The presence of clover in a turfgrass lawn is a common challenge for homeowners. Clover is a broadleaf weed that competes with desirable grasses for water and nutrients, leading many to seek preventative control. Pre-emergent herbicides are a popular tool for weed management, establishing a chemical defense against unwanted plants before they can become established. This approach raises the question of their effectiveness against clover.
How Pre-Emergent Herbicides Work
Pre-emergent herbicides function as a protective measure, targeting weed seeds before they can sprout and emerge above the soil surface. They create a chemical barrier in the top layer of the soil that interrupts seed germination. This barrier must be present when the weed seed begins to swell and absorb water.
The active ingredients in these products, such as Prodiamine or Pendimethalin, inhibit cell division in the germinating seed’s root or shoot tips. This disruption prevents the development of a root system, halting the seedling’s growth before it can establish itself. The timing of application is crucial; the herbicide must be applied and activated by irrigation or rain before the target seeds begin to germinate.
Because this mechanism relies entirely on intercepting the germination process, pre-emergents are not designed to eliminate existing, mature plants. The chemical barrier only affects new growth from seed. This makes them highly effective against annual weeds, such as crabgrass, which restart their life cycle from seed each year.
Clover’s Growth Cycle and Spread
Clover is a broadleaf plant that possesses characteristics making it resilient to standard preventative measures. The most common species, white clover (Trifolium repens), behaves as a perennial plant in temperate climates. This perennial nature means the plant does not rely solely on annual seed germination for survival.
A defining feature of clover’s spread is its use of stolons, which are above-ground, creeping stems that run along the soil surface. As these stolons elongate, they develop nodes that can sprout new leaves and adventitious roots. When a stolon successfully roots at a node, it creates a new, independent plantlet.
This vegetative reproduction allows clover to spread across a lawn without the need for new seeds to sprout. The taproot of the original plant may eventually die, but the plant persists and expands indefinitely through its network of rooted stolons. This growth habit bypasses the soil’s surface, where pre-emergent herbicides create their chemical barrier.
Addressing Clover: Limitations of Pre-Emergents and Effective Alternatives
Pre-emergent herbicides are largely ineffective at eradicating established clover patches. Since pre-emergents only stop new seeds from germinating, they cannot kill clover plants that are already actively growing or spreading via stolons. Applying a pre-emergent to an existing clover infestation will not kill the visible plants.
While they cannot kill existing plants, certain pre-emergent products containing active ingredients like Isoxaben can prevent the germination of new clover seeds. This can be a supplementary strategy for long-term control, but it does not address the main problem of established, perennial plants. For visible, spreading clover, a different approach is necessary.
The most effective way to eliminate established clover is through the use of post-emergent herbicides. These selective products are designed to be absorbed by the leaves of broadleaf weeds and then translocated throughout the plant to kill it. These treatments often contain active ingredients like 2,4-D, MCPP, and Dicamba, which disrupt the plant’s internal growth processes.
Beyond chemical controls, cultural practices are important for discouraging clover growth. Clover thrives in soil that is low in nitrogen because it possesses the ability to fix atmospheric nitrogen, giving it a competitive advantage over turfgrass. Applying a high-nitrogen fertilizer encourages a dense, healthy stand of grass that naturally outcompetes the clover for light and water. Increasing the mowing height to three or four inches helps the grass shade the low-growing clover, limiting its ability to photosynthesize and spread.