Tender perennials are attractive additions to the home landscape. Since they are not reliably winter hardy in Iowa, tender perennials must be planted outdoors each spring. Tender perennials include gladioli, dahlias, cannas, tuberous begonias, caladiums, and calla lilies, among other plants.
General Planting & Care | Planting & Care of Specific Species | More Information
General Planting and Care Recommendations
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Starting Indoors
Tender perennials' "bulbs" (actually tubers, rhizomes, and other structures) are usually planted directly outdoors in spring. They grow quickly in the warm temperatures of summer. However, these attractive ornamentals can be enjoyed for longer periods by starting them indoors in late winter.
Follow the potting and timing recommendations for different tender perennials below. When foliage appears, move the plants to a sunny window or under fluorescent lights. Before planting outdoors, harden or acclimate the tender perennials to outdoor growing conditions. Initially, place the plants in a shady, protected location and then gradually expose them to longer periods of sunlight. After they have been hardened, plant the tender perennials in their proper location.
Planting Outdoors
Plant those tender perennials started indoors in the garden after they have been hardened or acclimated to outdoor growing conditions. .
Tender perennials do not have to be started indoors. They grow quickly in the warm temperatures of summer. Most species will develop bold, large, colorful foliage and flowers by early July. Tips for planting the bulbs, or other geophytes like corms, rhizomes, or tubers, of tender perennials directly in the garden are provided below.
Summer Care
Water tender perennials regularly throughout the summer. Most are not drought tolerant so consistent watering, especially those growing in containers (and more prone to drying out), is essential.
To get the most out of these plants in the short amount of time they are in the garden, regular fertilization is important. Water-soluble fertilizers or slow-release granular fertilizers may be used. Apply soluble fertilizers mixed according to label directions every 1 to 3 weeks throughout the summer. Alternatively, soluble fertilizers can be mixed at quarter strength and applied every time you water. Slow-release fertilizers can be added to soil in early spring and may need a second application mid-season. Most benefit from fertilizers that can provide a lot of nitrogen. All-purpose fertilizers like 20-20-20 are good options. For those tender perennials that flower, use a fertilizer with a 1:2:2 or 1:2:1 ratio of nitrogen, phosphorus, and potassium to keep plants healthy and blooming well.
Remove spent flowers as they appear. Lower leaves that are dry or yellow can be pruned off as close to the ground or stem as possible.
Overwintering
Tender perennials are not winter hardy in Iowa. That means to grow them from year to year they must be dug up in the fall and stored indoors over winter. This is possible because these plants grow from bulbs or other geophytes like corms or tubers.
Learn more about overwinter tender perennials in this article: How to Overwinter Tender Perennials.
Specific Recommendations for Common Tender Perennials
The planting and care recommendations requirements for several widely grown tender perennials are below.
Canna | Caladium | Elephant's Ear | Dahlia | Gladiolus | Calla Lily | Banana | Tuberous Begonia
Canna (Canna spp.)
Start canna rhizomes indoors 4 to 6 weeks before your area's average last frost date. Plant the cannas in large pots. The rhizomes should be covered with 3 to 4 inches of potting soil.
Canna rhizomes can be planted outdoors in mid-May in central Iowa (about one week earlier in southern Iowa and one week later in norther Iowa). The rhizomes should be planted 4 to 5 inches deep.
Cannas perform best in moist, well-drained soils in full sun. During dry weather, water plants once a week. To promote growth, apply a balanced garden fertilizer, such as 10-10-10, to the area before planting and once or twice during the growing season. Remove spent flowers to maintain their attractive appearance and promote additional blooms. While cannas have a few insect and disease pests, none are considered serious.
Caladium (Caladium bicolor)
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Plant caladium tubers indoors 4 to 6 weeks before the average last frost date in your area. Tubers should be planted 1 to 2 inches deep. When planting the tubers, place the knobby side up. The knobs are the tuber's eyes or buds. After planting, place the containers in a warm, 75 to 80°F location to facilitate sprouting.
