Which weeds would you put on your top five “most wanted” list? Everybody’s list might be different. According to Bill Johnson, Purdue University weed control specialist, the five that make the list in his part of the world in Indiana are waterhemp, marestail or horseweed, Palmer amaranth, giant ragweed and common ragweed.
Each of these weeds has its own characteristics that make it worthy of appearing on a “most wanted” poster. Yet all five share a couple of things in common. “They’re often easier to control if you’re including residual herbicides as well,” Johnson says. “Take care of as many as you can before they come up.
“Second, if you’re going after them postemergence, your best bet is to spray while they’re small. That’s often 4 inches tall or less, which is smaller than most people normally visualize when they think about spraying weeds postemergence. Many labels specify spraying weeds that are a maximum of 4 inches tall, depending upon the species.”
5 tough weeds
Here is a closer look at Johnson’s five “most wanted” weeds. Descriptions are based on information provided by the Purdue University Corn & Soybean Field Guide. To see photos of these weeds, look at the accompanying slideshow.
1. Waterhemp. Plants are similar in appearance to pigweed and Palmer amaranth when small. Look for long ovate to lanceolate-shaped leaves, with short petioles and no leaf hairs.
“Waterhemp can continue germinating well into the season,” Johnson says. “You can’t wait for all to emerge. Adding a residual with the post treatment helps get later emergers.”
2. Marestail. While some Hoosiers call giant ragweed horseweed, weed scientists sometimes designate marestail as horseweed in literature. Leaves are alternate on the stem, linear and simple, with either entirely or slightly toothed margins. Leaves on mature plants don’t have petioles.
“Your best bet is to spray before they bolt,” Johnson says. If you’re hitting taller marestail with post products, the best you likely can hope for is burning them back.
3. Palmer amaranth. Similar in appearance to pigweed and waterhemp, leaves will be diamond to ovate-shaped. Compared to waterhemp, petioles attaching leaves to stems are much longer. You won’t find hairs on stems or leaf surfaces.
“They can jump in size overnight, so scout them out when small,” Johnson says. While Palmer amaranth can be anywhere, it shows up more often in northern counties, especially in drier years.
4. Giant ragweed. True leaves have three to five lobes and are slightly hairy. When plants are small, cotyledons are thick and fleshy. Stems grow coarse and hairy.
“Some giant ragweed populations are resistant to more than one herbicide family,” Johnson says. “Glyphosate resistance is common.”
5. Common ragweed. Look for deeply lobed, slender leaves. Cotyledons may be rounded, thick and short. Leaves may be hairy, and stems are rough to the touch.
“Common ragweed won’t usually get as tall as giant ragweed if let go, but they are still very competitive and can hurt crop yields if not controlled,” Johnson says.