Growing Tender Bulbs, Tubers, and Corms


What are they? Tender bulbs, tubers, and corms are plants that are hardy south of CT, usually in zones 7-10. Most are native to the tropics. They are planted in the spring in CT, grow and produce flowers all summer and into the fall. They are then dug up and stored for the winter and replanted again the following spring.

Examples: Dahlias, Calla lilies, Gladiolus, tuberous begonias, Acidanthera, Caladiums, Canna lilies, Ranunculus, Colocasia (elephant ears), Eucomis (pineapple lily), are all examples of these types of plants.

When do I plant them? These bulbs can be planted directly in the ground after the danger of frost is over. In CT, that is usually around the middle of May. If you use a soil thermometer, the soil should warm up to about 60 degrees before putting the bulbs in the ground. Ranunculus are an exception; they can take very cool temperatures, similar to pansies.

Should I/can I start them indoors ahead of time? For some plants, this makes sense, especially if you want to get a jump on the season. For others, they are usually just planted directly in the ground.

  • Dahlias can be started indoors 3-4 weeks before the last frost date. They are large tubers and should be planted in well drained, organic potting soil. Each tuber should be planted in a one gallon sized pot.
  • Tuberous begonias are often started ahead of time. The tubers are place hollow side up on top of the soil. They are often started in flats and later transplanted to pots. You can also start them in shallow pots. Cover the tubers with a mere inch of soil and place in a warm spot to encourage rooting and sprouting.
  • Caladiums can be started indoors 3-4 weeks before the first frost date. They are very susceptible to frost so make sure you wait until there is no danger of frost before planting them outside.
  • Gladiolus are usually planted directly in the garden, starting in the middle of May. You can stagger your plantings to prolong the harvest season for these excellent cut flowers. Acidinanthera are a fragrant form of dwarf gladiolus and are treated the same.
  • Canna lilies are large, dramatic bulbs. If you want to start them indoors, be sure to use a 1 gallon or larger pot. They are usually planted directly in the ground.
  • Colocasias are also very large bulbs and can be started ahead of time in 1 gallon or larger pots.
  • Ranunculus love cool temperatures. They are usually started in pots indoors in mid-late winter and used in containers and window boxes outside in the spring.
  • Calla lilies can be started indoors 3-4 weeks ahead of the frost free date and then planted out in mid-May. They can also be easily grown as houseplants as they are related to Philodendron plants. All of our tender bulbs come with instructions on exactly how to start them.

Do all of these bulbs need full sun? If you have dappled shade or morning sun, tuberous begonias and calla lilies are the bulb for you. Caladiums do best in shade and Colocasias will grow in shade or sun. All of the others prefer at least 6 hours of direct sun each day.

How do I amend the soil? Bulbs, corms, and tubers are underground storage organs and MUST be planted in well drained soil. Avoid soggy, wet areas. Add compost and Pro Gro organic fertilizer to the planting bed and remove all rocks and perennial weeds. Water well after planting with Organic Plant Magic and add 2” of organic mulch.

How deep do I plant them? They all are planted at different depths depending on the size and configuration of the bulb, corm, or tuber. Follow the instructions that come with the bulbs for each individual variety.

Do I need to stake them? Dahlias need to be staked, especially the large varieties. Put a sturdy hardwood stake in at the time of planting to avoid damaging the bulbs or roots later. The others are usually not staked or smaller bamboo stakes are added later as they are growing, especially for tuberous begonias which are brittle.

What other care do they need in the summer? As your dahlias and tuberous begonias grow, pinch the growing tips when young to encourage bushy, well branched plants. Feed all tender bulbs every two weeks with Dr. Earth Rose and Flower Fertilizer (granular or liquid, whichever you prefer).

How do I store the bulbs? After the first frost has nipped the foliage, dig the bulbs/corms/tubers very carefully. They will have increased in size underground so expect them to be a lot bigger than when you planted them! Lay them in a garage or shed for a few days until the soil has dried out. You can dry them in the sun on a nice fall day but don’t leave them exposed to cold night temperatures, rain, or dew. Once dry, carefully remove excess soil and cut off leaves and stems, leaving 1-2”. Store in a ventilated crate or basket (milk crate, plastic laundry basket, etc.) in slightly moistened sand, peat moss, or vermiculite. Store in the basement or a cool place that stays between 35 and 45 degrees during the winter. Do NOT store in an unheated garage or outbuilding. If they freeze all the way through, they will die. Check them on President’s Day. Trim any rotten spots off and discard any rotten bulbs. Mist slightly if very dry and shriveled.

Can I leave any of these plants in the ground and not dig them up? Some people find that gladiolus will overwinter in a sheltered spot, especially against a south facing wall or up against the house. Calla lilies have also been known to overwinter in southern CT in mild winters. But, to protect your investment in tender bulbs, plan on digging them and storing them inside for the winter.

Late winter/early spring…The cycle begins again!