They may have been deep enough when you planted them, but as the soil goes through the freezing and thawing process, the bulbs can actually work their way up in the ground. One way to keep your flower bulbs sleeping longer, which will protect them from freezing, is to mulch the bed. This layer of mulch will do a couple of things. It will maintain a higher moisture content in the soil, which is good, as long as the soil isn t too soggy. Well composted mulch also adds valuable organic matter to the planting bed.
Organic matter makes a great natural fertilizer.
It will keep the soil from freezing for a while, which is good because you don t want the bulbs going through a series of short cycles of freezing and thawing. Then when the temperatures drop below freezing and stay there for a while, the soil does eventually freeze. Then the mulch actually works in reverse and keeps the soil from thawing out too early. Keeping it in a frozen state is actually good because the bulbs remain dormant for a longer period of time. When they finally do wake up it is spring time, and hopefully by the time they emerge from the ground the danger of a hard freeze is past and they will not be damaged.