“Weed and Feed” is a common lawn care product that combines fertilizer with a weed control chemical, delivering nutrients to the grass while simultaneously treating unwanted plants. The single name, however, represents a large product category, leading to confusion about whether a specific bag contains a pre-emergent or a post-emergent herbicide. Understanding the precise chemical action is essential for correct and effective application.
How Herbicides Are Categorized by Action
Herbicides are chemicals classified primarily by the stage of weed development they target. This mechanism of action dictates when and how the product must be used for effective control. The two main categories relevant to lawn care are pre-emergent and post-emergent herbicides.
Pre-emergent herbicides stop weed growth before the seedling breaks the soil surface. They create a chemical barrier in the upper layer of the soil. When a weed seed germinates, its root or shoot absorbs the chemical, disrupting cell division and stopping development.
Post-emergent herbicides are formulated to kill weeds that are already actively growing and visible in the lawn. These chemicals are absorbed through the weed’s foliage or root system. Once absorbed, they disrupt physiological processes like photosynthesis or growth regulation, leading to the weed’s demise.
The difference between these two types is the fundamental distinction between prevention and treatment. A pre-emergent application is a proactive measure aimed at the seed bank in the soil, while a post-emergent application is a reactive measure targeting established plant tissue.
Decoding Weed and Feed Formulas
The term “Weed and Feed” is a market label for a combination product, not a single chemical formula, meaning it can contain either a pre-emergent or a post-emergent herbicide. The specific type of weed control included is the most important factor to consider before purchasing. Manufacturers use descriptive language on the packaging to indicate the herbicide type.
Products labeled with phrases like “Crabgrass Preventer” or “Season-Long Control” contain a pre-emergent herbicide. Their purpose is to establish a protective soil barrier to stop new weeds, especially annual grassy weeds like crabgrass, from sprouting. The fertilizer component is generally a lower nitrogen blend designed to slowly nourish the turf without causing rapid growth that could deplete the pre-emergent barrier.
Conversely, a Weed and Feed product featuring images of broadleaf weeds like dandelions and clover, or using phrases like “Kills Existing Weeds,” contains a post-emergent herbicide. These formulas are designed to stick to the leaves of the target weed after application. The fertilizer encourages actively growing weeds to absorb the chemical more quickly, accelerating the kill rate.
Some seasonal formulas may include both types of herbicides, though this is less common due to conflicting timing requirements. The logistical difficulty of combining fertilizer with a pre-emergent is that the fertilizer encourages immediate growth, which is not always desirable during the pre-emergent application window. Reading the active ingredient list on the bag is the only reliable way to know the product’s true function.
Timing Application for Maximum Effect
The effectiveness of any Weed and Feed product depends on applying it at the correct time for the herbicide it contains. Misapplication can render the product useless. Pre-emergent formulas must be applied before weed seeds begin to germinate, a timing dictated by soil temperature, not the calendar.
To prevent the emergence of summer annual weeds like crabgrass, the product must be applied just before the soil temperature consistently reaches 50 to 55 degrees Fahrenheit. This temperature signals the beginning of germination for these seeds. Applying the pre-emergent after the soil has warmed past this point means the target weeds have likely already sprouted, and the barrier will fail.
The timing for post-emergent Weed and Feed is different, as it is applied when weeds are already visible and actively growing. The best window for application is when air temperatures are between 65 and 85 degrees Fahrenheit. It is also important that the weed foliage is moist at the time of application, which helps the granular herbicide stick to the leaves for absorption.
Applying a post-emergent product too early, before weeds are actively growing, wastes the chemical. The active ingredients require the plant’s metabolic processes to be in full swing for absorption and transport throughout the weed. Applying any herbicide when temperatures are too high, above 90 degrees Fahrenheit, can cause the chemical to volatilize and damage the desired turfgrass.