Selective grass killers offer a solution for gardeners frustrated by unwanted grass growing among desirable garden flowers. These specialized products eliminate grassy weeds without harming broadleaf ornamental plants. This selective control is possible due to fundamental differences in the biology and internal chemistry of these two plant groups. Understanding these distinctions is the first step toward effective weed management.
The Biological Difference Between Grass and Flowers
The ability to target only grass comes down to a major division in the plant kingdom: the difference between monocots and dicots. Grasses are classified as monocotyledons (monocots), while nearly all traditional garden flowers, shrubs, and vegetables are dicotyledons (dicots). This distinction begins at the seed level, with monocots having a single embryonic leaf (cotyledon) and dicots having two.
These two groups have distinct physical structures that herbicides exploit. Monocots, like grass, have leaves with parallel veins and a fibrous root system, whereas dicots typically have branching veins and a taproot system. More importantly for chemical control, the growing point in grasses is often located near the soil surface, protected by the surrounding leaf sheaths, while the growing points in dicots are exposed at the tips of stems and branches.
The selectivity of the modern grass-killing chemicals is primarily based on a metabolic difference rather than just physical structure. Selective herbicides target specific enzyme systems that are structured differently in monocots and dicots. Dicots can either metabolize the herbicide into a harmless compound or possess a version of the target enzyme that the chemical cannot bind to, allowing them to remain unaffected.
Active Ingredients That Selectively Kill Grass
The chemicals that selectively kill grass while sparing flowers are known as graminicides, meaning they specifically target grasses. These are post-emergent herbicides, designed to kill grass that has already sprouted and is actively growing. The most common and effective active ingredients found in consumer products belong to a class of chemistry known as Acetyl-CoA Carboxylase (ACCase) inhibitors.
Two of the most widely available ACCase inhibitors are Sethoxydim and Clethodim. These chemicals work by interfering with the grass plant’s ability to synthesize lipids, which are fatty acids necessary for cell membrane formation and growth. By shutting down this lipid production pathway, the grass cannot create new cells, leading to a cessation of growth and eventual death.
Once absorbed through the leaves, these active ingredients are systemic, moving throughout the plant to the meristematic regions—the areas of active growth in the roots and shoots. Symptoms in susceptible grass weeds, such as yellowing and tissue death, usually become visible within one to three weeks after application. The selective action is ensured because the ACCase enzyme in broadleaf plants, including flowers, is structurally different and insensitive to these inhibitors, or the flower can rapidly break down the chemical.
These chemicals are distinct from the selective herbicides used to kill broadleaf weeds in lawns, like 2,4-D, which would kill flowers but leave the grass unharmed. The goal here is the opposite: to kill the monocot (grass) and protect the dicot (flower). Graminicides do not control sedges or broadleaf weeds, but only grasses.
Safe and Effective Application Methods
Successful grass control in a flower garden relies heavily on precise application to ensure the selective chemistry works as intended. When treating grass within a flower bed, spot treatment is the most effective and safest method. This involves using a pump sprayer to apply the chemical directly onto the target grass leaves, minimizing the chance of overspray onto desirable plants.
It is necessary to apply the selective grass killer only when the grass is actively growing, as the herbicide must be absorbed and translocated throughout the plant’s system. Application during periods of drought or stress will reduce the chemical’s effectiveness. Always follow the manufacturer’s directions precisely regarding mixing ratios and dosage. Applying too much can sometimes overwhelm the protective mechanisms of nearby flowers, even though the chemical is selective.
Weather conditions during application are also a significant factor in preventing damage to flowers. Apply the product on a calm day with little to no wind to prevent spray drift from carrying the herbicide onto the leaves of the flowers. Check the product label for the required “rainfast” period, which is the time the chemical needs to remain on the foliage before rainfall or irrigation will not wash it away. Thoroughly clean all spraying equipment after use to prevent accidental contamination. Residue from a previous, non-selective chemical could easily damage flowers during a future application.
Active Ingredients That Selectively Kill Grass
Two of the most widely available ACCase inhibitors are Sethoxydim and Clethodim. Once absorbed through the leaves, these active ingredients are systemic, meaning they move throughout the plant to the meristematic regions—the areas of active growth in the roots and shoots. Symptoms in susceptible grass weeds, such as yellowing and tissue death, usually become visible within one to three weeks after application. The selective action is ensured because the ACCase enzyme in broadleaf plants, including flowers, is structurally different and insensitive to these inhibitors, or the flower can rapidly break down the chemical.
These chemicals are distinct from the selective herbicides used to kill broadleaf weeds in lawns, like 2,4-D, which would kill flowers but leave the grass unharmed. The goal here is the opposite: to kill the monocot (grass) and protect the dicot (flower). It is important to note that graminicides do not control sedges or broadleaf weeds, but only grasses.
Safe and Effective Application Methods
Successful grass control in a flower garden relies heavily on precise application to ensure the selective chemistry works as intended. When treating grass within a flower bed, spot treatment is the most effective and safest method. This involves using a pump sprayer to apply the chemical directly onto the target grass leaves, minimizing the chance of overspray onto desirable plants.
It is necessary to apply the selective grass killer only when the grass is actively growing, as the herbicide must be absorbed and translocated throughout the plant’s system. Application during periods of drought or stress will reduce the chemical’s effectiveness. Always follow the manufacturer’s directions precisely regarding mixing ratios and dosage; applying too much can sometimes overwhelm the protective mechanisms of nearby flowers, even though the chemical is selective.
Weather conditions during application are also a significant factor in preventing damage to flowers. Apply the product on a calm day with little to no wind to prevent spray drift from carrying the herbicide onto the leaves of the flowers. Additionally, check the product label for the required “rainfast” period, which is the time the chemical needs to remain on the foliage before rainfall or irrigation will not wash it away. Finally, thoroughly clean all spraying equipment after use to prevent accidental contamination, as residue from a previous, non-selective chemical could easily damage flowers during a future application.