I am so jealous of your ferns - where I live it's too hot and dry for
an extensive bed except in deep shade...
I totally understand that you're frustrated with the way your fern
beds look in the dormant season. This is a common problem with
perennial beds. Mulching is not only a good solution, it'll be good
for the ferns. Mulch helps retain moisture and heat during the
winter, and as the mulch decays it returns nutrients to the soil that
may leach out over time.
Here are some websites that talk about mulching (and general care of) hardy ferns:
Yardener.com: Caring for Hardy Ferns
"Ferns benefit greatly from mulch. A 2 to 3 inch layer of of an
organic mulch, such as chopped leaves or chopped straw, spread on the
soil over their shallow roots cools the soil in summer. This will also
discourage weeds and help keep soil moist. Remove the mulch in the
fall after the fronds have died back. Then, after the ground has
frozen solid, lay a 2 inch layer of organic mulch to protect the roots
from winter heaving. Remove this mulch in the spring, about four weeks
before the last expected frost, so the soil can warm up faster."
Munchkin Nursery: Care of Ferns
"Always mulch after planting and keep the ferns mulched. Chopped
leaves, hardwood, compost, or a combination of these materials are
essential for retaining moisture and helping to keep the root zone
cool. Both are basics to success with ferns."
Clemson Extension: Hardy Ferns
"A 2- to 3-inch layer of leaves or pine straw, applied in the spring
and in the fall, is an excellent mulch for ferns. Ferns grown in
wooded areas benefit from the falling leaves and pine needles in the
Perennials.com: Succeeding with Hardy Ferns
"It is a good idea to mulch around your ferns with compost or leaf
litter once a year. This will improve the soil, keep the roots cool
and help to retain moisture. Where winters are very cold, cover ferns
with boughs or mulch in the fall to protect them."
I found these websites by doing a Google search for "ferns mulch".
These were the top ones, and I liked them because they come from a
variety of sources. I tend to read the extension agency materials for
my area for gardening advice, so you might want to check out
http://ohioline.osu.edu/lines/hygs.html, the Yard and Garden section
of OhioLine (bulletins, fact sheets, and information from the Ohio
State University Extension Agency and the College of Food,
Agricultural, and Environmental Sciences).
I hope these will help you take care of your fern bed. Enjoy, and
happy spring! Please feel free to let me know if you need/want more
information on this topic -