Broadleaf weeds, such as dandelions and clover, are dicots, which are anatomically distinct from lawn grasses, which are monocots. This biological difference allows specialized herbicides to target the weeds while leaving the grass unharmed. Herbicides are chemical tools designed to interrupt the growth process of unwanted vegetation in a lawn.
How Broadleaf Herbicides Work
Broadleaf herbicides are selective, meaning they are formulated to kill certain plant types without seriously damaging others. This selectivity relies on the biological differences between dicots and monocots, allowing the chemical to attack the weed’s unique metabolic pathways while the grass remains unaffected. Conversely, a non-selective herbicide, such as glyphosate, targets metabolic processes common to nearly all plants, killing any plant it contacts.
The effectiveness of these herbicides is further categorized by their action within the plant structure: systemic or contact. Systemic herbicides are absorbed by the leaves and translocated throughout the plant’s vascular system, moving down to the roots. This mechanism is necessary for controlling perennial weeds, like dandelions, which can regrow from deep root systems if only the top growth is killed.
Contact herbicides only kill the plant tissue they directly touch and do not move internally. This action makes them faster acting, often causing visible damage within hours. However, they are less effective against perennial weeds with robust root structures. Contact products are best suited for controlling annual broadleaf weeds that complete their life cycle in one season.
Understanding Key Active Ingredients
Effective broadleaf herbicides are generally blends containing multiple active ingredients, often two to four compounds, because no single chemical controls the full spectrum of common weeds. These ingredients primarily function as synthetic auxins, chemicals that mimic natural plant hormones and cause uncontrolled, abnormal growth that ultimately destroys the weed.
The most common component in these blends is 2,4-D (2,4-Dichlorophenoxyacetic acid), which is highly effective against common weeds with taproots, such as dandelions and plaintains. It is often combined with MCPP (Mecoprop) and MCPA to broaden the range of weeds controlled. MCPP is particularly useful for targeting weeds like chickweed and clover, which 2,4-D controls less effectively.
Dicamba is frequently included in three-way blends, managing tougher, deep-rooted broadleaf species such as clover and ground ivy. Dicamba has greater residual activity and soil mobility than some other ingredients, which is an important consideration near trees and shrubs. However, its effectiveness is sensitive to high temperatures, which can cause the chemical to volatilize and drift to non-target plants.
For difficult-to-control or woody weeds, such as creeping charlie (ground ivy) or oxalis, Triclopyr is often the preferred ingredient. Triclopyr is excellent at controlling woody plants and vines and is frequently included in specialized blends for clover and difficult weed control. Combining these varied chemical modes of action ensures a single application can manage a diverse population of broadleaf invaders.
Matching the Herbicide to Your Lawn and Weeds
Selecting the appropriate herbicide blend requires considering the specific grass species, as turfgrass tolerance varies significantly. Cool-season grasses like Kentucky Bluegrass, Tall Fescue, and Perennial Ryegrass are generally tolerant of common three-way synthetic auxin blends (2,4-D, MCPP, and Dicamba). These grasses can usually handle standard application rates without significant damage.
Warm-season grasses require a specialized approach due to their sensitivity to many common broadleaf herbicides. Turf types such as St. Augustinegrass and Centipede grass are susceptible to injury from standard three-way blends, especially when temperatures are high. For these sensitive lawns, specialized products like atrazine or those with reduced rates of active ingredients must be used to prevent turf damage.
Herbicide effectiveness is also tied to accurately identifying the weeds present. Perennial weeds, which return each year from established roots, require a systemic herbicide blend containing ingredients like Dicamba or Triclopyr to kill the entire root structure. Annual weeds, which complete their life cycle in one season, are generally easier to control with a broader range of products.
Herbicide products are available in several physical formats:
- Liquid concentrates are often the most cost-effective solution, allowing for customized mixing and application rates, though they require a sprayer.
- Ready-to-use liquid sprays offer convenience for small areas or spot treatments, eliminating the need for measuring or mixing.
- Granular products combine the herbicide with a fertilizer carrier, offering the dual benefit of feeding the lawn while treating weeds. They must be applied when the foliage is damp so the granules stick to the leaves.
Application Timing and Avoiding Collateral Damage
Successful weed control depends on application timing as well as product selection. Herbicides must be applied when broadleaf weeds are actively growing, generally in the spring and, most effectively, in the fall. Fall applications (mid-September through early November) are highly effective for perennial weeds because the plants are translocating carbohydrates downward for winter storage, pulling the systemic herbicide deep into the root system for a complete kill.
Temperature is a significant factor, as most broadleaf herbicides work best when the air temperature is between 60 and 75 degrees Fahrenheit. Applying herbicides when temperatures are expected to exceed 85 degrees Fahrenheit within 24 hours increases the risk of turf injury and can cause chemicals, such as Dicamba esters, to volatilize and drift. Spraying should also be avoided when winds are above 5 miles per hour to minimize spray drift onto garden plants and shrubs.
To maximize absorption, the lawn should not be mowed for two to three days before and after application. Furthermore, to prevent the product from being washed off, no rain or irrigation should occur for at least 24 hours after a liquid application. Following US EPA guidelines for personal safety, including wearing gloves and eye protection, is necessary when handling and applying concentrated or liquid herbicide products.