I ask because i left a container of one (R**nd*P) in the shed, over the winter. I tried using it this Spring, and the weeds seem unaffected.
Does the active ingredient break down and become useless (as a herbicide)?
It does, but I’ve had pretty good luck with the product you’re refering to after just one winter in the shed. (Two winters really does weaken it.)
Here’s a couple of things you’ve probably already tried:
Make sure the surface of the weeds is dry before you spray. If it rains within, say, 12 hours you may have to spray again.
Make sure you really, really shake it up before use.
Check and see if there are crystals or sediment in the jug.
If it calls for mixing with water, use less water than called for.
Even if the herbicide does seem to have broken down. Don’t throw it out. Instead, hit the weeds every day with it until they die or you run out.
I don’t know about glyphosphate specifically, but other herbicides are broken down by microorganisms in the soil. I did my final year dissertation on the bioavailability of 2,4-D (2,4-Dichlorophenoxyacetic acid, a commonly used broadleaf herbicide) in soil and found that 2,4-D was broken down quite readily in soil by the natural microbial flora (I didn’t have to inoculate the soil with 2,4-D eating bacteria). I actually used a solution of 2,4-D as growth media, so depending on how concentrated the solution of herbicide is in your bottle, you could have a bug in there using it as a nutrition source. It was over 10 years ago now, so I can’t be anymore specific in the info without searching the loft for my old lab book (I’ve no idea where it is).