Every summer, the big plant companies that develop and sell wholesale plants hold trials in different regions of the country. For example, retailers in the Midwest attend trials in Michigan, where the new plant introductions are grown outdoors en masse so that the different varieties can be viewed and compared.
I have seen photographs of the colorful displays. Retail nursery owners visit and then compile lists of the plants that they want to order for the following spring season. Their chosen varieties of patented plants arrive in the retailers' greenhouses the following winter as plugs that the greenhouse staff will grow on until they are mature enough to be sold to home gardeners to plant outdoors once all danger of frost is past.
Retailers usually order a mix of tried-and-true annuals, such as wave petunias and dragon's wing begonias, as well as the newest varieties of annuals that have just been patented. Nursery owners cannot lawfully take cuttings if a plant is still under patent; they have to buy plugs of those.
Regulation of the plant industry is strict, and inspectors arrive without warning at greenhouses to inspect the stock to ensure that there are no patent violations. Since it takes years of work for plant propagators to develop new varieties, those that turn out to be popular with the public must be under patent for a number of years in order for those who developed them to make a profit.