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Herbicide labels often recommend spraying weeds at three to four inches. Here are tips on identifying small weeds.

Tom Bechman 1, Editor, Indiana Prairie Farm

February 2, 2016

3 Min Read

When giant ragweed is six feet tall and full of pollen, it’s pretty easy to know what weed you have in the field. When Palmer amaranth is fully grown with a 20 inch seedhead or longer, it’s easy to know you have  a problem. What about when these weeds are smaller? How do you identify waterhemp, for example? Bill Johnson, Purdue University Extension weed control specialist, expects it will be one of the problem weeds in Indiana this year.

How do you tell it from a close cousin, redroot pigweed? And why do you care which one it is anyway?

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The last questions is easiest to answer. Waterhemp may be tougher to kill than redroot pigweed with certain herbicides, for example. And waterhemp is resistant to some herbicides that still control pigweed.

To help on identification when weeds are small, get a copy of Purdue University’s Corn & Soybean Field Guide. The 2016 edition is available. You can order it at: edustorr.purdue.edu, or by calling 1-888-EXT-INFO.  Inquire about versions for iPads, and newer versions coming for use on smartphones. The iPad version is especially useful in identifying weeds since you can see them in a larger format than in the Guide book. There is a charge for downloading the app that puts the Guide on your iPad.

Here is a look at some young weeds you may find this spring, and which you will want to identify. Identification information is taken from the Purdue guide. Pictures were provided by Purdue’s Johnson.

How to identify weeds at very early stages of growth

Welcome to waterhemp- It resembles both pigweed and Palmer amaranth, but hairs. Leaves are long, ovate to lanceolate shape and have short petioles to the stem.

Many herbicide labels for post-applications talk about spraying weaterhemp by or before it is three to four inches tall. The problem is many of us aren’t used to identifying plants this small. It’s also easy to underestimate plant height of weeds at this stage unless you actually get out a tape measure. Learn to recognize the weed at this stage.

How to identify weeds at very early stages of growth

Meet redroot pigweed - Even at this stage, it has identifying marks. True leaves are rough with prominent veins . Cotyledons, when they first emerge, are lanceolate in shape.

This weed can still be controlled with many soil-applied residual herbicides, and does not have resistance to many post products that its’ cousins do. It’s relatives include waterhemp and Palmer amaranth.

Stems are reddish, erect and also rough. Leaves typically have a notched tip that is visible if you look closely.

How to identify weeds at very early stages of growth

Palmer amaranth leaf - The trademark that separates Palmer amaranth from its cousins most clearly is the long petiole. This is the section from the base of the leaf that attaches back to the stem. Note how long it is on this young leaf. Palmer amaranth leaves are diamond to ovate shaped. This species does not have hairs either on stems or leaf surfaces. If you find a plant that resembles a pigweed with leaves that look like this one, you’ve won a lottery of sorts you don’t want to win! Palmer amaranth is a mean competitor with individual plants capable of producing several hundred thousand seeds per plant.

About the Author(s)

Tom Bechman 1

Editor, Indiana Prairie Farm

Tom Bechman is an important cog in the Farm Progress machinery. In addition to serving as editor of Indiana Prairie Farmer, Tom is nationally known for his coverage of Midwest agronomy, conservation, no-till farming, farm management, farm safety, high-tech farming and personal property tax relief. His byline appears monthly in many of the 18 state and regional farm magazines published by Farm Progress.

"I consider it my responsibility and opportunity as a farm magazine editor to supply useful information that will help today's farm families survive and thrive," the veteran editor says.

Tom graduated from Whiteland (Ind.) High School, earned his B.S. in animal science and agricultural education from Purdue University in 1975 and an M.S. in dairy nutrition two years later. He first joined the magazine as a field editor in 1981 after four years as a vocational agriculture teacher.

Tom enjoys interacting with farm families, university specialists and industry leaders, gathering and sifting through loads of information available in agriculture today. "Whenever I find a new idea or a new thought that could either improve someone's life or their income, I consider it a personal challenge to discover how to present it in the most useful form, " he says.

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