Homeowners often face the dilemma of whether to apply fertilizer or weed control first to achieve a lush, weed-free lawn. Sequencing the products correctly maximizes the effectiveness of both the fertilizer, which feeds the turf, and the weed killer, which targets invaders. Getting the order right prevents one treatment from limiting the benefits of the other. The correct approach depends entirely on the specific type of weed control product used.
How Product Type Dictates Application Order
The decision to fertilize or treat weeds first is determined by whether the weed killer is a pre-emergent or a post-emergent application. Pre-emergent herbicides stop weed seeds from germinating by forming a chemical barrier in the top layer of the soil. Since fertilizer feeds established grass roots below the surface and does not disrupt this barrier, these two products can often be applied at the same time. Many manufacturers offer combination products that include both pre-emergent herbicide and granular fertilizer in a single application.
The sequencing changes significantly when using a post-emergent herbicide, which is designed to kill visible weeds like dandelions and clover. These products are typically liquid sprays that require the weed to be actively growing to efficiently absorb the chemical through its leaf tissue. The recommended strategy is to fertilize the lawn first to encourage growth in both the grass and the weeds. Boosting the weeds’ metabolism ensures they are actively “drinking” and will take up the herbicide more quickly and completely when applied later.
Essential Wait Times Between Treatments
Once the application order is determined, the time interval between treatments becomes the next important factor. If applying fertilizer before a liquid post-emergent spray, waiting 7 to 14 days is advised. This waiting period allows the nitrogen to stimulate the weeds into vigorous growth, making them more susceptible to the herbicide. The increased growth rate improves the translocation of the herbicide throughout the entire weed plant, including the root system.
If the order is reversed and you apply a post-emergent weed killer first, wait approximately one week before following up with fertilizer. This delay gives the weed control chemical sufficient time to work on the target plants without the stress of a sudden nitrogen boost. Applying fertilizer too soon after a herbicide treatment can place undue stress on the entire lawn, including the desirable turfgrass.
Preparing the Lawn Before Application
Proper preparation of the lawn directly affects how well the products work. Before applying a granular product that contains a post-emergent weed killer, mow the lawn one or two days prior to application. This timing ensures the weeds have enough leaf surface area to catch the granules, but the grass is not so long that it shields the weeds. The leaf surface must also be slightly damp, usually from morning dew or a light misting, so the granular particles stick to the foliage.
For liquid post-emergent herbicides, do not mow for several days before spraying to maximize the leaf surface area available for chemical absorption. Application should happen when temperatures are within the range specified on the product label, typically avoiding times when the temperature is above 85 degrees Fahrenheit. Extreme heat can cause the liquid to evaporate too quickly or stress the turfgrass, leading to potential damage.
Post-Treatment Watering and Mowing Rules
If you have applied a granular fertilizer, it must be watered into the soil to begin breaking down and feeding the roots. This process also prevents the nitrogen salts from burning the grass blades. Watering should occur relatively quickly after application, often within 24 hours, unless the product also contains a post-emergent herbicide.
When liquid or granular post-emergent weed killer is used, the most important rule is to avoid watering or rainfall for a specified period, usually 24 to 48 hours. This allows the herbicide to fully absorb into the weed’s vascular system before being washed off the leaves. Wait two to four days before mowing the lawn after a weed killer application. Mowing too soon removes the treated leaf tissue before the chemical has fully moved down into the weed’s roots, which greatly reduces the overall effectiveness.