Giant Ragweed Control
The most effective approach on giant ragweed is to make an initial post application when plants are small – 4-8 in. – and then make a second application about three weeks later. Delaying the initial application until plants are larger in order to control late emerging plants usually results in less effective control. OSU research has shown that the single-application approach on larger plants results in a higher frequency of plant survival, and higher numbers of seed returned to the soil seedbank. In populations that still mostly respond to glyphosate (at least 80% control/suppression with the first application), use a glyphosate rate of 1.5 lbs. ae/acre in the first post application, and 0.75 lb./acre in the second application. Populations resistant to glyphosate are usually also resistant to ALS inhibitors, and the most effective strategy for these will be to mix glyphosate with Flexstar (or use Flexstar GT). Use the highest labeled Flexstar or Flexstar GT rate for your geography, and include crop oil concentrate or a methylated seed oil. Flexstar is usually about an 80% herbicide for control of giant ragweed, so it’s possible that a later application of Cobra may be needed.