The mop cypresses are threadleaf varieties of Japanese false cypress (Chamaecyparis pisifera). Their loose, floppy habit and global form give them their common name. Depending on variety, mop cypress grows in U.S. Department of Agriculture plant hardiness zones 3 through 7. This evergreen mostly takes care of itself and requires only occasional pruning.
While mop cypress doesn't require annual pruning, mid- to late spring is a good time to remove the odd stray branch that obstructs a walkway, obscures a view or otherwise doesn't contribute to a pleasing, round form. Using sharp bypass pruners, cut the errant branch all the way to the center of the plant or to a healthy, outward-facing shoot. Sterilize the pruners with disinfectant spray or rubbing alcohol before and after each use to prevent the spread of disease.
Shrubs that have grown leggy or out of control, as well as those that have lost their needles due to disease, will benefit from a hard pruning. In spring, remove the oldest and gnarliest one-third branches all the way to the center of the plant. Prune the remaining branches back about one-third, cutting at outward-facing side branches. This will likely leave a bare spots, but they will fill in with spring's growth spurt.
Shearing Is for Sheep
If you want to shape mop cypress for hedging, avoid shearing, which results in a dense, leafy canopy supported by bare, spindly branches underneath. Cut by hand in mid-spring and mid-summer. Each year, you should remove a total of about one-fourth of the branches almost to the center of the plant. This exposes the center to sunlight and encourages leafy growth along the interior branches. Colorado State University Extension recommends that the bottom should flare out wider than the top so the whole plant gets even sunlight. Expect a loose, billowy hedge, rather than a tightly clipped wall.
University of Minnesota Extension recommends a complete fertilizer slightly higher in nitrogen than phosphorus and potassium. N-P-K ratios of approximately 10-8-6 do the job. For a mature mop cypress, scratch 1 cup 10-8-6 into the mulch beneath the canopy. Use half that for younger specimens. An inch of organic mulch or compost shades plant roots, keeps the soil moist and enriches the soil as it breaks down. Remove dead or diseased branches at any time of the year, as soon as you find them. This prevents spread of the disease and makes room for new growth come spring.