how soon after putting weed killer down can I sow seeds?

[Jodee]

there are too many weeds to completely de-weed by hand, I want to put some sort of weedkiller down and then throw down some flower seeds, if I don't completely get rid of the weeds I won't be able to tell the difference between weed and shoot when they come through - any advice please?

Depends what weed killer you use. Best bet is probably to get something with a glyphosphate base (roundup is a name that springs to mind, but there are plenty of others) as this is a systemic weedkiller that is absorbed through the green growth it's applied to. It will take a while (10 days or so) for the top growth to die off, and the root systems should follow suit. Depending on what weeds you have you may need repeat treatments (brambles, nettles, bindweed, ground elder and creeping thistle are all pernicious things that will keep coming back for more...)

As for differentiating weeds and seedlings, best bet is to plant seeds in rows. They should all germinate at a similar time, so you'll get rows of seedlings all looking the same - c/w the random scatter of seeds blown in from weeds. You do start to recognise weed seedlings pretty soon, but if you're really concerned then stick the seeds in trays and plant them out later as mini-plants.

Good luck

Thank you Tangle!!