Take 6-inch cuttings from semi-mature wood in summer with clean, sterilized pruners. To use the cuttings for the propagation of bottle trees, you need to pinch off the leaves on the lower half of the cutting and remove any flower buds. When you are growing callistemon from cuttings, cover the cuttings with plastic bags to hold in moisture. Watch for roots to form within 10 weeks, then remove the bags. At that point, move the cuttings outdoors in springtime.