Get them as soon as you can identify them!

Roundup or similar poisions work best on green growing plants. If the pLANT
IS HEAT STRESSED, OR IS GROWING POORLY, THEM A WATERING A COUPLE OF (sorry
'bout that!) days before can increase kill rate, but once they have gone to
seed, the seed can remain viable and you will have another bumper crop next
year. If they have set seed and you want to kill them, a spade and cut them
out is the only bloody way!

If you are a regular coffee or tee drinker and only have a small problem,
tipping the left over boiling water from the kettle onto the weeds works
wonders, and is even green to boot!

I have a 400 metre long driveway and the cars tyres carry the seeds up and
down it. I use a 6 litre garden sprayer with a modified tip to give a water
pistol like spray so I don't get spray drift in high winds, and can "drown"
each plant I want to selectively hit, and I only have to do it a couple of
times a year, now that the weed numbers are down. Mind you, I also spray a
few more times each year on other areas of my 160 acre farm for other weed
types.

Hope this helps,
Peter