HOW TO GROW ROSE OF SHARON & OTHER HARDY HIBISCUS VARIETIESBring the tropics to your garden with these plants that can overwinter even in temperatures below zero
Hibiscus plants aren’t only for the tropics. In fact, some hardy hibiscus varieties, like the popular rose of Sharon, can overwinter outdoors in temperatures as cold as 20° below zero.
There are two popular types of hardy hibiscus, with the main distinction being size—of both the plant and the flower. Rose of Sharon varieties grow into a large shrub, usually 8 to 12 feet tall and 4 to 6 feet wide and produce abundant small flowers. The mallow varieties can produce giant, dinner-plate-size flowers, yet grow to a smaller overall size of 3 feet tall and wide on average (although a few mallows can reach ten feet in height). Although the 2 types differ in size, much of their growing and care requirements are the same.
These pink rose of Sharon flowers have a deeper colored eye. Photo by: kim gyu sik / Shutterstock.com.
Generally 5-8, with a few mallow varieties hardy to zone 4.
Rose of Sharon varieties can grow 8 to 12 feet tall and 6 to 10 feet wide. Mallow varieties are typically between 3 to 5 feet tall and 2 to 4 feet wide; however a few mallows can reach heights of ten feet.
Full sun to partial afternoon shade (best flowering in full sun).
Prefers moist, yet well-drained, acidic soil; mulch to help retain moisture.
Mallow varieties will start blooming in June, with rose of Sharon varieties blooms coming a little later in July; both will bloom into fall or until first frost.
Often white, pink, or red; sometimes with a contrasting colored eye or throat.
Both varieties are deciduous; with medium-green, heart-shaped leaves, although there are a few exceptions.
Hardy varieties also known as rose of Sharon, rose mallow, swamp mallow, or althea.
When to plant:
Rose of Sharon and other hardy hibiscus varieties can be planted in spring or fall, as long as there is no danger of frost.
Where to plant:
Full sun to partial afternoon shade. An area with good air circulation produces best flowers, strongest stems and is the best environment to prevent disease. Keep an eye out for hot, scorching afternoon sun that may burn the foliage; as well as provide protection from damaging wind.
How to plant:
Seeds can be started indoors and then transplanted, but it is more common to plant potted plants purchased from a nursery.
Hardy hibiscus can be planted in three easy steps:
- Dig a hole twice the size of the pot and set the plant in
- Make sure the crown of the plant rests just at or above the soil line
- Fill in the hole with the loose dirt and water the plant well
When properly planted and cared for you can get many flowers from one hardy hibiscus plant (shown is a type of Hibiscus moscheutos). Photo by: billysfam / Shutterstock.com.
Hibiscus need a fertilizer high in potassium (K), low in phosphorus (P), and with a medium amount of nitrogen (N) - too much phosphorus will kill a hibiscus. Hibiscus also respond well to organic fertilizer and a good layer of compost once a year is generally sufficient. Don’t fertilize after July as this can push new growth that will be damaged by frost.
Hardy hibiscus prefer moist soil and do not tolerate drought. They should be watered regularly and up to twice a day in hotter weather, especially if being grown in a container. Watering can be cut back a bit in colder weather. Water-stressed hibiscus may drop their buds or become more susceptible to insects and disease. They should not be left in standing water as this can promote root damage/root rot.
Hardy hibiscus require little actual pruning and is generally only needed to maintain appearance, size, and shape. The main rule of thumb for when to prune your hardy hibiscus is to do it just before a warming trend, in early or late spring when new growth has started. Pruning before very cold temperatures or in extreme heat can be very stressful to your hibiscus and do more harm than good. Look for new leaf nodes on the branches and prune about ¼ inch above a node that is pointing in a direction you want the new branch to grow. Tip pruning can stimulate branching, and since more branching means more flowers, a little trim late spring to early summer can be a good thing.
Hardy varieties will die back to the ground in the winter in colder regions, but new growth will develop in spring from the root tops. Don’t panic however, as hibiscus are heavy sleepers and new growth can be slow to emerge. Cover the root ball with mulch to insulate in colder winter climates. Some varieties require freezing to promote new growth.
Hibiscus can be divided in spring or seeds can be collected from the seed pods that form in fall and early winter. Hibiscus grown from these seeds, however, may not be true to their parent plant. Hibiscus may re-seed abundantly, and plucking new hibiscus from areas where they are not wanted may become a new chore. To help prevent an over-abundance of seed pods from forming, remove dead blooms from your hibiscus (deadhead) in mid-to-late fall. Stem cuttings tend to root easily if taken prior to flowering.
Diseases and Pests:
Hardy hibiscus can fall prey to a number of pests, including Japanese beetles, whiteflies, aphids, and ants. Ants can be more than a mere annoyance on your hibiscus, as they will actually bring aphid eggs to the hibiscus so that they can feed off of the byproducts the aphids produce from sucking the sap from the leaves and flowers.
Keeping your hibiscus healthy, free of pests, and in an area with good air circulation are the best defenses against disease. Hibiscus may be susceptible to dieback disease, wilt disease, and leaf fungus.
Designing with Hibiscus
- Mallow varieties are native to swampy areas and can be useful in low spots or wetter areas, along streams or ponds.
- Excellent foundation planting, anchoring structures to the landscape; planting close to buildings also provides some cold protection for the plant.
- Hibiscus are some of the most colorful plants in the landscape, especially mallow varieties with their large blooms, so be careful not to overwhelm an area or have clashing tones.
- Rose of Sharon, generally multi-trunked shrubs, can be trained as a single-trunk ‘tree’ or espalier.
- Hardy hibiscus are not good for cut flowers, as their blooms only last a day or 2. Their dried seed pods can provide an exotic look and last longer in an arrangement.
Hibiscus Varieties to Try
Hibiscus syriacus ‘Blue Bird’
(Rose of Sharon)
Blooms mid-summer to frost, 3 to 5-inch flowers, easy to care for.
Hibiscus syriacus ‘Marina’
(Rose of Sharon)
Also known as Blue Satin, this hibiscus is easy to grow and is heat and drought tolerant when established. Perfect for borders and mixed beds. This is a sterile cultivar that won't produce viable seed.
Hibiscus syriacus ‘American Irene Scott’
(Rose of Sharon)
Another sterile cultivar, more commonly known as Sugar Tip, this rose of Sharon has variegated foliage and double flowers.
Also called Confederate Rose, this hibiscus blooms summer through fall. Its large white blooms turn pink after a few days.
Hibiscus ‘Kopper King’
A compact rose mallow hybrid, ‘Kopper King’ features copper-red foliage and flowers up to 12 inches across.
This selection has striking green foliage with purple highlights. A good choice for gardens with extreme heat and humidity.
Hibiscus ‘Blue River II’
This hybrid cultivar has deep green leaves, with a tinge of blue. Great for use in a night garden.
Is Rose of Sharon Invasive?
Hibiscus syriacus can be invasive in the following areas:
- Most of Indiana
- Parts of the South
- Coastal areas of the Mid-Atlantic
To see if rose of Sharon is invasive in your area check out this map from InvasivePlantAtlas.org.
If you still love this plant, but don’t want to worry about vigilantly plucking new seedlings, try a cultivar that produces few if any seed pods.
Sterile cultivars that don’t spread include:
- Chiffon series
- ‘Marina’ (Blue Satin)
- ‘American Irene Scott’ (Sugar Tip)