I have found them difficult to germinate too. Though 100 of them is quite a bit!! This year I decided to sow the rest of some seed I bought 4-5 years ago and only 2-3 of them germinated. I did have much better germination the first couple of years, but I remember that it wasn't impressive.

They are pretty delicate for a long time. I remember reading that its best to plant them somewhere protected the first season, and move then to their permanent residence in the fall. I assume that that is if you are planning on using them in a pathway. Once established they are pretty tough. Mine have come back strong and have spread fairly well over the past 4 years (in zone 4)

That doesn't answer your question - they can certainly take some cold weather once they are hardened off, so I think planting out time will depend more on how you are using them and how exposed they will be to the elements.

Very informative, thank you! I started mine (very) early indoors in trays under light and on gentle heat. None grew to over 3/8 - 1/2". I lost my patience and put them in the garden hoping for survival but they all died. Evanston, WY. Zone 4 @7,000'.
I guess they are slower and WAY more delicate than I thought so I will try another tray (7/2014) and keep them potted inside until spring 2015 when I can acclimate them on my deck.
Thank you for the knowledge on delicateness!