The Hibiscus is a genus from the mallow family (Malvaceae) and is native to Mediterranean, tropical and subtropical areas all over the world.
There are three types of Hibiscus that I would like to mention:
1. the Hibiscus rosa-sinensis (Commonly named tropical hibiscus or indoor hibiscus and sold as houseplants in countries with cold climates)
2. the Hibiscus moscheutos and H. coccineaus (A hardy perennial hibiscus commonly named rose mallow or swamp hibiscus)
3. the Hibiscus syriacus. The Hibiscus syriacus is native to Asia. It is the only shrub from this genus that tolerates frost and is commonly named rose of Sharon, outdoor hibiscus or althea shrub. That last name stems from the fact that it used to be a member of the Althaea family.
The Hibiscus syriacus will grow in any soil as long as it is well-drained, light and humus-rich. It prefers a spot with morning sun. The morning sun is good for the development of flower buds. If the soil is too wet the leaves may turn yellow and flower buds may drop. This can also happen if the soil is too dry. The rose of Sharon is tolerant to heat, humidity, poor soil and some drought. It is best not to let the soil dry out completely. In southern USA it is best to place the rose of Sharon in a spot with morning sun and protection from intense afternoon rays.
The best time to plant or move the Hibiscus syriacus is in spring. In the southern regions of the USA fall is the ideal time to plant or move your rose of Sharon. It can happen that the leaves will appear very late or that the shrub will hardly grow after planting. After the roots have developed it will grow fine in the next years. Spring growth is always slow to appear, the leaves develop in May or even as late as June.
The Hibiscus syriacus does not need regular or routine pruning. The shrub will grow upright and fairly stiff in the shape of a vase. You should prune out dead, diseased or damaged stems. You could cut back all stems to half the length in the first spring after planting, in order to stimulate more branching. You can also prune to keep the shrub in the right shape. The period for pruning is February/March.
The rose of Sharon flowers on current year’s growth. There is no need to remove faded flowers to stimulate the production of new flowers.
The Hibiscus syriacus is often grafted. It can happen that suddenly stems develop with different flowers colors. It is best to cut away these stems that grow from below the grafting point, because that lower part grows faster and will take over.
The rose of Sharon can propagate through seed, but not all hybrids produce seed. You can also propagate the Hibiscus syriacus with cuttings. In the summer you can cut off a young shoot and in fall you can take a hardwood cutting. Cut stems that are 4–10 inches (10–25 cm) long and remove all but the top few leaves and place it into the ground. You can use rooting hormone to stimulate root growth.
The Hibiscus syriacus ‘Ardens’ has beautiful double, purple flowers.
Attracts bees: yes
Characteristic: deciduous
Deer resistant: yes
Exposure: sun/part shade
Flower color: purple
Flowering time: August - September
Foliage color: green
Fragrant flower: no
Hardiness: -13 ºF/-25 ºC
Height: 8 feet/2.5 m
Soil: normal/moist