Here's the general guideline for the timing of fall bulb planting according to soil temperature:
Spring-blooming bulbs are best planted in fall when the soil temperature reaches about 55˚F. Generally the ground gets to this temperature once nighttime air temperature lows have reached 50˚F for about two weeks.
Once the soil reaches the optimum temperature of 55, bulb planting can then continue up until the ground freezes, which may take another six to eight weeks.
Once the ground has frozen, add some mulch to the top of the soil where you've planted your flower bulbs. This will help the soil maintain a consistent temperature even if there's a brief warming spell in midwinter. In turn it will prevent your bulbs from "waking up" and starting to grow in a warm spell. If they did, they'd be damaged when the temperature drops again.
