Most seeds from landscape plants have a very hard, outer protective shell. Under natural conditions most of these seeds do not receive the proper treatment in order for the seeds to germinate. In eastern Pennsylvania for instance, Rhododendrons and Mountain Laurel grow wild on the mountain sides. Here in northern Ohio Dogwoods grow wild. Of course only a fraction of the seeds produced in the woods actually make it to the point of germination and survival.
Many seeds go through a period of internal dormancy right after the fruit falls from the tree.
In some cases, there is actually a chemical barrier that prevents the seeds from germinating while they are still inside the fruit. The hard protective coating on some seeds was designed by nature to protect the seed, but in many cases this protective coating actually inhibits the germination of the seed because water and air can not penetrate the hard coating. Many of these seeds actually require a double dormancy period before they will germinate. In other words, the seeds must lay on the ground completely dormant for one full growing season, and then germinate the following growing season. During the first season the only thing that is taking place is the outer coating is being softened by the elements.