Which type of dogwood tree are you wanting? Dogwoods can of course be started from seed but cultivars must be propagated from cuttings and probably most are grafted onto rootstock. The most popular species of dogwood, Cornus florida is particularly susceptible to a number of pests and diseases. Many cultivars have been bred to be resistant to one or more of these. So, if you want a C. florida that is resistant to dogwood anthracnose or powdery mildew, you'll probably have to buy one. Many of the disease resistant varieties are patented.