Many people view clover as a persistent weed, while others see it as a beneficial plant that keeps the turf green. The question of whether nitrogen fertilizer can eliminate clover is common for homeowners and land managers. Applying nitrogen fertilizer does not act as a direct poison to clover, like a selective herbicide would. Instead, nitrogen works by altering the environment and the dynamics of plant competition, which ultimately suppresses the clover population.
Clover’s Natural Nitrogen Advantage
Clover belongs to the legume family of plants, a group defined by a remarkable natural ability to secure its own nitrogen supply. This unique trait is possible through a symbiotic partnership with a specific type of soil bacteria known as Rhizobia. These microscopic organisms reside within specialized structures called root nodules that form on the clover’s root system.
Within these nodules, the Rhizobia perform a process called nitrogen fixation. This complex biochemical reaction captures inert nitrogen gas from the atmosphere and converts it into ammonia, a form the plant can readily use for growth. This allows clover to essentially manufacture its own fertilizer. The ability to create its own nitrogen is why clover often flourishes in poor or neglected lawns, as it gives the clover a tremendous advantage over grasses that must rely solely on soil nitrogen.
The clover expends energy to maintain this symbiotic relationship, making nitrogen fixation an energetically costly endeavor. When the soil is naturally low in nitrogen, the investment is worthwhile for the clover to ensure its survival and growth. This biological self-sufficiency means clover does not need to compete with other plants for the limited nitrogen resources in the soil. It instead uses its energy to spread and thrive where other plants are struggling due to nutrient deficiency.
How External Nitrogen Shifts Competition
Introducing external nitrogen fertilizer into a mixed turf area fundamentally changes the competitive balance between clover and the surrounding grasses. When the soil is suddenly saturated with readily available nitrogen, the clover’s biological incentive to perform nitrogen fixation disappears. The plant recognizes it no longer needs to invest energy in maintaining the Rhizobia partnership and shifts its resources elsewhere.
While the clover adjusts its internal energy budget, the grasses receive a massive growth boost from the applied fertilizer. Grasses are highly adapted to rapidly absorb and utilize soil nitrogen, leading to vigorous growth and increased height. This rapid expansion of the grass canopy is the primary mechanism for clover suppression, as the taller, denser grass effectively blocks sunlight from reaching the low-growing clover plants.
Clover requires ample sunlight to perform photosynthesis, and being shaded by the rapidly growing turf severely limits its energy production. This light deprivation weakens the clover, making it less robust and less able to spread or recover. Additionally, the flourishing grass develops a denser root system, further dominating the soil space and outcompeting the clover for water and other nutrients, such as phosphorus and potassium.
Practical Management of Clover in Lawns
For those aiming to reduce clover in a lawn, the strategic application of nitrogen fertilizer is a tool that favors grass dominance. To achieve effective competitive suppression, a high-nitrogen fertilizer should be used, providing a strong initial burst of growth to the turf. Applying fertilizer in the early spring, as both clover and grass begin active growth, gives the turf a head start in canopy development.
A second application in the early fall strengthens the grass for the following season and further inhibits clover growth before winter dormancy. Choose a fertilizer with a high nitrogen content, represented by the first number in the NPK ratio, to maximize the growth differential between the turf and the clover.
This nitrogen-based approach is a slower method of control compared to using a selective broadleaf herbicide, which provides a faster kill. The competitive suppression offered by fertilizer is more of a long-term strategy. Consistent fertilization encourages a thick, healthy stand of grass that naturally prevents clover seeds from germinating and existing clover patches from expanding. Maintaining a dense turf canopy through regular nitrogen feeding is the most sustainable way to keep clover populations low.