“Weed and feed” products combine a turfgrass fertilizer with an herbicide, offering a dual approach to lawn care. This combination aims to both nourish the existing grass and control unwanted plant growth in a single application. However, the success of this product is entirely dependent upon precise timing, especially in the variable climate of Minnesota. The state’s distinct seasonal shifts and reliance on cool-season grasses mean that the window for effective application is often narrow.
The Science Behind Spring Timing
The initial spring application is the most important step for preventing the toughest annual weeds, particularly crabgrass. Crabgrass, a warm-season annual, begins to germinate only after the soil has reached a specific temperature threshold. The pre-emergent herbicide component of the product must be applied and activated before this germination begins.
The precise timing for this preventative measure is dictated by the soil temperature, not the air temperature or a calendar date. Crabgrass seeds start sprouting when the soil temperature consistently reaches 55°F at a depth of a few inches for several consecutive days. In Minnesota, this typically occurs between late April and mid-May, depending on the year and region.
A commonly used visual cue in the Twin Cities metro area and surrounding regions is the blooming of the lilac bushes. When these plants begin to flower, it generally indicates that the soil temperature is approaching the necessary 50°F to 55°F range for crabgrass. Applying the weed and feed product at this time ensures the pre-emergent barrier is in place before the weed seeds can successfully establish themselves.
Targeting Broadleaf Weeds in Summer and Fall
After the initial spring crabgrass prevention, subsequent applications shift focus to eradicating existing broadleaf weeds and preparing the lawn for dormancy. Applying fertilizer during the hottest part of the Minnesota summer, typically mid-June through late July, should be avoided. Cool-season grasses naturally slow their growth and can enter a semi-dormant state, and fertilizing during this heat stress can force growth, which may lead to turf burn or permanent damage.
If broadleaf weeds like dandelions or clover appear during the summer, it is generally better to use a spot treatment instead of a full lawn application of a weed and feed product. The next full application window opens in late summer or early fall, around mid-August through September. This timing is optimal because the cool-season grasses are actively recovering from summer heat and storing nutrients for winter.
The fall application is particularly effective for post-emergent broadleaf weed control. Weeds are actively moving carbohydrates down to their root systems in preparation for winter, which also pulls the applied herbicide deep into the root system. This final application, often called a “winterizer,” provides the grass with the necessary nutrients to survive the cold and ensures a healthier, greener start the following spring.
Essential Application Protocol
Once the appropriate timing window is identified, the method of application directly influences the product’s effectiveness. For post-emergent broadleaf herbicides to work, the weed killer must physically stick to the leaf surface of the target plant. Applying the granular product when the grass blades are slightly damp, often from morning dew or a light sprinkling, helps the weed control particles adhere.
After the application, the fertilizer and pre-emergent herbicide components must be activated by moisture. The product label will specify the required amount, but a light watering or rainfall soon after applying a pre-emergent is necessary to move the chemical barrier into the top layer of the soil. This creates the protective zone that prevents germinating crabgrass seeds from establishing roots.
It is important to follow the manufacturer’s instructions on spreader settings to ensure even distribution and prevent over-application. Applying too much product in one area can result in chemical burn, while too little will create gaps in the weed barrier. Avoid mowing immediately after application to allow the herbicide granules time to work on the broadleaf weeds.
Why Timing is Critical for Minnesota Success
The short growing season and temperature extremes in Minnesota amplify the consequences of improperly timed treatments. Applying the spring pre-emergent too early, while the soil is still cold, means the chemical barrier will break down before the crabgrass threat arrives. Conversely, applying it too late means the crabgrass seeds have already germinated, rendering the pre-emergent barrier useless.
The efficacy of the treatment relies on the herbicide being present at the exact moment the weed seed attempts to sprout. Missing the 55°F soil temperature window by even a week can lead to a significant crabgrass infestation that requires aggressive post-emergent treatments later in the season. Similarly, applying fertilizer when temperatures are consistently high can severely stress the cool-season grasses that dominate Minnesota lawns. The goal is to feed the grass when it is actively growing and storing energy, which primarily occurs in the spring and fall.