One of the best domestic weedkillers in existence is sodium chlorate. Very effective on a small area for a short time, no long term effects on the environment as it just degrades into salt.

Problem is it's rather easy to use it as the basis for seriously effective explosives (you don't want chlorate itself since perchlorate is more powerful and safer, but making perchlorate from chlorate is not difficult). So it's been made unpurchaseable under Prevention Of Things That Go Bang legislation. The last lot I used must have been sold more than 20 years ago.

I think it may be possible to make it yourself by electrolysis of hot salt water. I haven't tried but I'm sure ISIS and the EDL have figured it out.