I planted Bibb and Romaine lettuces along with kale from seeds outside on May 8, 2009.  These are all considered cold weather crops that can be planted well before the last frost date. 

In the past, at this altitude, I have protected the seedlings from the cold and closed the cold frame at night.  (You could also cover with floating row covers or hoops).  This year, I decided to test the timing of growth without doing any season extending practices.  

We have had a cool and very wet spring s far.  The greens have been very slow to get going.  They were very small during the hail storm last week and were crushed.  

However this week, we had several hot days and the greens are FINALLY starting to take off.  They have totally recovered from the hail storm.  

For comparison, lettuce is bolting in Boulder this week, after a prolific early season harvest in every garden I have seen.  

See the picture below.  My greens are planted inside this cold frame, but I have left it open all season so far.  It's main function right now is to protect plants from wind (which are strong) coming through my yard and from animals.