Prior to the 1940s, clover was widely sown in lawns across the United States. When 2-4D became a popular herbicide, the clover would be killed. So began the story of how clover in lawns came to be seen as a weed. But it’s starting to reappear, said Mark Carroll, University of Maryland, because it’s good for pollinators and soil fertility.
White clover, red clover and “microclover,” a dwarf natural selection from Europe, are being reintroduced for their nitrogen fixing ability. Microclover, a dwarf growing selection, is covered by the grass canopy, so it stays hidden.
Mark, a water quality expert, comes at the topic of lawns from the perspective of how to reduce runoff pollution caused by compacted soils and overfertilization. He spoke to the Turning a New Leaf audience.
Clover fixes about 6 lb. N/1,000 sq. ft./year. Out of that, about 0.8 lb. will be available to the lawn. For the typical lawn requiring 2 to 2.5 lbs. of N/1,000 sq. ft. that translates to two fertilizer applications rather than three, he said. You can get another 0.3 lb. N/1,000 sq. ft. from grass clippings, too.
Working with a multiple-institution group, Mark and his colleagues began testing clover mixes in Maryland, Pennsylvania and Virginia turfgrass trials in 2012. During his conference presentation, he focused on one residential test site—new construction by Compass Homes in a high-end suburban Maryland neighborhood. Prior to sowing, they added 2 in. of compost (300 cu. yds./acre) to boost rainwater infiltration through increased organic matter. No fertilizer or pesticides were used over the course of the three-year test.
After a slow start due to heavy rains in year one, (net/net for most rain events) the compost-treated lawn outperformed the control in terms of reducing runoff and, because of that, nutrient loss, too, since less runoff means less nutrients lost. A preliminary benchmark for the effectiveness of the compost/clover treatment is that runoff must be reduced by 50% or more before you see reductions in N and P running off the site, he said.
Microclover will persist in the lawn except when it’s really hot or really dry, he said. Summer blight can kill it in high temperatures. When the clover dies, all the N is flushed into the lawn. While microclover will crowd out fine leaf fescue, it’s good with Kentucky bluegrass and tall fescues. If microclover isn’t mowed, it will grow large like regular clover. Also, like other clovers, microclover will re-seed.
The cost was about $2,700 for 5,000 sq. ft. for the treatment. Unfortunately, using compost as a stormwater practice in Maryland isn’t allowed, as the Maryland Department of Agriculture interprets their fertilizer laws, such that they’re applicable to compost. The practice is an accepted BMP in Pennsylvania and Virginia, where builders can use it. At least two mid-Atlantic sod farms produce sod that includes microclover.