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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

Planting Tender Perennials in the Garden

This article was published originally on 5/6/2009

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. Examples of tender perennials include gladioli, dahlias, cannas, tuberous begonias, caladiums, and calla lilies. Tips for planting the aforementioned tender perennials are provided below.

Gladioli Glads should be planted after the danger of frost is past, about mid-May in central Iowa. (Gardeners in southern Iowa can plant about 1 week earlier. Plant 1 week later in northern areas of the state.) Make successive plantings every 2 weeks for continuous bloom throughout the summer. The final planting should be made in early July. When purchasing glads, 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. Glads 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 sidedressing about a month after planting. Staking will be required in windy, exposed areas.

Dahlias Dahlias perform best in a 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 soil back in the hole. The dahlias should emerge in about 2 weeks. To avoid frost or freeze damage, plant dahlias in mid-May in central Iowa.

Cannas Canna rhizomes can be planted outdoors in mid-May in central 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 prior to 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.

Tuberous Begonias 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. Use a well-drained potting mix when planting tuberous begonias in pots, window boxes, and other containers. When planting tuberous begonias, 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.

Caladiums 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. Use a well-drained potting mix when planting caladiums in containers. Plants 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 on a weekly basis 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.

Calla Lilies 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.

Year of Publication: 
2009
Issue: 
IC-500( 7) -- May 6, 2009