I have about a gallon of premixed Roundup in a two-gallon garden pressure sprayer that I mixed about six months ago and did not use. It has been sitting in my garden shed since June.
With the glorious warm weather we're having in SoCal this weekend I thought I'd hit a few weeds that sprouted during the recent rains, but I don't know if this six-month-old Roundup is still good. I've read the package instructions carefully and it says nothing about storing premixed product. It does say that Roundup becomes inert ("deactivated") when it hits the soil, but nothing about the effectiveness of leftover mix. I vaguely recall some of the groundskeepers at work mentioning that mixed Roundup slowly becomes inert with time, but my memory of those conversations is a bit fuzzy.
It's no biggie to mix up a fresh batch, but as expensive as this stuff is, I'd like to not waste it if it's still good. I thought I'd ask here before calling Monsanto.
Frank