Bulb Forcing

forcing-bulbs

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As a gardener, each day in each season holds its own special joys. Right now, we are enjoying a warm, glorious fall. The colors on the trees are magnificent. Except for the fact that it is dry and we desperately need rain, the days have been magnificent. Along with enjoying the moment, however, gardeners must also look ahead to the future and prepare for what is to come. I hate to mention it, but right around the corner are the cold, gray, dreary days of winter. NOWis the time to think about how you can get ready for that time. I am potting up bulbs for forcing in January, February and March–to take away the dreariness–and you can too!

I gave a workshop on this last Saturday and everyone in attendance just kept commenting that it was “so easy”. They were right. To prepare bulbs for forcing, you only need to grasp a few basic concepts. First of all, you are duplicating nature. Bulbs need a dormant period in the soil when they grow roots. In nature, this lasts anywhere from 12 to 16 weeks or more. To force bulbs, you must duplicate that dormant period. Early blooming bulbs like crocus, miniature iris, scilla, and chionodoxa require a 10-12 week chill period. Later blooming bulbs such as grape hyacinths, narcissus, hyacinths, and April blooming tulips need 14 weeks of dormancy. I do not pot up late blooming May tulips as they don’t force very well–stick to the early and mid season bloomers. How do you achieve this dormant period? Pot up the bulbs in a well drained houseplant soil. Water them thoroughly.

Label them with the date you potted them and then put them in a dark, cool spot that remains above freezing but drops down to between 38 and 45 degrees during the winter. An old refrigerator is ideal as you can control the temperature inside. I use the steps leading down into our cellar, beneath the Bilco doors, as the heat from the furnace directly across from the cellar door seems to warm the steps up just enough to keep them above freezing. At home, I can use my cellar as I have electric heat and don’t have a furnace to keep the cellar too warm. Some people dig a deep trench, line it with sand, sink the pots in the trench, and cover them with wire mesh and leaves or hay. I have also used an insulated cold frame placed in a shady spot – lining it with leaves and covering it with heavy blankets. The goal is to prevent the pots from freezing solid. You may think that in the ground, the bulbs do freeze, but actually, unless our winters are very severe (which is rare) the soil below the surface where the bulbs reside stays just above freezing. Check the bulbs for watering every few weeks. They should not go bone dry, but never keep the soil soggy as they are rooting in and growing very slowly and using up very little water. Watch for rodents who will consider your potted bulbs their winter treat. I will sometimes wrap the pots in hardware cloth to keep the rodents out (they love tulips, don’t bother daffodils) Using a refrigerator solves that problem. Be creative and come up with a method that works for you.

Mark on your calendar when you potted the bulbs and count the required number of weeks ahead. Mark that date on your 1998 calendar. Anytime after that, you can bring the pots indoors and begin to force them. At that point, you will know that they are ready because the bulbs will have grown roots. If you tug on them, they will give you resistance. They will also have grown shoots, white pointy growing tips pushing out of the soil. When you bring them indoors, water them and put them in the light. Depending on the varieties chosen, it will only take a few weeks for them to bloom. Stagger your forcing schedule–bring a few pots into the house each week. If you pot up enough bulbs now, you can have color all winter long. And believe me, there is NOTHING that beats the joy of spring blooming bulbs on the windowsill went your soul is sick of winter.

Potting and Cooling Temperatures Charts for Hardy Bulbs

Bulb

Crocus
Hyacinth
Iris
Narcissus (hardy)
Puschkinia
Scilla
Tulip

Potting depth

1 inch deep
tops of bulbs out
1/2 inch deep
1 inch deep
1 inch deep
1 inch deep
tops of bulbs out

Cold, dark period

10-12 weeks
12-15 weeks
12-15 weeks
12-15 weeks
10-12 weeks
10-12 weeks
14-16 weeks

Cool, light period

2-3 weeks
2-3 weeks
2-3 weeks
2-4 weeks
2-3 weeks
2-4 weeks
w-4 weeks

In an effort to provide horticultural information, these educational documents are written by Nancy DuBrule-Clemente and are the property of Natureworks Horticultural Services, LLC. You are granted permission to print/photocopy this educational information free of charge as long as you clearly show that these are Natureworks documents.