Issue: December 24, 2005

Problem forcing bulbs to flower

Question:

In early October, I put some tulip bulbs in a pot of soil. I kept them outside in the shade until the end of November. After I brought them indoors, they began growing but flowers didn't open. What did I do wrong?

Rio Rancho

Answer:

There are several possible reasons why your tulips didn't flower. Perhaps the tulips did not experience sufficient cold pretreatment after you planted them. They need from 14 to 15 weeks (or more) at a temperature of 35 to 45 degrees F. to fully form their roots and prepare to flower. Some other spring flowering bulb plants can be forced to bloom with less cold pretreatment. Your tulips may have bloomed well if left longer in cold treatment. However, the recent cold weather was too cold. Temperatures for the cold pretreatment of bulbs should be near 40 degrees F. If the temperatures are colder (down to 32 degrees) or warmer (up to 50 degrees), longer treatment is needed. At temperatures below freezing, the bulbs may be damaged if they alternately freeze at night and thaw each day. Below freezing, both the root development and the flower initiation process cease until the temperatures warm to just above freezing. Below freezing temperatures do not contribute to the pretreatment process. The cold treatment may be more successful in an unheated garage or a shaded cold frame where constant cool temperatures can be maintained and extreme temperature fluctuations avoided.

You can store the bulbs in your refrigerator for 3 or more weeks before planting in pots. This will reduce the pretreatment time in pots by 3 weeks. However, you must still pot them and allow their roots to develop in a cool environment before you can bring them indoors to flower.

Another possibility is that you warmed the bulbs too quickly. After cold pretreating the bulbs at low temperatures, place them in a slightly warmer location (50 to 60 degrees) for a few days to allow leaf development. As the leaves develop, move them to a place with bright light. If the bulbs are moved directly from the cold treatment into a warm home, the flowers will often "blast" and fail to open. Once the leaves have begun to open and turned green, you can move them to the location where you will display them. If the house is very warm, the flowers will not last very long. Cool nights in the house will prolong the flowering period.

It is also possible you bought tulip varieties that do not force easily. Some varieties are recommended for forcing. Choose those varieties. If the bulbs were too small, they will not flower well. If they were bruised or otherwise damaged, they are not good candidates for forcing.

Forcing spring flowering bulbs is an interesting garden activity but can also be more complicated than growing other houseplants.

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Send gardening questions to Southwest Yard and Garden, Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu or at https://www.facebook.com/DesertBloomsNM/. Please include your county Extension Agent (aces.nmsu.edu/county) and your county of residence with your question!

For more gardening information, visit the NMSU Extension Horticulture page at desertblooms.nmsu.edu.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505-865-7340, ext. 113.