The convenience of a “Weed and Feed” product, which combines fertilizer and herbicide into a single application, makes it highly appealing for lawn care. However, applying this combination too early to a newly established lawn can be detrimental, potentially destroying the young grass. New turf, whether from seed or sod, is extremely vulnerable because its root system is not yet robust enough to withstand the chemicals intended to kill weeds.
The Dual Nature of “Weed and Feed” Products
Weed and Feed products achieve their dual purpose by containing both a fertilizer, which is the “feed,” and an herbicide, which is the “weed” killer. The herbicide component is the primary source of danger for new grass seedlings. Most of these weed killers are selective, meaning they target broadleaf weeds like dandelions and clover without harming mature turfgrasses.
The issue is that young grass plants can be severely damaged by selective herbicides because they lack a strong root system and thick cell walls. Many post-emergent herbicides work by disrupting the weed’s cellular structure or mimicking plant hormones, causing the plant to grow abnormally until it dies. These powerful mechanisms can easily overwhelm and kill a tender grass seedling that lacks the maturity to tolerate the chemical stress.
The fertilizer component also presents a risk, though it is intended to help the grass grow. Fertilizers contain salts, and when applied in excess, these salts can draw water away from the roots and dehydrate the plant, a condition known as “fertilizer burn”. Since new grass roots are shallow, they are highly susceptible to this scorching effect. Choosing a slow-release or low-nitrogen formula is always a safer option for young turf to prevent overwhelming the delicate root structure.
Timing Guidelines for Newly Seeded Lawns
A lawn started from seed is the most vulnerable and requires the longest waiting period before applying a combined Weed and Feed product. The grass must achieve a level of establishment that allows it to tolerate the herbicide without injury. Applying the product too soon can inhibit seed germination or kill the newly sprouted seedlings.
The most common advice for seeded lawns is to wait until the grass has been mowed a minimum of four times. This “four-mow rule” indicates that the grass plants have developed a sufficiently extensive root system and are actively growing. An alternative guideline suggests waiting at least 60 days after the initial germination before applying any broadleaf herbicide.
This lengthy waiting time allows the seedlings to mature past their most fragile stage, developing the necessary leaf area and deep roots. If the Weed and Feed label provides a specific waiting period, it is always safest to adhere to the manufacturer’s directions. Even after the waiting period, applying the product during mild weather conditions (generally between 60°F and 90°F) reduces stress on the still-developing turf.
Timing Guidelines for Newly Sodded or Sprigged Lawns
Transplanted grass, such as sod or sprigs, presents a different scenario because the turf is already mature but lacks established roots in the new soil. When sod is first laid, the primary focus must be on encouraging the roots to “knit” into the underlying soil. The herbicide component of a Weed and Feed product should be avoided until this root establishment is complete, as pre-emergent herbicides can inhibit the root formation process.
A general timeline suggests waiting until the sod can no longer be easily lifted, which typically takes about three to four weeks. Professionals often recommend waiting a minimum of four weeks, or until the sod has been mowed four times, before applying any broadleaf weed control. This waiting period ensures that the roots have anchored securely enough to withstand the chemical stress.
The fertilizer component can often be applied sooner than the weed killer, sometimes immediately after installation, if it is a specific starter fertilizer. If using a combined product, the herbicide component is the constraining factor, meaning the whole application must be delayed until the roots are fully established (usually around two months or more). Applying a separate fertilizer first and waiting to use a weed killer later gives the turf the nutrients it needs without the chemical shock.
Immediate Weed Control Options While You Wait
While waiting for the new lawn to mature, weeds will inevitably emerge, and there are safe steps to manage them without resorting to a combined product. The simplest and safest method is gentle hand-pulling, especially for the shallow-rooted weeds that commonly appear in disturbed soil. This technique removes the entire weed, preventing it from competing with the new grass for water and nutrients.
Another effective and low-risk strategy is to adjust the mowing height. Mowing the new grass at a higher setting helps the turf blades shade the soil, which naturally suppresses weed growth by blocking the sunlight needed for weed seeds to germinate. Regular mowing also helps to remove the tops of existing weeds, preventing them from going to seed and spreading further.
For spot-treating individual, persistent weeds, a non-selective herbicide like glyphosate can be used, but extreme caution is necessary to avoid drift or overspray onto the young grass. The best approach is often patience, allowing the new grass to thicken and outcompete most weeds, making a full Weed and Feed application six months later far more effective.