Michael: As always, thanks for the reply!
If its happening because of cold temps:
Try growing it inside and see what happens. If you plant it outside, try starting it inside and then transplanting it after you harden it up.
If its happening because of hot sun light:
Try giving it some shade. I grow my personal stash on the back deck in pots in semi-shade. The stuff I am selling will also be shaded (but will bolt, much like lettuce after a while). Shade is the rule in summer time, so maybe put a tomato cage around it with some 30% to 50 % shade cloth?
I've never really had any problems with it and this past summer we've had 60 days of 90F plus and 10 days of 100F plus.
My guess is that you being much further north you are getting plants that have been or (after you plant them) are exposed to cold. Which does not kill them, but makes them bolt later on down the line (don't ask me a time frame, I do not know). The plant changes slightly to survive the temperature. I hope this makes sense. I am not a master gardener by any stretch, though I have been filling my time when not reading about bees with information of soil science and three/four season growing.
If I had to make a wager I'd say its cold air, but you would know better. My suggestion is to buy from seed start a test group of two or three pots inside and see what happens. Leave one inside the entire year, take one out in early-mid spring and one out in early summer (post frost) and see what happens. If you are buying your plants, they may have been exposed to cold but appear just fine...till you get them home, they grow and then have to deal with summer.
Back on topic:
I think lemon grass shouldn't be too hard to grow. I'll let everyone know how it goes. I just ordered some seed to start transplants for spring.