A giant amongst giants – even amongst weeds. That's what giant ragweed is. The huge leaves on the plant in this picture with five deep lobes resembles a hand with five fingers. It's even as big as a hand with five fingers!
Despite all the advances in herbicide technology, controlling giant ragweed remains a tough challenge in some systems, notes Bill Johnson, Purdue University Extension weed control specialist.
Many giant ragweed plants are tolerant to certain herbicides, including glyphosate. Recently Johnson and others discovered that a new strain of resistant giant ragweed has appeared. It shows yellowing and necrosis of leaves very quickly after glyphosate is sprayed, compared to the normal reaction to glyphosate.
However, within a short time, it begins to regrow. The plant survives to affect yields and cause havoc at harvest if it gets as big as this plant pictured here.
It's not known if this plant is even resistant to glyphosate, let alone if it possesses the rapid necrosis trait. What is known is that if there are many of these in a field, thinking about residual herbicides to get a good base for control would be an excellent choice.
You don't have to wait until spring to do that. There are herbicide options for both fields going to corn and fields going to soybeans that can help provide a clean seedbed in the spring, assuming planting happens at a normal time. They can also take down winter annuals which might blanket fields in some areas between now and spring.
Related: 5 steps to 2016 weed control
Even though the weather is cooling down, there is still time to make fall applications, weed specialists say. How late you can apply depends upon the herbicide and the weed you are after.
Consider fall herbicide applications as a possible way to get a head start on stopping tough weeds like giant ragweed next season.