Althea plants (Hibiscus syriacus), also known as Rose Althea or Rose of Sharon, are popular summer-blooming ornamentals native to Eastern and South Asia. This plant is the national flower of South Korea. Althea plants grow naturally in a shrub form, with a mature height of eight feet and a breadth of about four feet. However, as they have a vase-shaped growth habit that tapers toward the bottom of the plant, pruning the lower limbs will create an Althea tree. Althea trees are excellent for use along foundations and beside entryways, in borders, and as isolated specimens. They are rated to USDA Hardiness Zones 5 to 8.
Althea trees are famed for their showy flowers, which bloom prolifically and continuously from late July to September. The four-inch, large-petaled flowers take on deep and solid colors, including red, purple, blue, and white. Several cultivars feature multi-colored flowers. The flowers are so large, and are produced so prolifically, that even large stems can bow under the weight of the full bloom. The flowers are short-lived, but are replaced daily by fresh, new blooms. Althea trees are deciduous and have deep green, three-lobed leaves that turn a mottled green-yellow in the autumn.
The Althea is typically available from nurseries and garden centers throughout its growing range, ready for planting in your yard. Seeds or vegetative cuttings can propagate it.
The Althea tree can adapt to a wide range of soil conditions, though it prefers a rich, well-drained soil. It can adapt to both full sun and partial shade environments, though it requires at least five hours of direct sun a day for best flowering results.
After the Althea has been allowed to mature and fill out somewhat, it can be pruned to create a tree form. It is best to do this in late winter or early spring. In most cases, numerous stems emerge from the ground, with a center trunk in the middle of the plant. Find the center trunk and remove the smaller, outer stems. The center trunk of the Althea branches very close to the ground. To create a tree, simply remove limbs from the trunk to a height that satisfies you. To maintain the tree form, knock off any shoots from the trunk and prune away any stems that emerge from the ground.