The 15-20C is the optimal temperature. Temperature extremes in general have greater disparity in North America than in Europe and Asia, so it's difficult to know what really hot is for you. where I am in zone 4, Minnesota, summer temperatures reach 100F (38C) about every third year, with 1.5-2 weeks straight with highs in the 90sF (32-37C) every day, and scattered days in the 90sF at other times. I have started martagon seeds in pots where they germinated in (probably) May, and next spring they sent up their first leaf right on schedule. The heat I have didn't hurt them. What will probably happen is the tiny seed bulblet will grow in the spring and early part of the summer, rest in the hot heat, and resume when it cools toward fall. Opposite of what you might think, when bulbs are resting in the hot time (and this includes mature bulbs that die back in summer), they like to be kept much drier.
You could start them inside (remember, DH seeds don't need light), say, April 1, and move them outside when temps warm in May and leave them outside for the rest of the summer (in the shade, of course).
