Hyacinths are a Fall Planting Bulb. I learned from the Netherland Bulb Co that in the summer Holland growers box up their bulbs and ship them out to places like the US to to plant before the first hard frost. This site also says that bulbs need to be planted within 6 months of purchase.
Colorado State says that:
September and October are the best months for planting bulbs because they can become well rooted before the ground freezes. Bulbs planted after October may not have time to root adequately and therefore may not flower uniformly in the spring.Doityourself.com says, however, to plant "after the first frost, so the bulbs will stay cool throughout the fall and winter."
This link from Cornell University shares the details about planting bulbs in the Fall for spring planting (although the information is targeted toward the New York area):
- In Zone 4 and 5, plant by late September to early October.
- In Zone 6, plant by mid-October.
- In Zone 7, plant by early November.
