Small Bulbs:

Most of the earliest bulbs are less than an inch in depth (5 cm). Select crocus, miniature daffodils, scillia, Emperor tulips, miniature hyacinth and small iris. Outside, these little bulbs erupt through cracks in soil that is still too hard to dig in. They can tolerate frost and snow and because they are close to the ground level, they are successful in the south as well as further north. These bulbs take about 2 months of chilling and about 3 weeks to bloom.

Mid height bulbs: Mid-season daffodils, early tulips and scented hyacinths are the most common of these bulbs. The bulbs are 1-1.5(5-8 cm) inches across and are correspondingly planted a little deeper than the small bulbs. These bulbs need about 10-12 weeks chilling and take a little over a month to bloom.

Late season bulbs are the largest and tend to be too large for indoor forcing but do well outside in the center of a layer arrangement. These are the daffodils that rise 2-feet (50cm) or so outside and do not do well outside, in the south. Tulips start later than the earliest bulbs and can be as tall as some of the daffodils. These bulbs need longer chilling time – up to about 3 months for some. They also take longer to break dormancy and over a month to flower.

Professionals:

Professionals play with the temperatures to hold or advance the bulbs. You can adjust the timing if you have a special occasion that you want the forced display for. If the bulbs come out of dormancy a week or two early, put them back in the cold fridge for a few days to retard the growth. If the bulbs are looking like they are taking too long, bring them out of the cool 60⁰ room and place in a warmer room to hasten development.

Growing bulbs in container and forcing them into bloom in the middle of January is a great way to keep the thumb green during long, or short, winters.

A great comprehensive book on forcing bulbs is Bulb Forcing for beginners and the seriously smitten by Art Wolk : 2012. Art displays his award-winning blooms at the Philadelphia Flower Show.