaches 50°F to 55°F for several consecutive days. This temperature threshold is the most important factor because it marks the beginning of the germination cycle. Applying the preventer too early allows the chemical barrier to break down before the seeds sprout, reducing effectiveness.
Applying the product too late means the seeds have already begun to germinate, rendering the pre-emergent useless against established seedlings. The goal is to have the herbicide barrier fully activated just before the 50°F to 55°F threshold is reached. A helpful visual indicator is the blooming of the forsythia shrub, as its bright yellow flowers often coincide with warming soil temperatures. Soil temperature changes more slowly than air temperature, providing a reliable measure for timing the application.
Practical Application Windows Across New Jersey Regions
New Jersey’s north-to-south latitude means the ideal application window varies considerably across the state. The spring warming trend begins earliest in the southern counties, moving progressively northward. Calendar dates serve only as estimates; local soil temperature monitoring remains the most accurate method.
Southern New Jersey
Southern New Jersey, including counties like Cape May, Atlantic, and Cumberland, experiences soil warming sooner than the rest of the state. Residents should generally target an application window starting in early to mid-April. This earlier timing is necessary because sun exposure and milder coastal air accelerate the soil warming process. Applying the herbicide during this period ensures the barrier is active before the earliest wave of germination begins.
Central New Jersey
Moving into Central New Jersey, encompassing areas such as Mercer, Middlesex, and Monmouth counties, the application window shifts slightly later. The best time for application in this region is typically from mid-April to late April. This window accounts for the slightly delayed spring warm-up compared to the southern coast. Residents can monitor local soil temperature resources or look for the full bloom of forsythia as a reliable sign.
Northern New Jersey
For Northern New Jersey, including counties like Sussex, Warren, and Bergen, colder winters and later spring mean the soil takes the longest to reach the critical temperature threshold. The most appropriate time for a pre-emergent application in this cooler zone is generally from late April into early May. Waiting for this later window prevents the product from degrading too soon, ensuring the chemical remains potent when the crabgrass seeds finally begin to germinate.
Preparation and Best Practices for Application
Effective control starts with proper preparation of the lawn before application. Clear the turf of any heavy leaf litter or debris that could block the granules or liquid from reaching the soil surface. Aeration and dethatching must be completed before application, as disturbing the soil afterward breaks the protective barrier and allows weeds to sprout.
When using a granular product, accurately calibrate the spreader according to the manufacturer’s instructions to ensure even distribution. To prevent streaks and missed areas, apply half of the required material in one direction (e.g., north to south) and the remaining half in a perpendicular direction (e.g., east to west). This cross-hatch pattern helps create a uniform chemical seal on the soil.
The pre-emergent herbicide must be watered into the soil to activate the chemical barrier. This requires approximately one-half inch of water from rainfall or irrigation within one to ten days of application. If the product is not activated, it remains on top of the grass blades where sunlight can break it down prematurely. After activation, delay mowing for a few days to allow the product to settle fully.
Addressing Late Application: Post-Emergent Strategies
If the optimal pre-emergent window is missed and crabgrass seedlings have already emerged, the strategy must shift from prevention to elimination. Pre-emergent products are no longer effective once the crabgrass plant has visibly sprouted above the soil line. At this stage, a post-emergent herbicide is required to kill the actively growing weeds.
Post-emergent herbicides contain active ingredients like quinclorac or mesotrione, designed to target established plant tissue. These products are most effective when the crabgrass plants are still small, ideally before they develop more than four tillers (side shoots). Treating mature, large crabgrass plants later in the season is much more difficult and often requires multiple applications.
When using a post-emergent product, the crabgrass must be actively growing. Application should be made when temperatures are below 85°F to minimize the risk of damaging desirable turfgrass. Post-emergent applications only kill existing weeds and do not prevent future seeds from germinating. Therefore, a missed pre-emergent application may necessitate a series of post-emergent treatments throughout the summer to manage the infestation.