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?
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.
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.
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.
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.