Posted: 19/05/2015 at 07:00
The only daft question is the one you don't ask
Broad leaved plants are those that don't have narrow leaves like grasses - so virtually anything that isn't a grass or similar.
Most weedkillers were designed initially for use by farmers - they needed weedkiller that would kill the weeds with big leaves but that would leave the wheat and barley unaffected. This means that they also work for lawns - we can treat the lawn with a weedkiller that will kill dandelions and plantains (broad leaves) and leave the grass to grow.
The problem comes when we use the weedkiller (lawn treatment) near plants that we want - the chemical cannot distinguish between them and the weeds - it affects all non-narrow leaved plants - dandelions, docks and delphiniums are all the same to it.
What I do when I use the lawn treatment that contains a weedkiller is to leave a gap around the edge of the lawn where I don't apply it - any weeds in that area I treat with a spot weedkiller that I can just dab on the offending weed (e.g. Roundup Gel). This way I can hopefully avoid poisoning my pansies
While we're on the subject I'll just add that this is the reason why the first 3 mowings of grass following the lawn treatment shouldn't be put in the compost heap - just in case it doesn't totally break down by the time the compost is used in the garden - don't want the compost to kill our cabbages
Hope that helps