How long does it take for weedkiller to work?

Mrs B here. Having just finished a paper on AgriChemicials I am going to disagree with you there.

Yes, Mrs B here. Just completed a diplomo in Horticulture and one of the papers was on Agri Chemicials. We did a tuturial on the proper use of sprays and it was an eye opener, especially for this gardener of 30+ years. My Dad was a slap dash poisoner of everything, never ever wore gloves or any protective equipment, and he was not great at following directions or taking advice. He died of cancer and he was not old.
I agree with anyone who has said read the labels (that is what they are there for) and follow the directions. Don't make up your own. I have learned a lot and will be being very, very careful in future after what I have learned. There are always people who are going to be careless with these chemicals, which is a shame, because spray drift harms other people's prize gardens, and over use of chemicals can lead to pests and diseases becoming immune to certain chemicals.
One of the very interesting things about reading the labels, brands like the Just brand contain exactly the same ingredients as the more expensive brands. We learned that the actual chemicals have not changed much in the last 20-30 years, its just that the old way was to spray everything at regular intervals, nowadays spraying is to be used as a last resort.
In out huge garden we never spray any veges, we use companion plantings, and plants that create chemicals insects hate. I am lucky if I spray my roses 1-2 times a year and I have a massive, beautiful rose garden. A little knowledge has certainly made a huge difference to my gardening habits and the state of my garden. A lot of the "knowledge" I was handed down doesn't work in todays gardens.

NO! it really does speed up the "kill"
Been gardening for well over 50 years and while I am careful have never felt the need to use gloves and a mask when mixing weedkillers like "Just" in my residential garden.

PATIENTS like others said a week or so may be longer

I was watching Maggie's garden show a squillion years ago and some guy said to always follow the quantity instructions so the poison will move through the plant to kill it. If it's too strong it will destroy the tissues it's sprayed on without being conducted down to the roots. It sounded logical and I've taken it on board when spraying @#$/thistles

Now one week later, I can see the weeds and grass turning yellow.

Another dumb theory from some "tv expert"
If it takes several days to destroy the plant why would it not get down to the roots in that time.

http://and.lternet.edu/lter/pubs/pdf/pub446.pdf

IF you could be bothered to read the attached paper you might just understand the complexity with which some chemicals work. However, the logic applied in your response indicates that an unlikely scenario. I'll watch with interest to see if the penny drops.

That paper is 50 years old quite probably not very relevant anymore.

Yup! +1

Well I suppose you could say that as I'm over 50 years old - I'm not really relevant any longer either. But you'd be wrong.

It's from a 1974 - I make that 41

Lol, I poisoned a whole lot of weeds around 3 weeks ago before we went away on holiday. Came home after some serious hot weather and yes all dead, but a new crop emerging from the earthquake cracked earth. Patience is a virtue in gardening!

Of course - weedkillers don't kill weed seeds.