Caladiums can be grown in beds, borders, and containers. Plants perform best in moist, well-drained soils in partial shade. Caladiums can be planted outdoors in mid-May in central Iowa (about one week earlier in southern Iowa and one week later in norther Iowa). Use a well-drained potting mix when planting caladiums in containers. Plants started indoors should be placed at the same depth as previously grown.
While caladiums enjoy warm weather, they don't tolerate dry conditions. In beds and borders, water plants every week during dry weather. To help conserve soil moisture, apply a 2-inch layer of mulch around the plants. Caladiums in containers should be checked regularly and watered when the potting soil begins to dry out. Caladiums have a moderate fertility requirement. In beds and borders, apply an all-purpose garden fertilizer and incorporate it into the soil before planting. Fertilize caladiums in pots with a soluble fertilizer every 2 to 4 weeks.
Elephant’s Ear (Colocasia, Alocasia, and Xanthosoma)
Start elephant's ear indoors 6 to 8 weeks before the average last frost date in your area. Plant the tubers 1 to 2 inches deep. Foliage may not appear for several weeks as elephant's ear tubers are often slow to sprout.
Elephant ear are normally grown from large, round to oval, brown tubers or from rooted plants. The tubers can be placed directly outdoors between 2 and 3 inches deep after the danger of frost has passed .
Elephant ear are normally grown from large, round to oval, brown tubers or from rooted plants. The tubers should be planted 2-4 inches deep in fertile soils once the danger of frost has passed (mid-May in central Iowa, one week earlier in southern Iowa, one week later in northern Iowa). Plants prefer part shade and often tolerate full sun (especially with adequate moisture). They grow best in moist, highly organic soils. Colocasia performs best in almost boggy soil conditions. Alocasia and Xanthosoma prefer a more well-drained soil but do not tolerate drying out. Water frequently to keep plants from wilting or developing yellow leaves. Fertilize regularly with a high-nitrogen fertilizer. The large leaves tatter easily in the wind. Locate plants or containers in protected locations to keep leaves looking good.
Dahlia (Dahlia spp.)
Plant dahlias indoors 6 to 8 weeks before the intended outdoor planting date. Pot up the dahlia tubers in large pots using a well-drained potting mix. Cover the tuber with 1 inch of soil, but leave the dahlia's crown exposed at the surface of the potting mix. Dahlia tubers are susceptible to rotting. Keep the potting mix moist but not wet.
Dahlias perform best in well-drained soil in full sun. (Plants that require full sun should receive at least 6 hours of direct sun each day.) Dahlias often rot in wet, poorly drained soils. Tuberous roots radiate out from the dahlia crown like the spokes of a wagon wheel. Viable tubers must have an eye originating from the crown portion in the center, plus a neck that connects the crown to the body of the tuberous root. Carefully divide the dahlia crown with a shape knife, scissors, or pruning shears.
Before planting tall cultivars, drive a sturdy support such as a metal fence post or wooden stake into the ground. Do this prior to planting to ensure that the tubers are not damaged by the post or stake. To plant tubers, dig holes about 6 to 8 inches deep. Place the tuber horizontally in the ground. Then, place the soil back in the hole. The dahlias should emerge in about two weeks. To avoid frost or freeze damage, plant dahlias in mid-May in central Iowa (about one week earlier in southern Iowa and one week later in norther Iowa).
Gladiolus (Gladiolus spp.)
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To get blooms earlier in the season, corms can be started indoors 4 to 6 weeks before your area's average last frost date. Corms should be planted 1 to 2 inches deep. After potting, water well and place the containers in a warm, 70 to 75°F location. Plant a portion of the corms in successive plantings indoors every two weeks. The remainder can be planted in succession directly in the garden to allow for continuous summer bloom.
Gladioli should be planted after the danger of frost is past, about mid-May in central Iowa. (Gardeners in southern Iowa can plant about one week earlier. Plant 1 week later in northern areas of the state.) Make successive plantings every two weeks for continuous bloom throughout the summer. The final planting should be made in early July.
