From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156704-372483
Question: My 3-year-old, 4-foot-tall arborvitae look terrible. The
branches are long and somewhat bare near the trunk and they just don't
have that bright green healthy look they had when I bought them. I
occasionally give them a shot of fertilizer in the spring and summer.
I also tie them up to protect them in the winter. Should I be pruning
them too?
Answer: First of all, I'll describe some of the plant's requirements.
Arborvitae should be planted in fertile, moist, well-drained soil.
They like full sun, though light shade is acceptable. (In very shady
spots they'll lose their internal foliage and become airy and open.)
Certain cultivars are particularly susceptible to winter damage in
your zone, despite winter protection. Now let's talk a bit about
pruning. When the tree is small, pruning is generally corrective, such
as pinching buds and redirecting branches to get the plant to grow in
a strong, attractive shape. In the prime of life the shrub would
probably benefit from some corrective pruning for rejuvination, beauty
or usefullness. In its old age, you just want to keep it healthy
enough to live long, and well. At three years, I would assume yours
fall kind of in between "young" and "prime"! This means you need to be
helping it look good. Evergreens make their growth from buds formed
the preceeding year. Proper pruning or shearing temporarily stops the
active growth of the large new buds, but stimulates the sprouting and
growth of many of the smaller ones that lie dormant all over the
branches. Because of this, when you shear off the new growth, the
tree's energy which would normally be directed towards those few
large, buds, now is redirected into the thousands of little twigs, and
you force the tree to grow bushier! For the best results, begin the
first shearing soon after the growth starts in the spring. Don't allow
the tree to make a lot of growth before you shear it, or the smaller
buds will remain dormant. Also, when you cut during the early part of
the growing season the cuts will heal quickly and new buds will form
where the cut was make. These new buds will grow in the spring, hiding
the shearing wounds. If you shear too late in the season, the newly
formed buds will be cut off leaving unsightly cut stubs showing all
year. Finally, consider mulching the plants each spring with a layer
of compost.