By Jeff Rugg-
Q: There is an outbreak of clover on my lawn and in my flower beds. It has been spreading for several years. I tried a pre-emergent weed killer, and it didn’t help. What do you recommend to prevent it from taking over the whole lawn?
A: There was a time when the pre-emergent weed killer may have helped, but that time is long past. Clover is a perennial plant, and it won’t be affected by the pre-emergent once it has started growing.
Let’s start at the beginning. An annual weed starts as a seed, grows leaves, roots, flowers and seeds all in one year, and then it dies. It must come back from new seeds every year.
There are two types of annual weeds: summer and winter. Summer annuals sprout from seeds as the soil temperature warms up. They grow during the summer and produce a new set of seeds before they die in the fall. Crabgrass is the best-known summer annual.
Winter annuals develop during late summer or in the early fall. They survive the winter as a tiny plant. They produce seeds as summer approaches, and they die when the weather gets warm. Common chickweed and annual bluegrass are common winter annual weeds.
Biennial plants live for about two years. They usually sprout in the spring and grow a small plant during the summer. They grow a rosette that survives the first winter and feeds a fast-growing flowering stalk the second spring. Once they produce seeds and flowers, the biennials die around the second fall. Queen Anne’s lace, burdock, mullein and some thistles are biennials.
Your clover is a perennial. Once these plants sprout, they can live for many years. Some perennials are solitary plants that are not connected to nearby plants. Dandelions and plantains are common examples.
Other perennials spread by underground branches called rhizomes or above-ground branches called stolons. Yellow nutsedge and Canada thistle spread by rhizomes. Clover and Bermudagrass spread through stolons.
The proper method of weed control will be determined by the life cycle of the weed. Annuals can be prevented using pre-emergent weed killers if used at the proper time of year, which depends on whether it is a summer or winter annual. If there are not very many annuals, hoeing, mowing, burying under mulch or pulling by hand may keep them under control without herbicides.
To get the best control over biennials, the taproot must be removed or killed. Herbicides are often the most efficient way to kill biennials. Since biennials and solitary perennials must start from seeds, they can both be controlled with pre-emergent herbicides applied at the proper times. Once the biennial or perennial plant is growing, pre-emergent herbicides will have no effect.
The longer a perennial has been growing, the larger the plant will be. There may be many above-ground plants spreading perennials through stolons, and there may be many feet of below-ground rhizomes. Hand-digging established perennials can be very difficult. Weed-killing products that list the plant on its label are often the easiest and most effective way to control perennial weeds. Your clover is a spreading perennial that will need to be controlled with an herbicide that lists clover on the label. Clover is often strong enough to take one or more applications of an herbicide. Follow label directions for the best results.