Fir trees have a natural conical shape that extends to the ground, making them excellent trees for planting as part of a windbreak. Pruning techniques for fir trees differ according to your goal. It can be as simple as removing the lowest branches for mowing clearance or going higher for visibility to a street or sidewalk. Use sharp, clean pruning tools and wipe them clean often to remove the sticky pitch that accumulates while pruning. Never prune firs in summer when they're actively growing.
Clean all pruning tool blades by dipping them in boiling water for 30 seconds or wiping them with a cloth dipped in rubbing alcohol. This will keep you from introducing bacteria, fungus and insect eggs to pruning wounds.
Prune fir trees in late winter or early spring, before new growth appears on the branch tips. Fir trees are susceptible to fungal infections. Fungi are dormant at this time of year, so infection will be less likely.
Put on gloves and safety glasses, and walk around the tree. Look for dead or diseased wood. Slice into the bark of a branch you suspect is dead with a sharp knife. If you don't find white, moist wood, prune the branch off at the trunk.
Prune small fir branches at the trunk with long-handled pruning shears. Angle the blades at 45 degrees, cutting down and away. Leave a 1/2-inch angled stub that will drain moisture and decrease the chance of infection.
Cut branches more than 3 inches in diameter with a pruning saw, using the three-cut method. Make the first cut one-third of the way through the underside of the branch, 12 to 24 inches from the trunk. Make the second cut through the top of the branch, slightly out from the first cut. The branch will snap from its own weight. Make the third cut at the trunk, as in Step 4.
Remove the lowest rang of fir branches for mowing clearance. Take off the lowest three rungs of branches for better visibility around and under the tree. Avoid cutting more than one-third of the tree's foliage in any given year for best results.
Cut up prunings and chip them for mulch, or discard them in yard waste bags. Clean and sterilise pruning shear blades before storing or using them on other trees, particularly if the fir tree is diseased.
Train a young fir tree into the classic cone shape by cutting the top of the central leader, or trunk stem, to a length of 8 to 12 inches. If the tree has several competing leaders, trim the others back to a tapered shape, leaving the strongest one as the tallest.
Never "top" a fir tree by cutting off a whole section of its top. This will deform and weaken the tree and perhaps kill it.