While bright green shamrocks (Trifolium dubium) may be a beloved symbol of all things Irish, they don't always find favor with gardeners. Because shamrocks--also called little hop clovers--increase soil nitrogen levels, shamrock seeds often find their way into commercial turfgrass mixes. These enthusiastic growers, however, often outperform their turfgrass companions to become invasive. With proper application, some herbicides let gardeners control shamrocks without harming other grasses or plants.
Shamrock Benefits and Disadvantages
Shamrocks work with a soil bacterium called rhizobium. Combining their efforts, the germ and plant draw nitrogen from the air into the soil, ensuring the shamrock's roots of a steady supply. As the shamrocks age and decline, their dying roots release the nitrogen into the soil to the benefit of surrounding turfgrass.
Shamrocks also bring disadvantages to turfgrass lawns. The plants grow a different rate, with texture and color noticeably different from those of turfgrasses. Stings from bees attracted to the shamrock's flowers are a possibility.
Pre-Emergent Shamrock Herbicides
Tackling shamrocks with herbicides can be problematical. Many herbicides also damage the surrounding plants. Herbicides applied before the shamrocks emerge from the soil are most effective, advise Richard Smith and colleagues of the University of California Integrated Management Program. Professional landscapers prefer pre-emergents containing isoxaben to control shamrocks around trees, woody shrubs and most annual flowers.
Post-emergent Shamrock Herbicides
Controlling sprouted shamrocks in landscaped areas with herbicide is challenging. Post-emergent herbicides with glyphosate work on shamrock seedlings shorter than 3 or 4 inches, but may damage surrounding plants. Taller seedlings resist herbicides.
Shamrock Herbicides for Turfgrass Lawns
Post-emergent herbicide control for shamrocks in turfgrass lawns is acceptable when no other plants are at risk. The ingredients in these herbicides harm ornamentals. The type of turfgrass dictates the choice of herbicide. Cool season turfgrasses--including Kentucky bluegrass and fescue--survive applications of any shamrock-controlling herbicide Zoysia, Bermuda and other warm-season grasses handle those with dicamba or mecoprop.
Alternatives to Herbicides
Hard-coated shamrock seeds lie dormant in the soil for many years before sprouting. Manually controlling the plants requires a continuing effort. Preventing their return is relatively easy. A manual control program consists of pulling plants up or hoeing them. Applying a 4-inch layer of organic mulch s where shamrocks have previously grown, or a 6-inch, light-blocking layer over new seedlings, is also effective. Annual re-application of mulch to maintain a 4-inch layer keeps the plants in check.
A program of pulling up or hoeing shamrocks, re-seeding cleared areas with grass, and adding nitrogen and phosphorus to the soil is the best alternative to herbicide use on lawns.