Roselle will typically begin producing flowers in the fall. October - November is a good rule of thumb when planning your garden as you will not pull up this plant when your summer garden winds down. You can tell when a roselle plant is getting ready to yield after it starts blooming.
Harvesting is fairly easy and straight-forward. After your plant begins blooming, the petals will drop and, between day six and ten, and you will see that several deep, red calyces have formed along the branches. To ensure you arent picking them prematurely, you can look at the pointy, red calyces. When the calyx is an inch wide around the seed pod, it is time. These can be harvested with scissors or pruning shears and placed in a bowl to process. The more you pick, the more your general yield will increase.
If you are planning on making tea, you dont dry the whole cutting as the calyx is on the outside and the inside includes a seed pod which is too hard to be useful. Simply remove the outside leaves by peeling and dispose of the pod.
Roselle does best if it is dried immediately after harvesting. The longer you wait, the higher chance it will lose its texture and develop mold. Best practice is to begin processing within 24 hours.