Every year, the National Garden Bureau, a non-profit organization promoting the pleasures of home gardening, selectsfive special plants to celebrate. Let’s look one of the plants chosen for 2021: the hardy hibiscus.
Hardy hibiscus adds a splash of tropical flair to your perennial garden.
Fast-growing and fabulous, these plants explode with pinwheel-like flowers the size of dinner plates in late summer. Often confused with their tropical cousins, these plants are actually capable of surviving temps as low as -30 °F (-34 °C).
North American gardeners can feel assured knowing they are planting a native perennial. Hardy hibiscus, also called perennial hibiscus and rose-mallow, are hybrids that can be primarily traced back to the species Hibiscus moscheutos. This species can be found growing naturally in wetlands and along riverbanks throughout the Midwest and East Coast, from Ontario down into Texas and Florida.
Know Your Hibiscus
Hibiscus is both the common and botanical name for a few different popular classes of the plant. Most think of Chinese hibiscus (Hibiscus rosa-sinensis) when they hear the word hibiscus, while horticulturists may note the shrub Rose of Sharon (Hibiscus syriacus) also falls in this category. However, neither of these plants are commonly considered to be “hardy hibiscus.”
Here is a quick overview of how they differ:
· Hardy Hibiscus
First, our subject and superstar: hardy hibiscus. This set of plants from the Malvaceae family is composed hybrids of several species native to North America, mostly Hibiscus moscheutos, but also other species such as H. palustris and H. laevis. These plants are perennials, but with a woody base and semi-woody stems and may pass for shrubs. They produce huge flowers and leaves of variable shape. USDA hardiness zones 4–9 (AgCan zones 5–9).
· Rose of Sharon Hibiscus
The name Rose of Sharon refers to the species Hibiscus syriacus and its hybrids, a plant which is native to southern and central Asia. Rose of Sharon is a woody shrub and blooms on the same branches each year. It tends to be the largest, with some varieties reaching 15′ (5 m) tall. USDA hardiness zones 5b-9 (AgCan zone 6b-9).
· Chinese or Tropical Hibiscus
Chinese hibiscus, or Hibiscus rosa-sinensis, is from southeast Asia. It’s a shrub, like its cousin, rose of Sharon (H. syriacus), though significantly less hardy. Since it is not frost tolerant, this species is planted outdoors only the mildest climates (hardiness zones 9–11), in the extreme south of the US, for example. Elsewhere, it’s grown as a patio plant or houseplant.
Common Hardy Hibiscus Varieties to Keep Your Eye Out for:
Hardy Hibiscus comes in shades of white, pink, red, and yellow, with different eye patterns and streaking through the petals. The leaves of the plants can also vary in color from green to bronze and near-black.
Some of the most popular series available include:
Hardy hibiscus have a history of growing in wet areas such as along riverbanks and around inland lakes. They perform best with consistent watering, particularly if they have been recently transplanted. If your hibiscus is losing its lowest leaves or aborting buds, you may need to up the water!
This water-tolerant characteristic makes them perfect for areas of the garden that periodically flood, or as a star plant in rain gardens.
Another characteristic of the genus is they are late to break dormancy in spring. They’re not dead! When they do wake up, they’re off to the races, growing more than an inch (2.5 cm) a day. Depending on the year, hardy hibiscus may stay dormant through the end of May, even well into June in the northern parts of its range.
Instead of tearing your Hibiscus out and starting over, try planting them with tulips, daffodils, or other spring-blooming bulbs who will be out of bloom when hibiscus are ready to emerge. As a bonus, the tired foliage of the spring-blooming bulbs will quickly be covered by the wide-spreading leaves of the hibiscus.
Once your hardy hibiscus gets going, be sure to leave them plenty of space to grow. Mature hibiscus can reach 5–6′ (1.5-1.8 m) wide and grow quickly. If you take a week’s vacation in June, you’re likely to come back to a plant twice the size you left it.
Full sun is a must. In too much shade, the otherwise sturdy stems stretch and become floppy. Over shading will also lead to a decrease in bud count and diminished flowering performance. UV light (full sun) will also bring out the color in cultivars with dark foliage.
For denser growth and a bushy habit, pinch back growing tips when they reach 8” (20 cm) and again at 12” (30 cm), although this will delay blooming.
In fall, cut the stems of your hardy hibiscus back to about 4 inches (10 cm) above the soil. Don’t cut them right to the ground as, unlike most perennials, which grow back from a ground-hugging or underground crown, the hardy hibiscus resprouts in spring from the base of the previous year’s stems.
Winter protection. In the colder parts of its range, it’s always wise to thoroughly mulch hardy hibiscus over the winter, offering this rather delicate plant a bit of extra protection against the cold. In the spring, pull back the mulch from around the base of the plant so the soil warms up faster. This will encourage faster sprouting… and earlier bloom.
Pot Culture for Earliest Bloom
In areas where spring temperatures remain cool into June and summers are short (and the two conditions usually go together!), hardy hibiscus can be discouragingly late to bloom, sometimes coming into first flower only days before the first fall frosts. In such climates, consider growing them as patio plants, buying plants already in bud or bloom in late spring that will then bloom for the entire summer. Or start your own patio plants from seed indoors in February so they come into bud by late June.
It would be a shame to miss the spectacular giant flowers of the hardy hibiscus when the solution is so easy!
You can multiply hardy hibiscus by crown division in spring or by stem cuttings during the summer. Seed strains can be started indoors in February, ideally under lights, as sunlight is very weak at that time of year. Soak the hard seeds overnight in tepid water: seeds that sink to the bottom are viable. Sow ¼ inches (5 mm) deep and keep warm: 70–75˚ F (21–24 ̊C). Germination can take 7 to 30 days, but after that, seedlings grow very quickly.
Almost all hardy hibiscus are of hybrid origin, so will not come true from seed. If you harvest seeds from your garden plants, you should expect some variability in the upcoming generation.
Those #@$%&* Japanese Beetles!
Yes, hardy hibiscus has a problem with Japanese beetles early in the summer. In regions where this pest is present, try spraying with neem oil and knocking them into a bucket of soapy water to keep them under control.
Hardy hibiscus: possibly the showiest of all the plants in your summer garden!
This article was based on the Year of the Hardy Hibiscus fact sheet from the National Garden Bureau as prepared by Walters Gardens. Unless otherwise mentioned, photos supplied by members of the National Garden Bureau.
Interested in buying hardy hibiscus to grow for yourself? Click the here to shop members of the National Garden Bureau.