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Questions about Microclover

Questions regarding microclover in lawns in the Mid-Atlantic Region

What potential benefits may occur from including microclover in a typical lawn?

Microclover is a legume. Legumes can convert atmospheric nitrogen into organic nitrogen forms that can be eventually be utilized by turfgrass species. The nitrogen fixation process takes place in microclover root nodules formed by Rhizobium bacteria. Thus, it is anticipated that significant reductions of nitrogen fertilizer applications can be achieved when microclover in mixed with conventional lawn grasses, while maintaining good turfgrass quality. Estimates range for a potential fertilizer reduction of 1 to 2 pounds of nitrogen per 1000 square feet annually.

Additionally, microclover has a dark green color that many homeowners enjoy. Turfgrass stands that include microclover often appear darker green due to the inherent color of microclover, and maintain this darker green appearance more uniformly over the entire growing season.

What differentiates microclover from regular white clover?

White clover (Trifolium repens) was a common component of lawns before the introduction of broadleaf weed herbicides in the 1950's. Although the most common targets of these herbicides were weeds such as dandelion, broadleaf plantain, buckhorn plantain, and other broadleaf weeds, white clover was also either damaged or killed by these applications. In addition, white clover tended to form large clumps, which suppressed desirable grasses. When white clover was flowering, the lawns attracted bees, which many homeowners, particularly those with children, considered objectionable. As a result, white clover was considered a undesirable weed by a significant number of homeowners.

Microclover ( Trifolium repens L. var. Pirouette) is a selection from white clover that has smaller leaves and a lower growth habit. When seeded at an appropriate rate, it mixes better with most turfgrass species than common white clover, without forming clumps and without excessively competing with desirable turfgrasses. Thus, the lawn also tends to have a more uniform appearance.

Which types of lawn grasses will mix successfully with microclover?

Microclover has been reported to successfully mix with most cool season turfgrass species, including Kentucky bluegrass, perennial ryegrass, turf-type tall fescue, and the fine fescues. However, in a study conducted at the University of Maryland, microclover totally dominated when mixed with chewings fescue and hard fescue. This problem may have been due to excessive microclover seeding rates or due to environmental conditions in the Mid-Atlantic region.

What percentage of microclover should be mixed with lawn grasses?

In University of Maryland trials, microclover was mixed with hard fescue, chewings fescue, turf-type tall fescue, and Kentucky bluegrass. The percent of coated microclover seed in these mixtures was either 5% or 10% by weight.

Microclover totally dominated when mixed with either hard or chewings fescue, resulting in little or no desirable turf. Further work needs to be done to determine if 1-3% microclover would produce an acceptable mixture with hard or chewings fescue in the Mid-Atlantic region.

Microclover at both 5% and 10% mixed well with both turf-type tall fescue and Kentucky bluegrass. Although initially (first 6 months) the microclover seemed to be predominating, a good balance between the microclover and these turfgrass species was ultimately achieved. No difference was observable between the two microclover seeding rates. Further research is needed to determine if lower rates of microclover might be acceptable.

Can other broadleaf weeds be controlled in lawns without removing microclover?

Many of the herbicides that will control the most common objectionable broadleaf weeds in lawns will also injure or kill white clover. More research is needed on screening individual broadleaf weed herbicides to determine if programs can be developed to kill weeds such as dandelion and plantains, while allowing microclover to persist. It is possible that adjustments to normal rates and timing of application of these herbicides will minimize damage to microclover while controlling other broadleaf weeds. For example, 2,4-D is known to damage white clover leaves, but the amount of white clover actually killed can vary greatly depending on many factors, including application rate. Also, the broadleaf weed herbicide MCPA generally has a minimal long-term impact on white clover.

Cultural practices can also have a major impact on reducing potential broadleaf weed problems, thus reducing or eliminating the need for broadleaf weed herbicide applications that may injure microclover. Proper soil preparation prior to seeding, including adjusting soil pH and phosphorus as recommended by soil tests, will help ensure rapid establishment and maximize lawn density, thus minimizing weed encroachment. Using recommended turfgrass cultivars when seeding turfgrass-microclover mixtures will help maintain stand density. Also critical to reducing the potential for weed encroachment is avoiding mowing the lawn too short. Mowing at a height of 3 inches versus less than 2 inches has consistently been shown to greatly minimize weed populations.

Are there any disease problems of microclover?

While disease problems of microclover are infrequent and have not been widely reported, the disease southern blight (Sclerotium rolfsii) severely damaged microclover during the summer of 2011 in University of Maryland research plots. Large patches of microclover (up to 2 feet in diameter) were killed during hot, humid weather in July. This disease has not occurred in other years on research plots, and probably occurs infrequently.

Southern Blight on Microclover
Microclover killed in patches by Southern blight disease

What happens to microclover after a frost or during the winter?

Microclover leaves tend to die back during the winter months, appearing as if the plants have died. However, the roots, crowns, and stolons are alive, and new leaves emerge as the weather begins to warm in late winter and early spring.

Is microclover heat and drought tolerant?

Microclover will survive most summers in the Mid-Atlantic region without major problems. However, under unusually severe conditions, significant amounts of microclover clover may die. During the very extreme heat and drought experienced at the University of Maryland Paint Branch Turfgrass Research Facility during the summer of 2012, approximately 95% of the microclover in test plots was killed. It should be noted that after the heat and drought conditions subsided, plots that had contained microclover exhibited pronounced nitrogen response effects (darker green color) until early spring 2013, compared to plots not originally seeded with microclover.

Is microclover shade tolerant?

In research studies at the University of Maryland, microclover did not persist in either medium or full shade locations. In the Mid-Atlantic region, it appears that mixtures of tufgrass and microclover will be predominately for use in full sun situations.

What is seed coating and is it necessary for microclover establishment?

Microclover can be purchased as coated or uncoated seed. Coated seed normally consists of limestone, an adhesive, and the bacteria Rhizobium leguminosarum biovar. Trifolii. Nitrogen fixation by white clover only occurs when the roots of the plant become infected with this specific strain of bacteria. The bacteria are normally present in soils that previously contained Rhizobium leguminosarum biovar. Trifolii infected plants. In such situations it is not necessary to use coated seed to obtain the nitrogen fixing benefits of clover. However, many lawns in new residential developments are established in soils not having a past history of clover use. In such situations coated microclover seed should be used. Bacteria viability on coated seed is limited. Accordingly, avoid using coated seed that has a seed label date that is more than six months old.

Microclover seed can be inoculated with Rhizobium leguminosarum biovar. Trifolii without coating the seed. In this case the bacteria is mixed directly with the seed and a small amount of water. The viability of uncoated freshly moisten bacteria is only a few hours, Thus freshly inoculated seed should be broadcast as soon as the seed dries.