How do you prevent a weed from getting 2, 3 or 4 ft. tall? You kill it when it is 2, 3 or 4 in. tall. That is the bottom line on weed control, even though the arsenal available to you is potent and efficient. However, the secret on weed control is strike early and hard.
Timing is critical for postemergent weed control, says Purdue Weed Specialist Bill Johnson, as he reminds you that most herbicide labels indicate the greatest efficacy of the product when the weeds are 4-6 in. tall, and grasses even smaller. His latest fact sheet says the taller the weed gets above the label requirements, the more dissatisfied you are going to be with the performance of the postemergent herbicide.
Johnson’s explanation for bigger weeds to repel a toxic shower will soak in for a lot of folks. As spring turns into summer, the weeds grow and the temperature gets warmer. As the heat rises, the weed increases the thickness of the cuticle, which is the nearly impenetrable covering on the plant. And as the cuticle thickens, your expensive post emergent herbicide drips off the weed with little impact.
Your plan for this year should be to take advantage of the additional rain that has caused germination of so many additional weeds, and try to control them while they are still small. But it is necessary to scout your fields and record the species of weeds you have and ensure that your postemergent herbicide is up to the task. That possibly includes additional applications as the label and your crop maturity allow. Johnson says consider how much of the spray will be collected by a corn plant. The bigger the plant, the more spray it will collect and shift around to the parts of the corn plant that are undergoing development at that time. It could be brace roots, tassels or ears, and any of those could be deformed by the increased volume of spray it collects. He says a 4-in. corn plant will collect much less than a 4-ft. corn plant.
Johnson says years like this one with a lot of rain will be difficult to manage weeds, since they will germinate when they want to. However, he says catch them while they are young, and your herbicide will perform much better, something that might even be enhanced with drop nozzles.
What kinds of weeds are in your fields? Johnson and his Purdue colleagues collaborated to come up with 13 of the most likely weeds to emerge late in corn and soybean fields this spring. That list includes: morningglories, waterhemp, burcucumber, fall panicum, common ragweed, giant ragweed, yellow nutsedge, shattercane, crabgrass, lambsquarters, smartweeds, nightshades and barnyardgrass.
The weed specialists say, “A single application, or even a second application of glyphosate or Ignite may not be enough to control some of these weeds. Depending on the weed population mix in an individual field, additional herbicide choices may be needed to control these weeds.” But they add there is a large choice of candidate herbicides that are labeled for them. You just have to apply them at the right time for the weed and the crop. They offer some help with that challenge. But they warn that this may be a year when you apply herbicides twice this summer.
Unusually wet weather has presented a challenge on weed control, by not getting to apply herbicides in a timely manner, but fostering a longer period of weed germination. This season may be a year when more than one herbicide application is made. The smaller the weed, the easier the kill, primarily because it’s physiological defenses have not yet been built up.