When purchasing gladioli, select bulbs (corms) that are 1½ inches or larger in diameter. Large-sized corms will produce attractive flower spikes. Corms smaller than 1½ inches in diameter will produce foliage but may not bloom. Planting depth varies with the size of the corms. Large corms should be planted 4 to 6 inches deep and 6 to 8 inches apart. Small corms should be planted at a depth of 3 inches.
Gladioli require well-drained soils and perform best in a sunny location. Gladioli typically bloom about 8 to 10 weeks after planting. Gladioli require little special care during the growing season. Control weeds by frequent shallow cultivation or by mulching. Water weekly during hot, dry weather. A 10-10-10 fertilizer may be applied as a side-dressing about a month after planting. Staking is often required, especially in windy, exposed areas.
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Calla Lily (Zantedeschia spp.)
Plant calla lily rhizomes 1 to 2 inches deep in a well-drained potting mix about 6 to 8 weeks before the average last spring frost in your area. After potting, water well and place the containers in a warm, 70 to 75°F location. Keep the potting mix moist but not wet.
Calla lilies should be planted outdoors after the danger of frost is past. Calla lilies perform best in moist, well-drained soils in part sun (4 to 6 hours of sun) to part shade (2 to 4 hours of sun). Before planting, apply an all-purpose garden fertilizer to the area.
Calla lilies like a consistent supply of moisture during the growing season. Water weekly during dry weather
Banana (Musa spp.) & Abyssinian Banana (Ensete spp.)
This plant can be overwintered as a houseplant allowed to stay green and grow all winter. For these plants, be sure to acclimate them over a period of 7 to 10 days before moving them to their summer location outdoors. Learn more about acclimation here: Moving Indoor Plants Outdoors in Spring.
Those stored as dormant plants can be potted up 4 to 6 weeks before the average last frost date in your area. Place them in large pots and provide bright, indirect light and warm temperatures. Often these plants are so large it can be difficult to start them early indoors. Plants grow quickly once the warm summer temperatures arrive, so starting early is not necessary for success.
Plants do best in full sun, and they tolerate part sun. Plant in well-drained, moist, humusy soil rich in organic matter. Their fast growth requires abundant moisture and regular fertilization. Never allow the soil to dry out and use a mulch around the base to conserve soil moisture. Fertilize plants every week with a water-soluble fertilizer. The large leaves tatter easily in the wind. Locate plants in a protected site to preserve their appearance.
Tuberous Begonia (Begonia xtuberhybrida)
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Plant tubers indoors about eight weeks before the average last spring frost in your area. When planting the tubers, place the concave or indented side upward. The rounded side is the bottom. Cover the tubers with 1/2 to 1 inch of potting soil. Water well. Then, place the containers in a warm, 70°F location. Since the tubers are susceptible to rotting, keep the potting mix moist but not wet.
Plant tuberous begonias outdoors after the danger of frost is past. Tuberous begonias perform best in moist, well-drained soils in partial shade. (Partially shaded sites typically receive 2 to 4 hours of direct sun each day.) In the home landscape, areas that receive morning sun and afternoon shade are often excellent planting sites. The planting site should also be sheltered from strong winds. When planting tuberous begonias started indoors, place the plants at the same depth they grew in the container. Planting them deeper than previously grown may encourage the buried portion of the stem to rot.
Once planted, watering, fertilizing, and deadheading are the primary maintenance chores. Tuberous begonias should be watered weekly in beds and borders in dry weather. Plants in containers will have to be watered more often. Check container-grown plants regularly (daily or every other day) and water as needed. In garden areas, apply an all-purpose garden fertilizer, such as 10-10-10, to the area before planting. Fertilize container-grown tuberous begonias every 2 to 4 weeks with a dilute fertilizer solution. Remove spent flowers to improve their appearance and encourage additional blooms.