Garden Tips: Control annual grassy weeds

It would be wonderful if weed-free lawns could stay that way forever. However, weeds get their foot in the door when lawns are subjected to stress from the environment or poor management practices, including watering, mowing and fertilization.

Weeds will eventually appear in lawns as they age, even in ones that are well maintained.

Much has been accomplished in the field of weed science for controlling broadleaf lawn weeds, like dandelions and clover. Whether chemicals, digging or pulling are employed, most broadleaf weeds in lawns can be controlled relatively easily.

Grassy weeds are a different story. It is difficult to pluck most grassy weeds interspersed with regular grass. Using chemicals to control weeds is difficult because many chemicals that kill the weeds will also kill grass. Before considering chemicals, you first need to understand how grasses grow.

There are two main types of grassy weeds: annual and perennial. Annual grasses die and come up again from seed each year. Crabgrass and annual bluegrass are the two most common annual grasses that cause problems in our area. Excessive watering, frequent shallow watering and consistently mowing a lawn too short make it easier for crabgrass start. Bluegrass prefers compacted soil as well as excessive watering. Correcting these problems and making a lawn as healthy and dense as possible with proper maintenance makes it difficult for these grasses to persist.

Chemicals are available that can help manage these two grasses. Pre-emergent herbicides chemically prevent seed germination and are applied before the seed of the annual grasses have the opportunity to germinate and grow.

Crabgrass seed germinates in the spring, and pre-emergent herbicides, or crabgrass preventers, are only effective if the application is made before seed germination. The right timing for applying pre-emergent herbicide is when the soil temperature at a depth of 1 inch is greater than 55 degrees for at least a week. This typically occurs when the yellow-flowering forsythia bush has been in full bloom for a week or two.

While some bluegrass seed germinates in the spring, most germinate in early to mid fall. It grows rapidly during mild winter and early spring weather, and then it flowers in the spring and summer, producing lots of seed. Pre-emergent herbicides applied before crabgrass germinates will not persist long enough to prevent most of the seed from germinating. However, the herbicides applied in the spring for preventing crabgrass will discourage early germinating annual bluegrass seed. For effective control of bluegrass, a pre-emergent herbicide should be applied in mid-August.

There are also post-emergent herbicides available to kill seedlings of crabgrass and annual bluegrass if a pre-emergent materials are not applied at the right time. However, these chemicals are only effective if the plants are relatively young and small. These materials have the potential to injure lawn grass if not applied correctly, so follow directions before use.

Next week, we will talk about the bigger challenge of managing perennial grassy weeds in lawns, like the dastardly Bermuda grass.

This story was originally published February 28, 2016 at 10:30 AM with the headline "Garden Tips: Control annual grassy weeds."