JoAnne Skelly: Let’s talk herbicides

My friend John wants to know if it’s time to spray for weeds. Green things are definitely sprouting. This is a more complex question than it sounds, so I thought I would review information about weeds and herbicides.
Weeds can be annual plants that grow and die in one year, biennials that grow leaves the first year but bloom and seed the second, or perennials, which come back year after year. Some weeds are broadleaf (dicots), while others may be grasses (monocots). Not all weed killers, otherwise known as herbicides, kill all weeds, so it is important to understand these terms when you read a label.
Most herbicides are post-emergent chemicals, killing only weeds that are actively growing, not the seeds or seedlings still in the soil. It takes a pre-emergent herbicide applied at the right time of year to control those. Some herbicides have long soil residual and sterilize the soil for years. While that may sound like a good idea from a weed control standpoint, sterilants move readily through the soil with water movement and often damage, and even kill, trees, shrubs, flowers, vegetables or lawns.
Some herbicides are selective for broadleaf plants only or grasses only. Non-selective or broad-spectrum chemicals will kill many kinds of plants, but not necessarily every green plant. Plants controlled are listed on the herbicide label. Herbicides can be systemic where the chemical moves from the sprayed leaf surface throughout the plant, killing it. Or, they can be contact kill only where the chemical just kills the leaves it touches.
I asked John’s wife Gina what product they usually used on their acre. They use a common concentrated herbicide with the active ingredient “glyphosate.” This chemical is a post-emergent, non-selective, systemic herbicide with no soil residual activity. It kills many plants, both broadleaf and grasses, annual to perennial, trees, shrubs, lawns, flowers and veggies. Extremely cool or cloudy weather slows the activity of this product.
Plants shouldn’t be cut, grazed or mowed first or the product won’t work well. Rain or irrigation soon after application can wash off the product, preventing control. Do not spray on a windy day because drift can kill non-target plants. Weeds six inches or less in size are easier to control than larger weeds. Larger, more mature weeds require a more concentrated formulation, which is 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 four inches or more in height and the weather is above freezing at night for a week or so.
JoAnne Skelly is an Associate Professor & Extension Educator Emerita at the University of Nevada Cooperative Extension. 

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