Winter Care of Kimberly Ferns
Kimberly Queen fern is considered hardy only in Zones 9 and 10. Where you live in Zone 8, it likely would not survive outdoors unless it is in a very protected location. They might survive outdoors with heavy mulch as protection, but to be safest, you should dig them up and grow them as houseplants over winter.
There are some types of astilbe that grow only about 6 inches tall. Do you know what kind or variety that you have? It's not unusual for a perennial plant such as astilbe to grow little its first year from a bare root planting. It may take 2-3 years to reach good blooming size. Astilbes appreciate constantly moist soil. Were they watered well all summer long? Drought stress could also have limited their growth.