Northern Nevada gardening and landscaping: Let’s talk killing weeds with herbicides | Carson City Nevada News
Show full screen
A man applies herbicides to a weed while wearing the proper protective clothing. File photo, UNR Cooperative Extension
My friend John wants to know if it’s time to spray weeds. Green things are definitely sprouting. This is a more complex question than it sounds, so I thought I was reviewing information on weeds and herbicides.
Weeds can be annual plants that grow and die in one year, biennial plants that grow in the first year but bloom and sow in the second, or perennials that come back year after year. Some weeds are deciduous (dicots) while others can be grasses (monocots). Not all weed killers, also called herbicides, kill all weeds. Therefore, it is important to understand these terms when reading a label.
Most herbicides are post-emergence chemicals that only kill weeds that are actively growing, not the seeds or seedlings that are still in the soil. It takes a pre-emergence herbicide applied at the right time of year to control these. Some herbicides have long remains in the soil and will sterilize the soil for years. While this sounds like a good idea from a weed control standpoint, sterilants move easily through the soil with water movement, often damaging and killing trees, shrubs, flowers, vegetables, or lawns.
Some herbicides are selective only for deciduous plants or only grasses. Nonselective or broad spectrum chemicals kill many types of plants, but not necessarily every green plant. Controlled plants are listed on the herbicide label. Herbicides can be systemic if the chemical moves from the sprayed leaf surface through the plant and kills it. Or they can only kill contact if the chemical only kills the leaves it touches.
I asked John’s wife Gina what product they usually used on their mornings. You use a common concentrated herbicide with the active ingredient “glyphosate”. This chemical is a non-selective, post-emergence systemic herbicide with no residual soil activity. It kills many plants, both foliage and grasses, annual to perennial, trees, shrubs, lawns, flowers and vegetables. Extremely cool or cloudy weather will slow the activity of this product.
Plants should not be pruned, grazed or mowed first, otherwise the product will not work well. Rain or watering shortly after application can wash off the product and prevent control. Don’t spray on a windy day as drift can kill non-target plants. Weeds 6 inches or less in size are easier to control than larger weeds. Larger, more mature weeds require a more concentrated formulation, which is stated on the label.
The weeds John and Gina are trying to control are Kochia and Purslane, both of which are listed on the label. I suggest they wait to spray until the weeds are at least 2 inches high and the weather is above freezing for about a week at night.
– JoAnne Skelly is Associate Professor & Extension Educator Emerita at the University of Nevada Cooperative Extension. She can be reached at [email protected]