Fall is the time to tackle some tough weeds
I get a fair number of e-mails from yardeners who are completely flummoxed by a weed in their lawn that they have been fighting with little or no success. There are four weeds that most often are noted in these pleas: white clover, violets, yellow nutsedge and good old creeping Charlie (ground ivy). These are tough-to-kill weeds that are not effectively controlled by the common broad-leaved herbicides that contain 2,4-D. Nutsedge and ground ivy are best controlled in the spring when I will cover those details. Now is the time to attack clover and violets.
These weeds appear because we are doing something wrong. We are using too much water and fertilizer. We are not overseeding periodically. We are mowing our grass too short. Tall, thick grass that is watered and fertilized properly seldom has any of these weeds.
Some yardeners don't mind having some white clover with its little white blossoms in their lawn. It stays green and doesn't grow too tall. However, there are many who want just grass in their lawn and struggle to keep white clover at bay.
White clover is persistent because it is a perennial with two methods of spreading in the turf. Existing plants make above ground horizontal stems that creep along and produce roots as they move over the soil. To ensure their existence those white flowers produce a hard-coated seed that is capable of living in a dormant state for many years.
The best time to deal with this pest is late summer into early fall.
The familiar low-growing common violet, with its blue or purple flowers and heart-shaped leaves will grow in the sun or the shade. They are tough to control because they have a very vigorous root system and they reproduce with rhizomes or stolons or by seed -- a tough combination to control.
The best time to treat violets is mid-September to mid-October.
There are two approaches to dealing with these two tough weeds. You can kill them with an herbicide that contains glyphosate (RoundUp or KleenUp). This product kills everything it touches so you will also be killing the grass growing in among the weeds.
The other approach is to use a product that will kill both of these pest weeds but not harm the turfgrass. Ortho has two candidates -- Ortho Weed-B-Gon Max (Trimec) and Ortho Weed-B-Gon Chickweed, Clover and Oxalis Killer (Triclopyr). Bonide offers Bonide's Weed Beater Ultra (MCPA, Mecoprop-p, Dicamba, and Carfentrazone) and Bonide's Chickweed, Clover, and Oxalis Killer (MCPA, Triclopyr and Dicamba).
Weeds must be actively growing in order for any herbicide to work.
To be sure of success, it is likely that any of the herbicides listed above will need to be applied twice, separated by two weeks. Two weeks later, either overseed the whole lawn or spot seed in the spots that had the weed problem.