Well, don't know that we have a "roundup proof cultivar", but even the literature down here states that roundup may not be real affective on mature stands of poison ivy. The most affective "spray" period is mid-late March or early April, right after knew leaf development or immediately around bloom stage, with subsequent applications as needed, depending on stand establishment and maturity. Once the plant has been allowed to progress past blooming, the leaves and plant appear to be much hardier. There is a big difference between a well established stand and a stand that is in the beginning stages of development. My folks just recently spent 150+ dollars on round-up and all they did to an established stand on their lot is make it "kinda" sickly, then it was over. The few sick leaves that existed showed no evidence of chemical treatment after 2 days. They were to the point of wanting to spray it all down with gasoline, but that could be more hazardous than applications of roundup with wild abandon. I recommended they wait until this winter and cut every vine on the property, then hit the new leaf buds with diesel as soon as they appear. They have established vines on their place that are 5 inches thick. It will undoubtedly take them a few years to eradicate these vines from their property. And diesel has proven to be very affective when used in the "optimal" window for doing the most damage. > Leave it to Texas to have a Roundup proof cultivar of poison ivy. Roundup works > worth two or three flips on the vine here in Maryland. :-) > > Ed Flynn Md zone 6/7 But, ya know, things are always bigger and better and hardier in Texas!! ;-))))) Thanks Paul Reynolds Environmental Agronomist Austin Texas