I also grow monkshood or aconitum in my shaded bed and it thrives in rich, moist soil - if allowed to dry out this tall, blue meadow plant will get powdery mildew and root rot. Sounds like your plants may be suffering from a dry period in the summer. Late fall or early spring is the time to dig and divide the clump and restart the roots for a fresh start. Cut back the tall stems to ground level in late fall so no mildew spores will overwinter. Mix some compost into the soil and add a mulch in the summer to keep the soil moist. In the spring you get a chance at a fresh start and mildew-free plants. Isn't great that gardening offers the chance to redo and do it better? Keep growing, Marianne Binetti P.S. I grow hosta at the base of my aconitum as they both like shade and moist soil.