Posted: 21/01/2014 at 16:57
What sort of weed are you having a problem with? If they're perennial weeds then spraying with glyphosate when they are growing strongly and then leaving them to die down and go brown will kill the roots as well as the green tops. You can then plant your lavender bushes - glyphosate has no effect on the soil.
If you're struggling with lots of little germinating weeds from seeds in the soil (they'll look a bit like the mustard and cress you get in punnets from the greengrocer), then you need to hoe the beds regularly, at least weekly in the growing season, until most of the weed seeds have germinated and you've hoed and destroyed them - then plant your lavender. You'll never get rid of all the little weeds and will need to hoe gently between your shrubs, or get on your hands and knees and hand weed while the weeds are still small.
And yes, lavender needs good drainage - I find horticultural grit dug into the soil is better than sand.