In the spring of every year I get asked this question at least one hundred thirty-two times. After seventeen years as an extension agent this is an easy question to answer. The harder part is what really needs to be done to control or prevent them. Are you ready?
To know how to manage a particular weed you need to know something about its lifecycle. Henbit is an introduced winter annual. It has green to purple tender stems that are four sided. The leaves are opposite of each other on the stem. The leaves are egg-shaped with rounded indentions along the edge. The plant is a member of the mint family. It’s good that it’s not a perennial or we would all be in big trouble. Winter annuals like henbit grow from seed that sprout in the fall or winter. During the cold temperatures of winter the plants remain dormant. They just sit there, small and hidden by the stand of dormant warm season grass most of us have, waiting for warm weather of spring to arrive. Henbit thrives in thin lawn areas where there is plenty of moisture.
Everyone wants to know what to do about it when they see it. For most lawns, a lawn herbicide labeled for the grass type you have can be applied to help with control. Some examples are Southern Weed Killer for Lawns, Speed Zone Southern, Weed Out, and Weed Stop. The problem is once the weeds are large and flowering, like they are now, they are more difficult to kill with these products. These products are best applied between November and March (earlier is better). I would avoid treatments to centipede and St. Augustine lawns during green up. There are two other herbicide options available for most people but they must be done in the fall before you see the weeds. If henbit has been a problem in the past or is now, then this coming fall is a good time to take action. Consumer formulations of liquid atrazine may be applied in November or December to take care of henbit and work excellent (90 – 100% control). The other is a granular product containing pendamethalin, like Scott’s Halts (with no fertilizer) applied in August or September. This product gives good control (80 – 90%).
You can manage henbit and winter annual weeds by grass selection, site management, and maintenance practices. A thick, healthy stand of grass is the best way to prevent most annual weeds. Grasses grow well in full sun (at least 6 to 8 hours of unfiltered sunlight) and poorly at lower light levels. Soil test to make sure the soil pH is right for the type of grass you are growing. Centipede needs a soil pH of around 5.5. All the other grasses we grow prefer a pH of 6.5. The pH centipede likes is 10 times more acidic than the other grasses like. Do not fertilize centipede grass until June in Pitt County. Use a 15-0-15 fertilizer then at the rate of 3 pounds per 1000 square feet. The only other fertilizer needed on centipede is 5 pounds of 0-0-22 or 2 pounds of 0-0-60 per 1000 square feet applied in August. DO NOT FERTILIZE BERMUDA, CENTIPEDE, ST. AUGUSTINE, or ZOYSIA LAWNS NOW, IT IS TOO EARLY. This includes granular weed and feed products. Fertilize Bermuda in May and July with 6 pounds 16-4-8 (or similar analysis) fertilizer per 1000 square feet for a medium maintenance lawn and also in June and August with the same for a high maintenance lawn. Fertilize St. Augustine and zoysia with 3 pounds of 16-4-8 in May and July.
Mowing is an important management technique to prevent weeds. All the warm season grasses should be mowed to a height of 1 to 2 inches without scalping (leaving brown areas after cutting) the grass. I prefer to start as low as possible in the spring to help with soil warming and green up and raise the height as summer comes on to help with drought prevention. The higher the grass is mowed within its recommended range the more drought-tolerant it will be. If the lawn is not damaged by drought, it will be thick and have fewer weeds. To prevent drought damage, water only when you see signs of grass wilting and apply one inch of water. Do not water again until wilting occurs.
If you follow these recommendations you will have a lawn with fewer weeds like henbit this time next year. If you like the look of henbit in your lawn and enjoy the pretty purple flowers then ignore everything I have written here. If you need more gardening information contact the Pitt County Extension Master Gardeners at 902-1705 or email firstname.lastname@example.org. You can find more gardening information and links to our social media sites at http://pitt.ces.ncsu.edu.