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 lows have reached 50˚F for about two weeks. Bulb planting can 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. You do not want your bulbs to "wale up" and start growing in a warm spell, because they'd be damaged when the temperature drops again.
Image: "Crocus chrysanthus - dorothy" by This photography was created by Mariluna. Other photos see here. - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons.
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