The words "tender perennial" sounds a bit like a contradiction. The group of plants considered "tender perennials" in Iowa can be overwintered in gardens in warmer climates. However, in order to keep these plants from year-to-year in the midwest, the rhizomes, corms, or tubers need to be dug in the fall and stored indoors over the winter months. A few plants considered tender perennials include: dahlia, gladiolus, tuberous begonia, canna, calla lily, and caladium. Below are some tips on planting and growing tender perennials in your garden. See the 1996 Horticulture and Home Pest Newsletter, page 157, for information on digging and storing tender perennials in the fall.
Dahlias (Dahlia hybrid) are beautiful garden flowers and make excellent cutflowers. They are available in a wide range of colors, flower forms, and plant height. Bedding dahlias that are used in containers or border plants in a flower garden, can be started from seed and are typically treated as annuals. The large-blooming varieties are grown from tubers or cuttings. Select firm healthy tubers with healthy buds. Plant dahlia tubers in warm, well-drained soil. Don't be in a bury an plant them too early or the tubers will rot in the cool soil. Select a full-sun location that is well-drained. Plant tall-growing dahlia varieties 2 to 3' apart. Drive a strong 6 to 7' stake into the ground where each dahlia tuber is to be planted. Dig a hole near each stake and plant the tuber, sprout-side up, 3 to 4" deep. The tuber should be about 3" from the stake. Dahlia varieties that grow less than 2 1/2' tall will not require staking and are planted only 2 to 3" deep. After the third or fourth set of leaves appear, pinch out the growing point. This will encourage new side branches to form. Begin tying the stem to the stake when it's about 18" tall. Use strips of old nylon stocking or twine to tie the stem to the stake. Keep dahlias well-watered throughout the summer and remove spent blooms.
Cannas (Canna x generalis) add a tropical appearance to annual and perennial gardens. The large green, red or striped foliage make excellent backgrounds, temporary screens or focal points in the garden. Tall scapes of bright flower colors bloom from mid-summer into early fall. Cannas are available in standard (3 to 6' tall) and dwarf (2 to 3' tall) varieties. Plant firm, healthy canna rhizomes a week before the average frost date in your area. Select a full-sun location that contains rich, moist soil. Plant the rhizomes horizontally about 6" deep and 18" apart with the buds upright. Keep the plants well watered and fertilize once or twice a month with a balanced liquid fertilizer. Remove spent blooms.
Tuberous begonias (Begonia x tuberhybrida) are perfect selections for containers or window boxes. Their large brightly colored flowers, ranging from white through yellow and orange to deep red, will brighten decks and patios. They prefer partial shade and a soil mix with a high organic matter content. A soil mix containing 1/2 and 1/3 part sphagnum peat moss will help hold moisture in the soil. Tuberous begonias are planted as firm, round tubers. Plant them in the containers about 2" deep with the rounded side down ("scooped-out" side up). Keep them will watered and do not allow the soil to dry out. Remove faded, spent blooms.
Gladiolus spikes (Gladiolus xhortulanus) brighten bare areas in perennial gardens in mid-summer and make excellent cutflowers. They bloom in nearly every color of the rainbow. Gladioli are planted as corms from early May through early July. Do not plant all the corms at one time. Plant a few every 7 to 10 days to provide a continuous supply of blooms. Plant the corms in a full-sun location between 4 and 6" deep, depending on the size of the corm. Space the corms about 5 inches apart. Keep them well watered. The flower spikes appear two to three months after the corms are planted and last about two weeks in the garden. It may be necessary to stake the flower spike to keep it growing straight and upright.
Fancy-leaved caladiums (Caladium x hortulanum) produce large, colorful leaves that give midwest gardens a very tropical appearance. The foliage patterns accent the veins with shades and combinations of green, white, red, and pink. They prefer a shady to partially shady location and a moist, well-drained soil. Caladiums can be purchased as plants that have been grown in a greenhouse or as tubers. Plant the tubers around the last frost date in your area. Space the tubers 2 to 3 inches deep and 4 inches apart. Water them frequently.
Calla lilies (Zantedeschia aethiopica) also have a tropical appearance and grow 24 to 36" tall. The flowers consist of a center spike (spadix) with a single petal (spathe) that curves around the spike. The flowers come in a wide range of pastel colors. The arrowhead-shaped leaves may be solid green or green with white speckles. Calla lilies are excellent as marginal plants in water gardens and are attractive in bog areas or along streams. They prefer partial shade but will tolerate full sun if given ample moisture. Calla lilies can be started indoors in March or April or planted directly into the garden after the danger of frost is past. The soil should contain ample organic matter to help retain moisture. Plant calla lily rhizomes 4" deep and 18" apart.
This article originally appeared in the April 25, 1997 issue, pp. 52-53.
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