A few weeks ago I visited the nether regions of my back garden where the
dreaded horse parsley was getting a worryingly vigorous foothold. I sprayed
it with weed killer, and wherever I saw any that was flowering, I hacked it
down for disposal (because of the seeds). I decided to put all of the
hacked-down material in a large lidded bin, full of water, which will
hopefully result in some useful liquid fertiliser. Does submersion
generally kill seeds? If so, how long does it take?

Many thanks,

Al