Q: Is it a good idea to add clover to an existing turf sward?

A: White clover (Trifolium repens L.) is a stoloniferous plant that is commonly found in low-medium maintenance turf. In a turf situation, it is typically classified as a weed species and controlled with a selective broadleaf weed herbicide; however, there may be situations where a clover-turf mix is beneficial. Clover is a legume, which means that it can take nitrogen gas from the atmosphere and convert (fix) it into a nitrogen source that can be used by clover plants. Research has also shown us that clover can symbiotically transfer nitrogen to adjacent plants, like turfgrasses. The nitrogen fixation process in clover is done by rhizobium bacteria, located inside the root nodules of the clover plant. The symbiotic relationship between clover and rhizobia is particularly common in nitrogen-limited conditions, such as low maintenance lawns or athletic fields.

While white clover may be too obtrusive on a sports field, “microclover” is a clover species touted to be more diminutive in growth habit and produce less flowering, resulting in a better compatibility/quality component than traditional white clover. As its name suggests, it is a smaller version of white clover, with smaller leaves and a low-growing growth habit that does not produce clumps, but blends in nicely with the grass plants. Microclover seed can be purchased from many seed suppliers, and it typically comes coated with the rhizobium bacteria, since the bacteria may not already be present in urban soils.

So while we have historically treated clover in turf as a weed, clover is actually a very cool plant that can add nitrogen to the soil, feeding itself and adjacent plants. Clover can also grow in poor soils, which is a big benefit to turf managers managing native soil sports fields. And let’s not forget that clover flowers attract pollinators like bees and therefore improve upon the biodiversity of the turfgrass system. For these reasons, researchers and practitioners alike have been looking at microclover/turf mixes to see if it could result in a more sustainable turf system.

Results to date have suggested that microclover/turf mixes are viable systems that produce a good quality, uniform, dark green sward. The inclusion of microclover also reduces the amount of fertilizer, pesticide and mowing inputs. One study at Ohio State reported up to 50% cost reductions for fertilizer, 20% reductions in pesticide (namely herbicides) and 2.5% for fuel and labor costs. The reduction in herbicide was attributed to the fact that the clover shaded out germinating weed seeds. The study also observed that Kentucky bluegrass turf was significantly less drought stressed during dry-down periods when microclover was added to it.

A microclover/turf sward can be established much the same way as any new sward, so timing and soil preparation would be the same. The seed mix should contain no more than 5-10% microclover seed by weight, to make sure that turfgrass is the predominant species. This is important because even though it is hardy to USDA Hardiness Zone 3, clover will go dormant during the winter and lack of ground cover during that period can cause soil erosion problems and encourage winter weeds. Introducing microclover seed to an established turf sward can be done in a variety of ways: by slit-seeding it in, core aerating, vertical mowing or scalping. The most effective way to establish microclover weed is scalping, but that also causes injury to the turf and delays growth. Keep in mind that the more competitive the turf species is, the more it will crowd out the clover. Quick species like tall fescue and perennial ryegrass will dominate the sward, while slow-establishing/growing species like Kentucky bluegrass and fine fescues may contain more clover.

There are a few challenges associated with maintaining a microclover/turf sward. Firstly, it is difficult to control other broadleaf weeds like dandelion and plantain with herbicides without also removing the clover. There are several herbicides that microclover has shown some tolerance to, such as 2,4-D, quinclorac and MCPA, but the tolerance depends upon rate and type of clover (micro or white). There is still research needed in this area.

Like most turfgrasses, microclover grows best in full sun and does not do well in shade. Mowing heights need to be no lower than about 3-3.5 inches for the microclover to persist, which means it is suitable for lawns and recreational fields but may not be useful for high quality turf and sports that require ball roll and ball bounce. Be mindful that introducing pollinator plants to the sward will attract bees, which may cause issues for children and people with bee allergies. Lastly, the longevity and amount of the microclover in turf can be adversely affected by several factors; it does not persist well under heavy traffic, it is a short-lived perennial so needs to reseed itself adequately to maintain density (don’t keep mowing the flowers off), and if supplemental fertilizer is added, the turf may crowd out the microclover over time.