Many of my hostas Krossa Regal, Sum and Substance and others (are not as tall as they were last year. We have had a cold spring (zone 5B WNY). Does a cold spring, lack of sun, soil moisture, etc. effect the height of the plant? Why actually does. Hostas look healthy.
Yes, to all of the reasons you list. I'm also in a cold zone 4b and we are so far behind. Some of my Hosta have just barely broken ground. Provide ample moisture and give them time!