I would like to force some potted bulbs to bloom for Valentine's Day. I understand that they need to be chilled, can I do it in my garage? Can I chill them in their original packaging? How should I store them before I start the forcing process?
Minnesota Master Gardeners say:
Yes you can trick certain bulbs into blooming earlier than usual, it is called forcing. Bulbs that are candidates for forcing belong to one of two categories, those that need chilling prior to forcing and those that don't. For lists of bulbs that can be forced, see the FAQ on Bulbs That are Recommended for Forcing.
You can meet the chilling requirements of certain spring flowering Dutch bulbs if you have a cool basement or an old working refrigerator where you can keep potted bulbs at 35-48 degrees F. Another option is to dig a pit outdoors or use a coldframe, both of which must be well insulated with mulch to keep the bulbs from getting as cold as the air temperature during subzero conditions. If you use the outdoor method, protect the bulbs from rodents with wire hardware cloth. Unfortunately a heated garage is probably not cool enough, and an unheated garage in Minnesota typically gets too cold.
Different bulb species require different periods of chilling, averaging 12-15 weeks. It is during this chilling period that the bulb grows roots in preparation for the emergence of foliage and flowers in the spring. Bulbs cannot be forced to bloom if they are not well rooted in a medium like soil or water. Chilling the bulbs in their packaging won't do the trick.
Once you have chilled the bulbs for at least the minimal amount of time required (or longer), they can be brought into a warm light environment to begin the actual forcing. If all has gone well you should see roots emerging from the drain hole on the bottom of the pot and shoots emerging from the soil surface. At this point your bulbs will take 3-4 weeks to flower, given the right conditions. The longer you keep the bulbs in cold storage after they have completed their rooting period (i.e. the closer to spring it is), the quicker they will bloom when forced.
Knowing how long a bulb must be cooled, and knowing the date you want blooms, enables you to calculate when the bulbs should be started in pots.
Check out this fact sheet Forcing Bulbs for Indoor Beauty in Winter for more details on the forcing process. http://www.extension.umn.edu/distribution/horticulture/DG1116.html
or this PDF file from Kansas State, Forcing Bulbs:
http://www.oznet.ksu.edu/library/hort2/mf2077.pdf
Check this bulb forcing timetable, from The Netherlands Flower Bulb Information Center, to help you plan your winter show for a given date: http://bulb.com/spring/timetable.asp
With regard to forcing hyacinths, they are sold as either pre-cooled or uncooled; the former will take less time to force, as indicated in the above timetable.
Now, if you want to start some Iris reticulata for Valentine's Day, for example, they require a 15 week cool rooting period and 2 weeks (give or take) to come into bloom once they are brought into warmth and light. October 17th is the date you would start your pot of iris, 17 weeks prior to 2/14.
If you need to store bulbs in the fall before the cooling/rooting treatment they can be kept for several weeks in mesh bags or open paper bags either in a working refrigerator, or hanging in a cool dark aerated room. Keep the bulbs in mesh bags, with good air circulation and check them periodically to make sure they are not molding or drying out (shriveling). Storing bulbs in a crisper drawer together with fruits or vegetables is not ideal, especially in homes with small children, since *some* bulbs are poisonous. Secondly, ethylene gas released from ripening produce can potentially damage the dormant flower in bulbs. See this article, Ethylene Gas and Its Effects Upon Flower Bulbs and Cut Flowers, from The Netherlands Flower Bulb Information Center for more information: http://bulb.com/spring/ethylene.asp
It is desirable to use a refrigerator devoted just to bulb forcing, if possible.
