February 20, 2006
Posted online March 15, 2006

Gardening with summer bulbs

By Bob Sampson

While summer bulbs are summer-blooming plants that have some type of underground storage structure, most of them don't look like bulbs, said a University of Illinois Extension horticulture educator.

"This is because most of them are not true bulbs but some type of underground storage structure," said Sharon Yiesla. "In northern Illinois, the vast majority of summer bulbs are not cold hardy and will not survive the winter outside. They are often referred to as 'tender' bulbs.

"These plants need to be dug up at the end of the season and the storage structure kept indoors until the following planting season--after the danger of frost has passed."

Yiesla described various types of summer bulbs and what the gardener might expect to see.

True bulbs, she noted, are the structure with which most gardeners are familiar. They are the underground storage structures of some of our common spring bulbs--tulips and daffodils. A true bulb is round on the lower end and pointed on top. Summer bulbs that are true bulbs include pineapple lily.

"Corms are swollen stem bases modified into a mass of storage tissue," said Yiesla. "When the corm is cut in half, the storage tissue appears solid. From the outside, a corm looks like a bulb that has been squashed down from the top. Summer bulbs that are really corms include acidanthera, gladiolus, and tigridia."

Tubers are swollen stems that are modified into storage tissue. A tuber is distinguished by its nodes or "eyes" (like those seen on a white potato). Tubers vary a bit in their shape, but most of them are fairly plain and featureless, except for the "eyes" which may be very small. Summer bulbs that are really tubers include begonia, caladium, elephant ears, and ranunculus.

"Tuberous roots are enlarged roots that serve as storage organs," she said. "Buds are found at the stem end and fibrous roots form at the opposite end. Summer bulbs that are really tuberous roots include dahlia."

Rhizomes are swollen stems that grow horizontally under the surface of the soil or just at the surface level. Roots are produced on the lower side of the rhizome and shoots are produced on the upper side. Summer bulbs that are really rhizomes include calla lily and canna.

"Tender summer bulbs cannot be put into the garden until after the danger of frost is over," Yiesla said. "Many of them can be started indoors for a jump on the gardening season. Most of them just need a container of moist soil, some warm temperatures, and a sunny window.

"These bulbs vary in the care they receive in the summer. Looking at the end of the season, we need to be aware that these summer bulbs must be dug and stored inside for the winter. Most will be dug around the time of the first fall frost."

Yiesla reviewed some common summer bulbs and how they are grown.

Tuberous begonias (Begonia x tuberhybrida), grow from a tuber. The plants range from 12 to 18 inches tall and there are also trailing types. They come in a wide range of flower colors, from pastels to brights. Bloom time is from summer into autumn.

"Begonias should be planted about 1-inch deep, with the concave side of the tuber facing up and spaced about 9 to 12 inches apart," she said. "They do best in partial shade. Keep the soil evenly moist, but not wet. Avoid wetting the foliage since disease can be a problem with begonias.

"Dig tubers after the first frost. Dry the tubers in the open air for a few days, then store them in dry peat moss or vermiculite at 35 to 40 degrees."

Caladiums (Caladium x hortulanum or Caladium bicolor), grow from a tuber. These plants reach 1 to 2 feet in height and have colored foliage in a mix of red, pink, green, and white. These plants are grown for their beautiful foliage, not flowers. They should be planted about 1-inch deep, with the knobby side of the tuber facing up and spaced about 8 to 12 inches apart.

"They do best in partial shade, although there are some newer varieties that tolerate sun," said Yiesla. "Keep soil evenly moist, but not wet. Dig tubers in the fall before the leaves lose all their color. Dry the tubers in the open air for a few days, then store in dry peat or vermiculite at 70 to 75 degrees. Tubers may rot if they are stored below 70 degrees or if they get wet."

Calla lilies (Zantedeschia species and hybrids), grow from a rhizome that looks somewhat like a tuber. Plants grow 1 to 4 feet tall and have waxy, trumpet-like flowers in a wide range of colors. Bloom time occurs in mid to late summer.

