Only if it is wood ash not if it has coal or anthracite ash in it. Even with wood ash it isn't going to do a great deal of good.

Yes, only twigs would contain a little more potash which should be stored and used in the late spring. Be aware that wood ash should not be used on lime-haters as wood ash will contain some calcium.

A bit dicey for roses on limey soil, too--they could get chlorosis from too much alkali.