Forcing Spring Bulbs
Presentation for London Horticultural Society Meeting on February 24, 2025,
by Eva Norman-Vestergaard
Forcing spring bulbs using the cold treatment adds colour, a breath of spring to come and some have fragrance. You must act in the fall. Hardy bulbs for indoors should be started in the fall. They include crocuses, daffodils, grape hyacinths, tulips, snowdrops, iris reticulata, scilla and hyacinths.
In the fall buy healthy plump firm looking bulbs free from bruises and nicks and have no sprouted roots. The so called “bargain” bulbs and damaged bulbs may bloom poorly.
Bulbs except Paperwhites and Amaryllis require a chilling period at 35-45 degrees F to make them root and bloom. The chilling period varies by the type of flower. The smaller the bulbs, the easier it is to force them.
If the package you purchase has a “prechilled “notice, then avoid this stage of forcing.
Basic Forcing Procedures:
1. Select a clean container that is twice as deep as the bulbs and has drainage holes.
2. Partially fill the pot with moist potting mix, try to arrange the bulbs close to but not touching each other. Add enough soil to cover the bulbs, leaving the tips showing below the rim of the container.
3. Plant flat sided- bulbs like tulips, with the flat side facing the edge of the container, to allow the first large leaves to form a border around the pot.
4. When planting multiple bulbs in one large pot, make sure they all have the same bloom time.
5. Cover the pot with a paper bag and store in a cool dark place (35-45 degrees F) for chilling 12-16 weeks see chart. Use the refrigerator or cold cellar or an outdoor trench, do not allow the bulbs to freeze. Keep fruit out of the refrigerator, as ripening fruit produces ethylene, a gas that will stop bulbs from flowering.
6. Regular regime to check the soil to maintain damp ,but not wet soil.
7. Keep the potted container in cold storage until root formation occurs. Root formation must occur first before top growth. When removed from cold storage the pale sprouts should be showing, leave in cool area until the shoots turn green. Gradually move into a warmer area with more light.
Pre- chilled hyacinth bulbs may be grown in water -filled hourglass-shaped vases called hyacinth glasses. Maintain the water level at 0.3cm (1/8”) below the base of the bulb. Place in a cool dark place for the rooting period and move into gentle warmth when the growing tips are about 3” long. Prefer a cooler location at night to lengthen their bloom. Sometimes the bloom fails to stretch out of the rosette, thus cover the pot with a paper cone or large paper cup until the bloom has stretched. Check florist stores or thrift shops for hyacinth vases.
Chilling and Blooming Times
Crocus - chill 8-15 weeks, blooms in 2-3 weeks
Daffodil- chill 12-15 weeks, blooms in 2-3 weeks
Grape Hyacinth- chill 8-15 weeks, blooms in 2-3 weeks
Iris reticulated – chill 13-15 weeks, blooms in 2 weeks
Hyacinth-chill 8-15 weeks blooms in 2-3 weeks
Tulips- chill 10-16 weeks, blooms in 2-3 weeks
Paperwhites are the most common flowering bulb used, requiring no chilling. Wedge them in stones with the tips exposed. Paperwhites can be grown on sand, gravel, pebbles or marbles. Fill the container with water to ½ inch below the medium surface. Place container in a well lit room at 60 degrees F. Keep bulbs well watered. Blooms appear approx. 5-6 weeks.
References
https://www.gardenworks.ca/tip-sheets/forcing-bulbs
https://hgic.clemson.edu/factsheet/forcing-bulbs-indoors factsheet HGIC 1556
https://store.extention.iastate.edu/product4715/ sheet on forcing bulbs