Yiesla said calla lilies should be planted about 1-inch deep and spaced about 1 to 2 inches apart. The plants like full sun and should be kept in soil evenly moist to wet. In the fall, after the foliage withers, the plants should be dug. Allow them to dry and store at 50 to 60 degrees.

"Cannas (Canna x generalis), grow from a rhizome," she said. "The plants range from 2 to 7 feet tall, depending on the variety grown. Cannas are grown for both flowers and foliage. They come in a wide range of flower colors--red, orange, yellow, pink, and some bicolors. Some cultivars have colored foliage as well. Bloom time is summer into autumn."

They should be planted about 3 to 4 inches deep. Rhizomes should be planted horizontally and spaced about 18 to 24 inches apart. They prefer full sun, and the best growth occurs in a rich soil with a good supply of moisture.

"Dig rhizomes in the fall after first frost," she said. "Let the rhizomes dry for a day or two, then store in dry peat or vermiculite at 40 to 50 degrees. Do not allow rhizomes to freeze."

Dahlias (Dahlia hybrids), grow from tuberous roots. Plants range from 1 foot to 8 feet tall. There is also a wide variety of flower types, including anemone-flowered, ball, pompom, cactus, semi-cactus, and others. Dahlias also come in a wide range of flower sizes and colors. Bloom time is from summer into fall.

Dahlias should be planted with the crown--the end with the buds--just above soil level. Spacing of the tuberous roots depends on the mature size of the plant. Dahlias that will reach 3 feet or less should be spaced about 2 feet apart; larger plants should be spaced about 3 feet apart. Most dahlias will need staking.

"These plants prefer full sun," said Yiesla. "Keep soil evenly moist, but not wet. To obtain larger flowers, allow only one shoot to develop. Disbudding will also lead to larger flowers--flower buds come in threes so remove two side buds.

"Dig tuberous roots after the first frost. Before digging, cut the tops off so that a 3-to 4-inch section of stem remains attached. Tuberous roots tend to spread, so dig away from the main stem. Try to dig the entire clump to avoid injuring the individual tuberous roots. Remove the majority of the soil from the clump. Let the remainder dry and remove it once dry. Store in dry peat or vermiculite at 35 to 50 degrees. Packing material can be very slightly moist to reduce shrinkage of the tuberous roots."

Elephant ears (Colocasia esculenta), grow from a tuber. Plants are 3 to 3 1/2 feet tall, with large, tropical-looking leaves. They are grown for foliage, not flowers. Plant the tuber 4 to 6 inches deep with the blunt end down and spaced 2 to 3 feet apart. Elephant ears like full sun to partial shade and a moist to wet soil. Protect the plant from wind as the large leaves can be easily damaged.

"Dig the plants up in autumn after the first frost," she said. "Allow tubers to air dry and store in dry peat or vermiculite at 70 to 75 degrees."

Gladiolus (Gladiolus x hortulanus), grows from a corm. The plants range from 1 to 5 feet tall, and the flowers come in a wide range of colors. Bloom time is mid-summer. They should be planted 4 to 6 inches deep and spaced about 3 to 6 inches apart. Gladiolus likes full sun. The soil should be kept evenly moist, but not wet. They should be protected from the wind, and staking may be necessary.

Corms should be dug after the first frost. Cut the stems just above the corm and air dry the corms for a week or more. Store in mesh bags or slotted trays with good air circulation and store at 40 to 45 degrees.

Pineapple lilies (Eucomis autumnalis and Eucomia comosa), grow from true bulbs. The plants are 1 to 2 feet tall with a low rosette of strap-like leaves. The greenish flowers are in spike-like clusters atop a 1- to 2-foot stem.

"The flower spike is topped with a cluster of leaf-like bracts, causing the flower cluster to resemble a pineapple," said Yiesla. "Bloom time is mid- to late summer.

"Plant the bulbs 5 to 6 inches deep about one inch apart. Pineapple lily likes full sun to light shade and well-drained soil. Dig the bulbs before the first frost and store them at 55 to 65 degrees."

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Source: Sharon Yiesla (847) 223-8627
Contact: Bob Sampson
Extension Communications Specialist
Phone (217) 244-0225; rsampson@uiuc.edu