PlantLife Online

Answer

There are several types of cuttings that can be used to propagate plants however the best for Buddleia davidii would be soft wood cuttings or semi-ripe cuttings taken in the summer however hardwood cuttings taken in fall should give you the best chance for success with this particular shrub. 

After the leaves have dropped in the fall select stems about as thick as a pencil of this years growth, the cuttings should be 6-10 inches in length. The top cut should be just past a bud or pair of buds, the bottom should be just below a pair of buds. Using a rooting hormone dip the base (about 1 inch) in the mixture and place the cutting in your prepared rooting media either field or container will work. The best media is course and well drained, course sand with equal parts peat to retain moisture in containers is recommended. Place another layer of soil leaving only the top 1-2 inches of the cutting exposed and firmly pack the soil around the cutting. The cuttings should be protected from wind and other things that may disrupt the process, a cold frame or hoop house is recommended. If you are only propagating a few cuttings use a clear plastic bag with bamboo stakes to build a tent around the cuttings. It will take several months for the roots to develop so be patient.