Warm spells in winter can cause the soil to briefly thaw and send a signal to flower bulbs that it's time to wake up and grow. When the weather inevitably turns wintery again, the cold air and re-freezing soil can damage the bulbs' growth, making them unlikely or unable to bloom once spring truly arrives.
To prevent winter warm spells from waking your flower bulbs early, cover their planting sites with a loose mulch once the ground has frozen solid. This will insulate the earth, keeping it a more uniform temperature throughout winter, regardless of what the weather is doing. In spring, you can either pull the mulch back or let the bulbs growth up through it, which they will do at the appropriate time.
Image: "Shade garden evergreens in winter" by User:SB_Johnny - my photo. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons.
Learn all about gardening with flower bulbs with Horticulture's Bulbs CD.
Make planting flower bulbs quick and easy work,m even if you have many bulbs to plant, with the Bulb Planter by Clarington Forge.
Mix and match plants for your garden with the Trio of Garden Wheels: Bulb, Herb and Perennial.