Skip to Main Content

LuEsther T. Mertz Library
Plant & Research Guides

Bulbs: A Seasonal Cycle: Tender Summer Bulbs

Dahlia 'Bishop of Leicester' at NYBG; photo by Ivo Vermeulen
Dahlia 'Bishop of Llandaff' at NYBG; photo by Ivo Vermeulen

Dahlia (Dahlia hybrids)

- come in a wonderful range of heights, shapes and colors. Their flowers are always spectacular and may be shaped like anemones, peonies, cactus flowers or ping-pong balls. The plants can be as small as 1 foot tall or as large as 7 feet. Most of them fall in the range of 1 to 4 feet. Try the Gallery series for compact container plants; for dark foliage look for 'David Howard' (orange flowers), 'Bishop of Leicester' (lavender flowers), 'Bishop of Llandaff" (red flowers) or 'Fascination' (lilac-pink flowers).

Start them off early by planting the tubers in a pot 4 weeks before the last frost date, planting 2 to 3 inches deep. Once all chances of frost has passed, transplant them outside in a sunny spot in rich, well-drained, garden soil. Stake immediately so that you do not damage the tubers when trying to place supports later.

If you plant the tuberous roots directly into the ground, there are many techniques to follow. The most common is to dig a deep, wide hole to about 10 inches and amend with compost. Fill the hole to 6 inches and lay the tuber in the hole so that the eyes (new growth) are facing upwards; fill the hole 2 to 3 inches more. Once the dahlia starts to grow, slowly fill the rest of the hole.

Larger dahlias can be pinched when they are 10 to 12 inches tall for shorter, bushier plants. Water well during dry spells; deadhead to promote more blooms and feed every few weeks with a balanced liquid feed starting from mid-summer into fall.

Dahlias are indigenous to Mexico and Central America. In their native climates the tuberous roots (that look like oblong potatoes radiating out from the central stalk) stay in the ground. In the New York area, they need to be lifted after the plants begin to die back from frost exposure. Allow them to dry out for a few days then store them in a cool, dark place over winter. A protective technique is to store dahlia tubers in peat moss, vermiculite or Styrofoam peanuts.

Gladiolus (Gladiolus hybrids)

- these magnificent, cut flowers grow from corms. They can be planted in the garden starting in May and then planted every 2 weeks through the middle of June to produce a long show throughout the summer. Plant either in a straight row for cutting or in a small cluster of 5 to 7 flowers for a vertical accent in a border. For your floral arrangement, cut the stem when just 2 to 3 flowers are open.

"Glads" prefer to be grown in full sun in well-drained soil. Plant the corms pointed side up (they look a bit like a chocolate kiss). Plant 4 to 6 inches deep and approximately 6 inches apart. Most gladioli will need staking; if you stake when planting you will not damage any of the roots.

They are generally used as annuals. If you are planning to over-winter the corm, leave at least 4 leaves on the plant when you cut the flower. Cut the foliage down to an inch once the frost has cut the plant back. Dry the corms for up to 3 weeks.  Clean off the old, tired corms that are underneath the new corm that has formed during the season.

Store the corms in a cool, dark, well-ventilated area in a paper bag or an egg carton. If you have had an insect problem during the season (thrips is common) then add moth balls to the paper bag or soak the corms in 4 teaspoons of Lysol to 1 gallon water for 6 hours and then dry. If this feels like too much work, some of the species are hardy to Zone 6. Look for Gladiolus caucasicus and Gladiolus dalenii hybrids, especially  Gladiolus dalenii 'Boone'.

Flower of gladiolus dalenii; photo by Ivo Vermeulen

Gladiolus dalenii in The Ladies' Border at NYBG; photo by Ivo Vermeulen


Tuberous Begonia (Begonia x tuberhybrida Group)

- many people are familiar with the ubiquitous wax begonia (Begonia semperflorens Cultorum Group) and the angel wing begonia (Begonia coccinea) that graces every garden center. These adaptable plants have fibrous roots, grow in full-sun to part-shade, and flourish in well-drained soil that has been amended with organic matter. They are easy to grow and terrific for the beginner gardener. 

Once you have mastered the wax begonia, graduate to their tuberous cousins. Tuberous begonias produce huge flowers that come in a wonderful array of sorbet shades. Flowers can be shaped like a rose or a camellia. For maximum flower power, try the 'Nonstop' begonias. If you like frills, look for 'Picotee' begonias whose flowers are edged with a contrasting color. A new favorite of some growers for container gardening is the On Top series of begonias.

You can buy the plants in spring or buy the tubers. Place the tubers on a damp tray of peat moss in February with the round side facing down and the flat side up. Once they start to show shoots and roots, plant in a well drained potting mix (plant nice and shallow so the tubers don't rot - about ½ inch below soil line ). Plant them outside once the temperatures have warmed up and all dangers of a frost are gone.

Tuberous begonias like well-drained, rich soil and do best in bright, indirect light or partial shade. Feed them either at the beginning of the season with a slow release fertilizer or add compost to the planting mix and then give them a boost of fish emulsion every few weeks. At the end of the season, pull out the tubers after the first light frost; let them dry out for a day in the sun and then store them in peat in a cool, well-drained area.

Poppy anemone (Anemone coronaria)

- one of the early harbingers of spring is the diminutive, Grecian windflower (Anemone blanda). Its taller cousin Anemone coronaria arrives later in the spring, flowering in May and June. While the Grecian windflower is hardy in this area, the poppy anemone is only hardy to zone 7. They are tuberous rhizomes that perform best if soaked overnight before planting.

Plant Anemone blanda in the fall with the rest of your hardy spring bulbs. Anemone coronaria can either be potted up in fall and over-wintered in a cool greenhouse or potted up early in spring and then moved outside for a later bloom time. Anemone coronaria - like most geophytes (bulbs, corms, etc.) - like well drained soil. The poppy-like flowers come in red, white and purple/blue with rich black centers. The De Caen Group comprises popular, single varieties while the Saint Bridgid Group is a nice choice for double-flowering anemones. These short-lived, inexpensive tubers are best used as annuals: plant two inches deep and four inches apart.

Persian Buttercup (Ranunculus asiaticus)

- is another late-spring/early-summer showstopper. The tuberous roots look like a miniature bunch of bananas or a tiny octopus. Hardy to zone 8, this tender bulb is best planted a few weeks before the last chance of frost or potted up early in spring (February or March). Plant 2 inches deep and 6 to 8 inches apart in well-drained soil with the claw-like structure facing downward and the fuzzy head on the top.

Persian buttercups make a wonderful and especially beautiful cut flower. They look like graceful, miniature, double poppies with layers of delicate petals.  This geophyte likes the cooler weather, so get it outside as early as you can. It will go dormant with the summer heat. The tubers are difficult to store; you can store them in peat moss in a cool location, or better yet, just use them as annuals. Look for the Bloomingdale Series for sensational colors and a beautiful intricate rose-shaped flower. 

Ask a Plant Expert

Contact Us

with your plant questions by email

Find a Plant at NYBG

Find a Plant at NYBG

Map of NYBG

Noteworthy Books on Summer Bulbs