I think it all depends. Pre-existing plants and landscaping, it's pretty easy to do a top dressing of compost. I'd like to do it just before a big rain, in the root circle plant. Remember the sprinkle it inn a ring around the main stem so it hits the entirety of the root system. If you wanna fun project, you can make some compost tea and use your watering can to get it to the roots versus waiting for nature in the rain. Good luck.
