Hibiscus Species, Large-Flowered Hibiscus, Swamp Rose-Mallow, Pink Swamp Hibiscus

I live in Western New York (2.5 blocks north of Buffalo’s north border. We have always been considered Zone 5. Now, some maps put us so...Read Moremewhere in Zone 6. Our deep winter lows dip as low as minus 15 Fahrenheit.
Husband and I have what certainly appears to be Hibiscus grandiflorus growing enthusiastically against our west facing backyard back border wooden stockade fence. It also grows in the backyard of the suburban property behind us (our two clumps are part of our neighbor’s old clump). I wish air could see a way to include a photo taken within the past two days, but ai do not see a way to add it, here.
Our H. grandiflorus has the typical sweet baby pink blossoms with the red centers. The buds are pink with what appears to be spiral white stripes. Blossom petals are very much separated, one from the other, in contrast with the overlapping petals of H. moscheotus.
So, Dave, I propose that you add Zone 5/6 Tonawanda NY to the list of sites in which H. grandiflorus flourishes, in ground, with no winter mulch or other protection!

This plant is much more cold hardy than given credit for. It has survived -9F this past winter at Boone County Arboretum (northern Kentu...Read Morecky, zone 6a) with no extra protection other than 2-3 inches of mulch inside a flower bed.

Vistitors simply love this plant, even if it never bloomed it would be much admired for the handsome velvety foliage! When it blooms the plant is simply stunning.

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 N...Read Moreew 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...Read More 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.