Annual Dahlias. This tropical genus brings the mid-to-late-season garden a never-ending succession of flowers in glorious shades and shapes. Dahlias vary from under 12 inches to 7-foot-tall giants; their flowers may be demure buttons or huge, blowsy blooms. Most are sold as tubers and planted in well-drained soil after the last spring frost. In our garden, we like to install Dahlias when we pull out the spent Tulips, and sometimes we even remember to pot them up a few weeks earlier to advance the first blooms. Please take a few minutes to view our video, "All About Dahlias."