Even though the rose of Sharon is a hardy perennial which can live for decades with little care, this doesn't mean it doesn't benefit from care. If you're noticing slower growth and fewer blooms year after year, a little fertilizer may be all you need. It's easy to fertilize a rose of Sharon and with each rain or watering, the fertilizer is delivered right to the roots of your plant. While you may need a friend to help you measure your plant, the process of fertilizing your plant shouldn't take more than 30 minutes.
Measure the height of your rose of Sharon in feet. If your plant has recently been pruned, it may be only two feet tall, but un-pruned plants may reach twelve feet, so have someone help you if your plant is particularly tall.
Apply one tablespoon of granule 10-10-10 fertilizer for each foot tall in height. This fertilization should be done in the spring by sprinkling the fertilizer around the base of the plant.
Cover the fertilizer granules with a layer of mulch around the base of the plant. Typically two or three inches deep of mulch is sufficient.
Rake back your mulch in midsummer to reapply fertilizer before blooming. Measure the current height of the plant and use the new height to determine the amount of summer fertilizer to use.
Add more fertilizer again using one tablespoon per foot as your guide. Push the mulch back in place to prevent runoff of the fertilizer.