PlantFiles: Swamp rosemallow
Hibiscus grandiflorus

With a winter mulch of about 1 foot of salt-hay I was successfully able to overwinter Hibiscus grandiflorus in USDA Zone 6b in Northern New Jersey. In New Jersey the H. grandiflorus bloomed the first year and broke dormancy in the first week in May.

Mike

I have not grown this plant. Swamp rosemallow (Hibiscus grandiflorus) is native to Alabama, Florida, Georgia, Louisiana, Mississippi and Texas. It is also commonly known as giant rose mallow, swamp hibiscus, velvet mallow and giant hibiscus. It is native to fresh and brackish marshes and the edges of swamps, ponds, streams and ditches and can be grow where other plants succumb to salty soil. The up to 10 in (25 cm) long and across, 3- to sometimes 5- lobed, palmate leaves are a gray-green and have a velvety upper and underside surface. Unlike many hibiscus species, the plant has dense foliage. Because of its soft leaves, it is sometimes referred to as toilet paper hibiscus. Swamp rosemallow is evergreen in mild climates; however, in colder climates, it will freeze to the ground and re-emerge from the roots in the spring. Usually, each year it produces additional stalks from its root crown.

From late mid-summer through through fall (late fall in mild climates), it produces pink, pale pink-violet (rarely white or rose-purple) blooms with crimson to purple centers that have five, six-inch (15 cm) petals (10-inch bloom). The petals do not overlap. Borne singly or in branched clusters, they open in the late afternoon and persist until noon the following day. Swamp rosemallow blooms are the largest of any hardy perennial in North America.

It is easily grown from seed planted in the spring. The seeds should be collected from the dried hairy seed pods once the they brown and start to split. Seed can be sown indoors 12 weeks before the last expected spring frost. Seed can also be sown in situ outdoors after the last expected frost date or fresh seed can be sown in fall. To break through the hard exterior seed coat, scrape a seed with a file to expose a small area of inner seed. A needle can be used instead to poke a single hole into the seed. Soak seeds in very warm water for one hour before sowing. Plant the seed in a damp medium. They will germinate in a few days.

In addition, swamp rosemallow can be propagated by dividing the root mass in spring. Be careful working around the soft new shoots. Usually, it does not tolerate fall division or transplanting. Also, swamp rosemallow can be propagated by rooting cuttings. Cuttings can be rooted at anytime that new growth is available, however, rooting is usually quickest in spring. Take pencil thick, 5 to 6 inch long cuttings of firm new growth. Take off all of the lower leaves leaving 3 or so at the tops. Dip the cut ends in powdered root stimulator. Place the cuttings in a mix of three parts sand and one part peat. Keep the potting medium moist. This can be achieved by placing plastic bags over the containers and attach them with rubber bands. Or cut appropriate sized plastic soda bottles in two and shove the capped top portion upside over the cuttings. The caps can then removed later if water needs to be added. Place the containers in filtered shade. Roots should form within four to five weeks. (I have rooted other hibiscus species cuttings in a clear jar filled with water.) Once the roots are formed, the plants can be moved into a larger container or transplanted to a permanent location.

Although the plant is not drought tolerant, once established, it will grow in gardens that receive at least an inch (2.5 cm) of irrigation per week. Swamp rosemallow requires sun. In areas of high heat, the plant enjoys a little afternoon filtered shade. Cut back in the spring to encourage branching and/or to manage height. Swamp rosemallow may be grown in a large container to decrease its size by root compression; however, as with most hibiscus species, it attains its maximum beauty if planted in the ground.

A beautiful native, this is found in low lying swampy areas. One of my favorite plants with big pinkish flowers . Its a fairly upright plant sending tall shoots in the spring and flowering by July / August. In the fall it begins to decline but comes back in the spring even stronger.

Excellent for swamp control. It will spread until it runs out of swamp and suck up the water, reducing or eliminating mosquitos.