Here’s a trick for getting summer-blooming bulbs off to a good start.
Summer-blooming bulbs such as cannas are planted in spring. To get off to a good start, they need consistently warm soil. If you’ve bought summer-blooming bulbs at a nursery or through mail order, and your weather is not yet warm enough to plant them in the ground, trying planting them in pots indoors. You can then transplant them into the garden later. This will ensure the bulbs succeed and may lead to an earlier/longer bloom time.
Here is a rough guide for when to plant common summer bulbs. Remember that the lowest daily temperature almost always occurs just before dawn. At this time, the Earth has lost all of the heat from the day before to radiational cooling.
Canna — wait until the daily low temperature is consistently above 55˚F.
Crocosmia — wait until after the last frost.
Dahlias — after the last frost.
Gladiolus — after the last frost.
Rain lilies (Zephyranthes spp.) — when the daily low is consistently at least 60˚F.