Perennial of the Month: Hardy Hibiscus
The hardy perennial hibiscus, also called rose mallow or swamp rose, adds the beauty of a tropical hibiscus to the garden, but can withstand cold winter temperatures that kill the actual tropical varieties. Here’s how to grow hardy hibiscus in your garden!
Perennial hibiscus have big, disc-shaped, hollyhock-like flowers that can be 6 to 12 inches across. The perennial hibiscus species found in gardens are the result of hybridizing native hibiscus species, including Hibiscus moscheutos and H. coccineus.The larger, more shrub-like hardy hibiscus species, H. syriacus (aka Rose of Sharon), has similar planting and care to the smaller species highlighted in this article. It produces an abundance of smaller flowers and grows into a much larger shrub that doesn’t die back to the ground in winter.
How to Grow Hardy Hibiscus Plants
Plant taxonomy classifies the hardy hibiscus plants as Hibiscus moscheutos. They also go by such common names as rose mallows and swamp mallows. The hardy hibiscus is a cold hardy plant despite bearing large blooms that call to mind the tropics. The hues of the most common cultivars are white, bicolored, or various shades of red or pink, but other colors are now available.
Although hardy hibiscus plants seem woody in summer and function as sub-shrubs in the landscape, their stems do die back to the ground in winter, making them herbaceous perennials, technically.
Some of the most popular hardy hibiscus cultivars reach about four feet in height, with a spread slightly less than that, but the bloom size can be up to 10 inches.
Even cultivars with smaller blooms still produce impressive, saucer-size flowers. While each bloom lives only a day or two, they are quickly replaced by newcomers.
The species plant is indigenous to eastern North America. H. moscheutos cultivars can be grown in USDA plant hardiness zones 4 to 9.
Light
For your hardy hibiscus plant to bloom to its greatest potential, it needs about six hours a day of full sun. However, if you live in a hot and dry zone you should provide your hardy hibiscus occasional relief from the bright afternoon sun. Shade from other leafy plants placed nearby should help. Indoor hibiscus plants should be situated near a sunny (southwest facing) window and if that still doesn't provide enough light, you can augment with artificial lighting.
Soil
The species plant is a wetland plant, and hardy hibiscus flowers can be treated as plants for wet soils. So if your landscaping situation is a soggy area where most plants do not grow well, H. moscheutos might be the answer. This makes them useful around water features.
Water
If you are not planting hardy hibiscus plants in a wet spot, make sure they are adequately watered—but don't overdo it. A small plant with fewer leaves needs less water than a large leafy plant. In warm weather, you need to water your hibiscus plant daily but in the winter you should water it only when the soil is dry to the touch.
Temperature and Humidity
Hibiscus flower best in the 60 to 90 F range. Bring plants indoors before temperatures dip to 32 F, but be mindful that low humidity can dry them out. Mist the leaves daily or place each pot on a tray with a layer of gravel underneath. Add water up to the top of the gravel and as it evaporates, the humidity will rise around the plants. A humidifier may also help.
Fertilizer
Growing hibiscus plants need plenty of nutrients. Use either slow-release or water-soluble fertilizer but make sure the nutrients are balanced. For example, use a 20-20-20 or 10-10-10 fertilizer. You can use a diluted liquid fertilizer once a week, or a slow-release fertilizer four times a year: early spring; after the first round of blooming; mid-summer; and early winter.
Potting and Repotting
Repot in late winter and use houseplant potting soil or a soilless mixture. Your hibiscus can wait two to three years to be moved into a larger pot. Just remember to use one with good drainage.