Most of your iris types won't iris bloom in winter months at all. The bulk of them flower in spring and early summer once your ground warms up. But a few special types break this rule for you. Reticulata irises push blooms through cold soil in late winter. Iris unguicularis can flower from November through March if you live in a mild zone.
When I first spotted a winter flowering iris in my zone 6 garden, it caught me off guard. A small group of reticulata irises had pushed bright purple blooms right through February snow. Nothing else in my yard showed any life yet. Those little flowers stood just 4 to 6 inches tall and gave me the first color weeks before any crocus or daffodil showed up.
Your iris bloom timing comes down to stored energy and cold triggers. During the growing season your iris stores fuel inside its bulb or rhizome for you to enjoy later. Reticulata bulbs need a stretch of cold weather to trigger their blooms. Once they get enough chill hours, even a short warm spell in late winter tells the bulb to send up a flower stalk right away.
Iris unguicularis works in a different way. This plant from the Med region blooms during the coolest months and sleeps through summer heat. In zones 8 through 10 it puts out fragrant flowers from late fall through early spring. If you garden in zones 5 or 6, hard freezes will kill the flower buds before they open. You need a mild winter climate to make this one work for you.
The full iris bloom season runs from February through June if you plan it right. Plant reticulata bulbs for late winter color. Add dwarf bearded types for early spring. Follow with tall bearded irises in May. Finish with Japanese irises in June. This plan gives you four solid months of blooms with no gap between them. You won't find a better way to stretch your iris display through the whole season.
For the earliest blooms you can get, plant reticulata bulbs in fall at 3 to 4 inches (8 to 10 centimeters) deep. Pick a sunny spot that stays sheltered from harsh winds. A south-facing wall works great because it holds heat and protects the small flowers. Tuck 25 to 30 bulbs into a tight group and you'll get a strong splash of color when those first blooms break through the cold ground.
Read the full article: Iris Plant: A Complete Growing Guide
