I googled and found this:

Plant Allium bulbs in the soil in the spring, as soon as the ground can be worked. Plant them to a depth of twice the diameter of the bulb.

Here is a link that might be useful: how to grow ornamental alliums

I grow many ornamental alliums and I always plant in the fall. I've never heard of planting them in the spring before.

I never heard of planting ornamental allium bulbs in Spring also,in fact while in Illinois several weeks ago I saw some bulbs there and I just finished planting here in Kansas about 2 weeks ago to go with my others.

I plant in the fall.
kay

Plant in the fall here too, about 6 years now, always successful.