Leguminous weeds are broadleaf plants from the Fabaceae (bean) family. In tropical and subtropical lawns, these weeds typically grow beneath the height of the mower blade and are mat-forming. Although distinct patches can form, initially they tend to move through the turfgrass assisted by horizontal stems which peg roots from their nodes into the soil.
In some societies clover lawns are highly valued, staying green when the grass dies back due to drought, and adding stable forms of nitrogen to the soil via a unique symbiosis between the plant and rhizobial bacteria (rhizobia).
In poor fertility situations, some of these weeds have the capacity to develop root nodules that fix atmospheric nitrogen into a form available to the plant. This organic nitrogen source is in slow release form, less likely to leach away or to cause excess soil acidity. The process is activated by rhizobia and needs specific conditions. Hence, not all leguminous weeds have nodules and only nodules with a red interior are actively working.
It is estimated that a 5% clover lawn with active nitrogen fixing nodules produces enough nitrogen to supply half the lawn’s annual nitrogen requirement.
Despite the benefits of (sometimes) adding soil nitrogen, most dedicated gardeners scowl when leguminous weeds break the visual continuity of their grassy lawn.
Legumes in lawns can be painful, and for some individuals create the potential for a medical emergency. Clovers, in particular, attract bees which are readily trodden on, with severe consequences for those allergic to their stings. Lesser, but still painful and annoying, injuries are caused by burr medic, which has spiny fruits and sensitive weed, which has short curved prickles.
Reading your site
The presence of leguminous weeds in the lawn can be an indicator of low levels of nitrogen relative to phosphorus. Early weed identification, followed by hand weeding or spot treatments with glyphosate or a desiccant herbicide (such as a solution of vinegar and salt) can prevent a small problem becoming greater.
Shifting the balance
As the weeds listed below are all nitrogen-fixing to varying degrees, fertilizing the lawn with a nitrogen-rich N:P:K formulation or a N:0:0 fertilizer will assist the turfgrass to out-compete the weed. Suitable nitrogen forms include: ammonium sulphate (rapid release) or slower release sulphur-coated urea, resin-coated urea or natural organics. At a pH < 6.5 grasses are more efficient at taking up nitrogen, giving them an advantage relative to the leguminous weed. Try to avoid adding unnecessary phosphorus, as this element is known to favour the leguminous weed over turfgrass.
Ferrous (iron) sulphate is registered as a herbicide for clover control. Use strictly according to the directions on the label, and not on already acid soils. As an added advantage it kills other broadleaf weeds and will correct iron deficiency, which is endemic in tropical and subtropical lawns through the wetter months.
A range of registered herbicides are available for clover control, and are freely available at nurseries and hardware stores. These mostly contain MCPA (a phenoxy herbicide) in combination with one or two other herbicides.