The first thing I would notice when people use weed killers is that they use way too much and over apply the chemicals. This is both harmful to the environment and quite expensive. Most chemical herbicides are best applied by using a knapsack sprayer and when you purchase chemicals they come with detailed instructions on how best to use them and what application rate to use. These instructions can be difficult to follow. There is also a lot of advice and videos on how to mix weed killers on you tube and other websites.
Regarding dilution rates; it is best to dilute your chemical herbicide by 1 part chemical 100 parts water. This is a 1% mix and would be lower strength. For a stronger mix, i.e. to kill larger weed such as Scutch grass and thistles a 2” mix would be better. This is 2 parts weed killer 100 parts water. Working to these percentages will give the desired results without harming the soil or surrounding water ways and will also not cost too much.
An example dilution rate (Roundup / Rambo):
You have a 10 Litre knapsack sprayer:
10 Litres of water =10,000 ml of water
100 ml of Chemical herbicide required of a 1% mix
Or 200 ml of Chemical herbicide required of a 2% mix
10 litres of weed killer mix will be sufficient to kill a large amount of weeds. To apply using a knapsack sprayer spray an area approx 1M wide and continue spraying as you walk covering approx 1M per second (walking pace). Move the sprayer from side to side so as to cover an area 1M wide. When spraying keep the head of the sprayer approx 10 cm above the height of the weeds, this will ensure good coverage while not being too high that could see the weed killer begin blown elsewhere.
When using weed killer be sure that the weather is suitable so avoid using if there is a chance of rain in the next 12 to 24 hours. Also avoid using the weed killer when the plants are not actively growing as the effects will take longer and will not be as successful. I have used some herbicides as late as December and have had moderate success but it would be best to apply between the months of late February and Late October. Avoid using weed killers in windy weather as you may damage surrounding plants. Best time is in the mornings as the weeds then have al day to absorb the chemicals.
You should begin to notice the effects of the herbicide after a week and a half but most weeds will not be fully dead for up to eight weeks after application. It is important to leave the dying weeds in place until after this time to ensure that the weed killer has worked its way right down to the root system of the plants. If the weeds show some signs of life after six weeks then a second application would be recommended and this is quite common for tougher weeds and a certainty with Scutch grass