Weeds compete with your lawn for sunlight, water, and nutrients, diminishing the appearance of a healthy yard. Managing these unwanted plants requires the strategic use of herbicides, commonly available as sprays. Choosing the correct spray depends on whether you are preventing weeds or eliminating those already established. Success in maintaining a clear, healthy lawn is determined by the proper application of these products based on their chemical properties and intended purpose.
Preventing Weeds with Early Application
A proactive strategy against annual weeds, such as crabgrass, involves applying a pre-emergent herbicide. These sprays create a chemical barrier in the top layer of the soil that targets seeds immediately after they begin to germinate. The active ingredients interfere with cell division in the developing seedling roots, halting growth before the plant emerges from the soil surface.
Timing is a non-negotiable factor for the effectiveness of pre-emergent sprays, which must be applied before the weed seeds sprout. For summer annuals like crabgrass, the ideal window is typically early spring, when soil temperatures consistently reach 50 to 55 degrees Fahrenheit. Applying the product too late means the weeds will have already broken through and the spray will be largely ineffective.
After applying a pre-emergent product, water the area lightly to move the chemical into the soil, forming the protective barrier. This activation allows the herbicide to bind to soil particles and remain active for the intended 60 to 90-day period. Disturbing the soil after application, such as through aeration or raking, can break this barrier and allow weeds to germinate.
Treating Weeds That Are Already Growing
When weeds are already visible and actively growing, the solution shifts to using a post-emergent herbicide. These sprays are designed to kill established plants by being absorbed through the foliage and stems. Once inside the plant, the active ingredients move through the vascular system, disrupting essential biological functions like photosynthesis or protein synthesis.
Post-emergent sprays are often applied directly to the leaves of the weeds, making full coverage of the plant surface a requirement for maximum effectiveness. For lawns with only a few scattered weeds, a targeted spot treatment is the most efficient method. If the lawn has a widespread infestation, a blanket application across the entire area may be necessary.
The success of a post-emergent spray is highly dependent on environmental conditions at the time of application. Products work best when weeds are actively growing, which typically occurs when temperatures are between 65 and 85 degrees Fahrenheit. Applying sprays during periods of extreme heat or drought stress reduces the overall kill rate by causing less efficient chemical absorption. Ensure no rain is expected for at least 24 hours after application to allow the chemical time to be fully absorbed.
Choosing Between Selective and Non-Selective Sprays
A foundational concept in lawn weed control is the difference between selective and non-selective herbicide sprays. Selective sprays are formulated to control specific types of weeds, most commonly broadleaf varieties like clover and dandelions, while leaving the desirable turfgrass unharmed. These chemicals exploit physiological differences between the weeds and the grass, such as how they absorb or metabolize the active ingredients.
Common active ingredients in selective lawn sprays include 2,4-D, dicamba, and mecoprop-P (MCPP), which are often combined to create a product that targets a wider range of broadleaf weeds. The 2,4-D component, for example, acts as a synthetic plant hormone that causes uncontrolled, twisted growth in susceptible plants, ultimately leading to their death. Using a selective spray is the only way to eliminate weeds from a lawn without causing widespread damage to the turf itself.
Non-selective sprays are powerful chemicals that kill every plant they touch, including grass. These products function by disrupting cellular functions necessary for plant survival. The most widely known non-selective ingredient is glyphosate, which blocks an enzyme pathway plants need to produce essential amino acids.
Because they kill all vegetation, non-selective sprays are not used for general lawn maintenance. Their use is limited to areas where no plant growth is desired, such as cracks in sidewalks, along driveways, or for clearing a patch of ground before establishing a new garden bed or lawn. Care must be taken during application to prevent spray drift from contacting and killing nearby desirable plants.
Natural Spray Solutions
For individuals seeking alternatives to synthetic chemical products, horticultural vinegar is the most common natural solution. It contains a much higher concentration of acetic acid—typically 10% to 30%—than household vinegar. This concentrated acid works as a contact herbicide, quickly dehydrating the weed’s foliage and breaking down its plant cells.
Horticultural vinegar is most effective on small, young annual weeds and works best when applied on a warm, sunny day. The product only kills the parts of the plant it physically touches, meaning it does not move systemically to kill deep roots. Perennial weeds with robust root systems will often regrow after the top growth has been burned off, requiring repeated applications.
Like non-selective chemical sprays, concentrated acetic acid is non-selective. Therefore, it is best reserved for spot treatments or for areas like pavement and gravel paths where turf damage is not a concern. Due to the high acid concentration, protective gear should be worn during application to prevent skin and eye irritation.