There are actually two different types of plant commonly referred to as hibiscus. Tropical hibiscus (Hibiscus rosa-sinensis) is only hardy in climates without frost. In most places in the U.S. it is grown as an annual or in a container and overwintered indoors. Hardy hibiscus (H. coccineus or H. moscheutos) are hardy to U.S. Department of Agriculture planting zone 4 and are commonly grown as perennials. Both plants benefit from heavy pruning.
Pruning Hardy Hibiscus
Hardy hibiscus can actually take quite a bit of pruning and there are a few different theories about what to cut and when. You can deadhead the first flush of flowers to encourage new blooms later into the season. According to Clemson University, cutting the plant back severely, to one-third its height, after the first flush of flowers encourages a second flush. University of Illinois Extension horticulturalist Linda Naeve advises cutting hardy hibiscus back to 1 foot when they reach 2 feet -- before first blooming -- encourages more blooms when the plant finally does flower.
Other Tips for Profuse Blooms
Full sun is a must for vigorous blooming. According to Clemson University, hardy hibiscus grows in part shade, but it may not bloom as much. Well-drained, moist soil, rich in organic matter is also important. Use high-nitrogen fertilizers until July, then switch to fertilizers high in phosphorous to promote blooming. Cut your hardy hibiscus down to the soil after the first frost to ensure healthy growth the following season.
Pruning Tropical Hibiscus
Deadheading is not necessary on tropical hibiscus. According to the University of Minnesota, flowers and seed pods drop off on their own within a day or two of flowering. They do, however, benefit from heavy pruning. In late winter or early spring cut plants back by one-third. This promotes bushy growth, as opposed to leggy, spindly growth.
Care Tips for Tropical Hibiscus
Set tropical hibiscus outside when all danger of frost has passed. Initially, set it in a sheltered location for only a few hours a day to help it acclimate to the outdoors. Gradually increase its exposure before setting it in full sun. Reverse the process in the fall, gradually reducing sun exposure. Several cultural conditions, if not met, can result in bud drop and fewer flowers. Temperatures lower than 65 degrees Fahrenheit or higher than 75 F can cause bud drop. Too much, or too little water can also affect blooming. Keep the soil moist, but not waterlogged. Ensure the container you use has adequate drainage holes. Plants left in the shade also produce fewer blooms.