Yard and Garden: Starting Flower Tubers Indoors for Spring



AMES, Iowa – Winter weather still rules Iowa’s landscape these days, but spring isn’t far off on the horizon. It’s time to think about spring flowers and getting a head start by starting tubers indoors. It’s an excellent way to get plants growing even ahead of the final frost and beautify yards and gardens for summer.

Iowa State University Extension and Outreach horticulturists can help answer queries about starting tubers indoors. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

When should I plant tuberous begonia tubers indoors?

Plant tuberous begonia tubers indoors eight to 10 weeks before the average last spring frost in your area. All containers should have drainage holes in the bottom. Use a well-drained potting mix. When planting the tubers, place the concave or indented side upward. The rounded side is the bottom. Cover the tubers with ½ to 1 inch of potting soil. Water well.

Then place the containers in a warm, 70 degree Fahrenheit location. Since the tubers are susceptible to rotting, keep the potting soil moist, but not wet. Once the tubers sprout, move the plants to a sunny window or place under fluorescent lights.  

Harden (acclimate) the tuberous begonia plants outdoors seven to 10 days before planting. Initially, place the plants in a shady, protected location. Then gradually expose the plants to longer periods of early morning or late afternoon sun. Plant tuberous begonias outdoors when the danger of frost is past.

When should I plant caladium tubers indoors?

Plant caladium tubers indoors four to six weeks before the average last frost date in your area.  Tubers should be planted 1 to 2 inches deep. When planting the tubers, place the knobby side up. The knobs are the tuber’s eyes or buds. After planting, place the containers in a warm, 75 to 80 F location to facilitate sprouting. When foliage begins to emerge, place plants in a sunny window or under fluorescent lights.  

Harden the caladiums outdoors seven to 10 days before planting. Plant caladiums outdoors after the danger of frost is past.

When should I start dahlias indoors?

For earlier bloom, plant dahlia tubers indoors in early April. Fill an 8-inch-diameter pot about one-half full with a commercial potting mix, place a tuber horizontally on the potting mix surface, then cover with an additional 1 to 2 inches of potting soil. Water thoroughly. When growth emerges, place the dahlias in a sunny window or under artificial lighting.  

Plant dahlias outdoors after the danger of frost is past. Before planting, harden or acclimate the dahlias to outdoor conditions for several days.