I have a bag or really old simazine, 20 years? I sprinkled some on an area where I wanted to kill the weeds and eventually plant with flowers. I sprinkled it on full strength to the soil/weeds. I wasn't sure it if was still effective but it was. It has killed everything where I sprinkled it, which was what I wanted. But how long does the simazine last.

I want to use it in a few spots in my unused field, to eventually plant some rudbeckia and coneflowers. So how long after applying the simazine, can I safely plant my flowers.

Thanks to anyone for your input.

The label isn't on the bag anymore, but I know its granular.