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
"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,
"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
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
"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
"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
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
"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."
Source: Sharon Yiesla (847) 223-8627
Contact: Bob Sampson
Extension Communications Specialist
Phone (217) 244-0225;