Even if you use a liquid feed, something like Miracle Gro for roses, you'll still need to dilute in a watering can and water around the roses but not on them.

MG I think Moatrat is wondering if she applies granules on the surface of the bark whether they will need washing in with a hose. I'm guessing she already has the granules.

Yes I have the granules

Well they would wash in eventually with the rain but helping them along with the hose would speed things along. Just a thought - if the fertilizer remains in the bark it might encourage weeds to grow into it?

A local weather report in lieu of a garden hose would be best. Check for rain (good soaking) forecasts which will occur within 24 hours and do a light sprinkle of fertilizer every time and right before this opportunity occurs. As far as weeds and fertilizer my experiment where I placed fertilizer in one section of my garden and in another section no fertilizer was placed showed no difference in the growth and density of weeds. Weeds always win it seems.

Thanks to everyone

The best thing to do is gently rake away the bark around the base of the roses, apply the fertilizer, then rake the bark back again. I wouldn't, though, recommend fish blood and bone - it attracts cats, dogs, rats, foxes and any other creature that's around, and is best used under the ground. Your roses will be fine if you don't feed them at all, but the best thing for roses is actually Toprose, which is a powder/granular feed which you'd apply in the same way. That, combined with a topdressing of composted manure, is optimum for roses, though actually, a topdressing of just the composted manure will do on its own. I suspect that will be more difficult for you than scraping away some bark and sprinkling Toprose round the base though.