Montana State University Ag Specialist Guest Column: Weed Management for Lawns

Bozeman, Montana (April 04, 2011)--Many times, a weed-free lawn can seem like something that only occurs in televised golf tournaments. I don't know about you, but every spring I struggle to keep dandelion, nutsedge, and quackgrass from taking over my lawn.

While herbicides can help manage weeds in lawns, no single product will work against all weeds. For example, products that contain 2,4-D are designed for broadleaf weeds and won't work for grasses. Unless care is taken, herbicides can be dangerous to lawns, flowers, pets, and people. A successful weed management program takes advantage of many approaches including cultural practices, mowing, and, if necessary, herbicides.

The key to managing weeds is to outcompete them with a dense and vigorous lawn. Most weeds have very little chance of establishing if thick grass blocks sunlight, captures moisture and takes advantage of available nutrients.

A good fertilization program can help grow a dense and competitive lawn. Be aware that too much fertilizer, while helping to nurture a lawn, will also feed the weeds. Also, too little fertilizer can lead to a sparse and uncompetitive lawn. In Montana, one or two applications of fertilizer per year are usually enough. When purchasing fertilizer, choose one with a high percentage of controlled-release nitrogen to provide a slow, steady nutrient supply. Consider having soil tested as well, so you know how much fertilizer is needed.

Mowing can help manage weeds, but it can also hurt your lawn. Frequent mowing weakens grass by reducing its ability to capture enough sunlight and produce food. Moreover, a short lawn allows too much sunlight to reach the soil surface, helping weed seeds sprout and grow. Mowing at the highest possible level - usually between 2 and 4 inches - will help manage unwanted weeds.

Frequency and timing of watering are also crucial to healthy lawns. Usually, lawns need about one inch of water per week. As a general rule, the best approach is to water lawns infrequently and deeply. Providing a lawn with infrequent, deep soakings helps grow deep-rooted grasses and prevents the germination and growth of shallow-rooted weeds.

Identifying weeds is also essential to reduce their spread and abundance. Grass and broadleaf weeds that are found in lawns fall into two main groups: annuals and perennials. An annual is a plant that dies at the end of each growing season and new plants are produced each year from seeds.

Annual weeds, such as crabgrass and shepherd's-purse, are not that difficult to control. Preventing seed production in annual weeds by mowing or other means is often effective.

A perennial plant, on the other hand, lives for more than one year and grows back from the roots each year. Perennials also produce seeds, which can give rise to new plants. To effectively control deep-rooted perennials, such as Canada thistle or bindweed, concentrate efforts on managing their roots. Roots of perennials often have vegetative buds, growing deep in the soil.

These buds can give rise to new, independent plants. Certain herbicides, such as those containing glyphosate, can move within the plant, down into the root system. A fall application is usually recommended to maximize herbicide translocation to the root system.

If you are not sure of the identity of a weed, take a sample to your county Extension agent or county weed office. They will also help design a weed management program that integrates as many practices as possible.

Believe it or not, hand-pulling is still one of the best defenses against weeds. Pulling annual weeds before they flower and seed is the simplest way to prevent them from spreading. Hand-pulling is easier when soil is moist. A sharp spade or digger can help do the job.

When using an herbicide, choose one that is labeled for the type of lawn you have and is effective against the weeds you have. Before spraying any herbicide, read the label and follow directions carefully. Some herbicides work within a certain temperature range; others need to be applied at a specific time of year. When used incorrectly, herbicides can injure or kill turf and other desirable plants.

If your efforts to get a weed-free manicured lawn - one that is the envy of a professional golf-course manager - don't work, relax and enjoy the summer anyway. I don't know about you, but I'm learning to live with my dandelions.

Contact:

Fabian Menalled
406-994-4783
menalled@montana.edu