Rose of Sharon (Hibiscus syriacus), also known as shrub althea, is a very common deciduous shrub that is grown in USDA hardiness zones 5 to 8. It gains its name from the blooms, which some people find reminiscent of the prolific blooming pattern of shrub roses. In reality, this plant is a member of the mallow family and viewed up close, its flowers more closely resemble those of hollyhocks or hibiscus. This rangy shrub grows up to 12 feet tall and blooms vigorously from early summer and into the fall, and therein lies the problem. Many flowers mean many seeds, and rose of Sharon is known to self-seed prodigiously, which can result in dozens if not hundreds of volunteer seedlings that spring up all around the garden and lawn. These seedlings are fast-growing, with the result that rose of Sharon can be severely invasive if you don't take measures to control its spread.
Fortunately, there are several methods for limiting rose of Sharon's habit of rampant self-seeding.
It's also possible that you might want to grow some rose of Sharon plants into mature specimens. If so, the ease with which these plants self-seed also makes them very easy to propagate intentionally.
Preventing Self Seeding
There are several ways to prevent rose of Sharon from self-seeding and producing volunteer seedlings in the garden.
Choose Non-Seeding Cultivars
Most rose of Sharon varieties are by nature vigorous self-seeders, but if you are seeking one for planting, try to find a cultivar that produces few or no seeds. Some popular choices include a series developed by the U.S. National Arboretum, including 'Diane,' 'Helene,' 'Minerva,' and 'Aphrodite.' Another group from Proven Winners are also non-seeders: the Chiffon or Satin series.
Deadhead the Flowers
Seeds are found in the fruit or seed pods that are produced by flowers, and if you remove the seed-producing structure of the plant, you will also remove the source of the seeds. With rose of Sharon, the seeds are contained in small seed pods that appear just below the blooms. Removing the blooms and developing seed pods—a process known as deadheading—halts the seed production process in its tracks, and thereby prevents the rampant self-seeding for which rose of Sharon is famous.
When the flowers of your shrub are done blooming, simply deadhead them. This will nip seed production in the bud and eliminate all those annoying seedlings. Make sure you remove not just the flower, but also the developing seed pod at its base. With rose of Sharon, the seed pods develop in October and take 6 to 14 weeks to mature, so you have a little time once the blooms have withered. But if you do this carefully, you will be rewarded next spring with a garden free of unwanted seedlings.
If you fail to implement this preventive control measure, you will have a tougher row to hoe. The fully developed seed pods eventually dry out and split open, dropping the seeds near the parent plant. In this way, rose of Sharon readily forms clumps or colonies, which is why it can be useful as a hedge but also why it's considered invasive in some climates.
Apply Preemergent Herbicide
Some classes of chemical herbicides are intended to be preemergent, working by preventing seeds from germinating in the soil while not affecting existing plants. Products such as Preen are of this type. A preemergent herbicide applied on the ground around a rose of Sharon bush will prevent its seeds from germinating and taking root.
Be aware, though, that these preemergent herbicides generally work on all seeds, including those you deliberately plant in the garden. Applying a preemergent herbicide will prevent all seeds from taking root, whether they are wanted or unwanted. It is generally best to try non-chemical means of preventing self-seeding before reaching for a herbicide.
Careful and complete deadheading is probably the best way to prevent rose of Sharon from self-seeding, but if you're not able to do this, you'll be faced with dealing with those volunteer seedlings.
Practice Manual Removal
Pulling seedlings by hand is the most time-consuming and difficult way to deal with volunteer seedlings, but it is also the most environmentally friendly. Spotted early, the seedlings aren't difficult to pluck from the soil. Larger seedlings are a bit more difficult, as the root systems develop quickly, but they still easy enough to dig up with a trowel or shovel. You really don't need to bother with seedlings that sprout up in the lawn, because regular mowing will soon cause them to fade away.
Smother the Seedlings
Small areas of the garden can be covered with a tarp, sheets of plastic, or even cardboard or newspaper. This technique will smother the rose of Sharon seedlings (along with other plants) within a month. This is not the most attractive solution, though you can cover the tarp with wood chips or mulch to hide it while it does its work.
Use an Herbicide for Woody Plants
Of course, chemicals can be used to kill rose of Sharon seedlings. A herbicide formulated for woody plants, such as Bioadvance's brush killer or Ortho's Ground Clear will do the job. These are non-selective plant killers, though, so you need to be very careful in how you apply them. They will kill or badly damage any living plant they touch. The best strategy is to "paint" some of the herbicide on the leaves of the unwanted seedlings and wait for them to die. Avoid spraying if you can, since any overspray can be carried on the lightest breeze to other plants.
Propagating Rose of Sharon From Seed
The fact that rose of Sharon self-seeds so easily means it is also quite easy to propagate deliberately. You can simply let nature do the work for you. Just allow the seeds to drop on the ground in fall and winter of their own accord, and wait for them to germinate in spring. Then, dig up your new rose of Sharon plants and transplant them to your desired location. Or, transplant them into small pots to give away to others.
Alternatively, you can collect the seeds as they drop from the shrub's seed pods. Seeds must be fully mature to grow into new plants, so wait until they naturally drop from the seed pods. You can cover the pods with nylon bags to catch the seeds as they fall. In spring, plant the seeds 1/4 to 1/2 inch deep in humus-rich soil. Give them full sun and water them deeply. Watch out for birds eating exposed seeds.