Birch trees can be grown from seed, but it is a long process. North Carolina State University recommends the rooting of stem cuttings as a means of propagation for woody ornamental plants and some species of trees, including elm, crape myrtle and birch. With a little practice, any home gardener can do it, without the benefit of a greenhouse or growing room. If you're taking cuttings because a birch tree is in decline, be sure to take cuttings from the healthiest area of the tree.
Plan to take cuttings from soft, new wood in May or June, in early morning when the birch's sap is least mobile. Fill medium-sized containers with moist, soilless potting mixture and set them aside. Fill a bowl with ice cubes and moisten a few paper towels, and keep them on hand.
Cut several 6-inch, soft wood stems from healthy birch branches. Test twigs with your fingers. They should be flexible and light, greenish brown, where last year's wood will be stiffer and dark. Cut the stems at a 45-degree angle, not straight across, to leave a large surface for absorbing rooting hormone and moisture.
Remove leaves from the bottom third of the cuttings. Wrap cuttings in wet paper towels and stick them in the bowl of ice to keep them from drying out before you can root them. Don't let them stay on ice for more than a few hours.
Dip the bottom third of the cut ends in rooting hormone, and tap off the excess. Stick the birch cuttings into the soilless potting mix to a depth of 2 to 3 inches. Pat the mixture down with your fingers to eliminate air spaces and ensure good contact.
Place the containers into clear plastic bags and secure the tops with twist ties. Set the bagged containers in indirect light, in a warm room. Open and mist cuttings daily with a spray bottle full of water, resealing the bag to keep humidity high inside. Test the cuttings after four weeks. If they resist a light tug, they are establishing good roots.
Don't use bread bags for bagging cuttings. Residue from breadcrumbs will react with moisture and heat, causing mould to develop inside the bag.
Be careful to avoid direct sun on bagged cuttings. Sunlight will spike the temperature inside the bag and roast your birches quickly.