(WZZM) Greenthumb Rick Vuyst says it's sheer madness trying to remember which plants to trim back in the Fall and which ones to "leaf" alone.
Rick prefers to leave his alone, but if you want to trim back your Buddleia, Rick says it's fine. It's completely up to you. Often they'll act like a perennial and die-back down to the ground on their own.
When it comes to Hydrangeas, it depends on which type you have. A Macrophila Hydrangea you want to leave alone because next year's blooms are on this Fall's branches so if you cut it back now, you won't get any blooms next year.
But on the Pannacle Hydrangeas which bloom on the same year's wood, it's fine to prune it back right now.
Ornamental grasses can also go either way. Rick likes the way they look in the snow, so he recommends you leave them alone. Rick recommends pruning it back in March and April. But if you want to do it now, make sure you cut them hard because if you leave a lot of stubble on them, you will get the halo effect, which is shorter in the middle.