Your Katsura tree is a great one to build a garden under as the form of the tree casts a delicate shade that some perennials just love. I would suggest hellebores and snowdrop bulbs for winter and early spring color, primroses especially the more slug resistant English primroses such as 'Wanda' and the cowslip primroses along with corydalis for later spring color and astilbe and the Japanese Fountain Grass for summer color. Be sure to add compost and mulch the plants to conserve moisture and I can see you flower filled but low maintenance shade perennial garden being a success. Keep growing, Marianne Binetti P.S. I just made a video that will posted to YouTube soon about my shade perennial garden and it may be helpful to you.