Rose-of-Sharon: Beloved Ornamental or Pesky Invasive?

by Kathryn Dorman

Rose-of-Sharon (Hibiscus syriacus) is a deciduous shrub that can be a beautiful addition to many lawns and flower gardens.  Even though we didn’t have any Rose-of-Sharon on our property when I was growing up, they were and still are growing in many other locations in central Pennsylvania.  Some gardeners really like Rose-of-Sharon; others are concerned that these plants are becoming invasive.

Rose-of-Sharon was introduced from Asia and India to Europe in the 1500’s, and then to North America in the 1700’s.  Thomas Jefferson grew many of these plants from seed.  Rose-of-Sharon is hardy from USDA Hardiness Zones 5 (northern Clinton County) to at least 8.  It blooms in late summer, while most other woody plants have finished flowering by late spring.  It can be grown in containers and near buildings, as single specimens, or used as a hedge or living privacy screen (See Photo 1). 

Rose-of-Sharon isn’t a rose at all.  It is a member of the mallow (Malvaceae) family, and is closely related to both hardy and tropical hibiscus (H. moscheutos and H. rosa-sinensis respectively).  The flowers of H. syriacus resemble those of the hollyhock (Alcea rosea), which is another member of the mallow family.  

The large, showy, 5-petalled, trumpet-shaped flowers of Rose-of-Sharon range in color from white to pink to purplish-blue, with a red ring surrounding a column of stamens in the center.  Some varieties have double blooms.  The simple leaves have three lobes.  They are coarsely-toothed, grow alternately on the stem, and are typically light to medium green in color (See Photo 1).  The leaves will drop in the fall without much of a color change.  Depending on how it is pruned, a Rose-of-Sharon plant can be shaped like a cylinder or a vase.  It can have multiple stems, or can be trimmed to only one main stem, and can grow to a height of at least ten feet. 

Rose-of-Sharon is easy to grow – perhaps TOO easy to grow!  According to the Missouri Botanical Garden (MBG), this plant will tolerate poor soils, road salt, and air pollution.  It will grow in full sun or light shade.  Rose-of-Sharon can survive Pennsylvania’s typical summer heat and humidity, as well as some drought (https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=c522).

According to the United States Department of Agriculture Forest Service (USDA/FS), once it is about two years old, Rose-of-Sharon develops a deep taproot that can be difficult to remove.  Fruits persist throughout the winter, and easily sprout into new plants.  Although Rose-of-Sharon reproduces mainly by seed, and is a prolific seed producer, it can also be easily propagated by stem cuttings.  Rose-of-Sharon is considered to be invasive in Kentucky, Pennsylvania, Tennessee, and Virginia (https://www.invasive.org/weedcd/pdfs/wow/rose-of-sharon.pdf).  I have always thought I would like to plant Rose-of-Sharon, but wasn’t sure where to put it.  I never really thought about it being an invasive species until recently.  

There were many clues that Rose-of-Sharon could be invasive.  My first clue was given to me in the form of six Rose-of-Sharon plants from my sister’s backyard.  My sister claims to be an abysmal gardener, yet here were all of these extra plants, and she kept at least one plant.  These plants must have grown very easily!  Now I have a Rose-of-Sharon growing near my garage.  The second clue came from my church.  I had been asked to find new plants for our church grounds.  I suggested Rose-of-Sharon, which is mentioned in the Bible (Song of Solomon 2:1).  One gentleman in particular was passionately opposed to any Rose-of-Sharon shrubs on the church grounds.  He knew how prolific these plants can be.  Clue number three came from a fellow Master Gardener.  She has many Rose-of-Sharon plants of various sizes growing on her property, including a glut of new seedlings. 

What can be done to control Rose-of-Sharon?  Be proactive.  There are some cultivars (cultivated varieties) that are sterile, so if you choose to plant one of these, you will have flowers, but few, if any, seeds (USDA/FS).  If you do decide to plant non-sterile Rose-of-Sharon, plant only a few.  Be vigilant.  Deadhead the spent flowers before they go to seed.  Pull or dig out any seedlings as early as possible.  Consider applying heavy mulch, or even plastic tarp, around each desired Rose-of-Sharon plant, in order to smother any unwanted seedlings.  Rose-of-Sharon flowers on new growth, so you can prune it in late winter or early spring, and still get flowers.  Pruning again in late spring will lessen the number of flower buds, and keep the plant more compact.

Although it is beautiful, and the nectar is used for food by butterflies, other insects, and hummingbirds, Rose-of-Sharon poses a threat to native plants, and to the balanced ecosystems that they support.  “It has escaped intended plantings to invade, crowd out, and displace more desirable native plants (USDA/FS).”  I am going to keep a close eye on my Rose-of-Sharon, to make sure it doesn’t start a whole grove of these shrubs!   

 

Kathryn Dorman has been a Penn State Extension certified Master Gardener in Clinton County since 2017.  She can be reached at clintonmg@psu.edu, or through the Clinton County Master Gardener Hotline at (570) 858-0198.