In reply to pebbles: it's fairly clear:
"The early season freeze, often with a lack of snow, usually in December through to early January.
The mid-season freeze, usually in the first three weeks of February.
The end of season frost period, when high and north-facing routes often come into their best condition, usually in the middle of March."
I was there with Steve during the last research visit before the guide was published - the 2012 winter which is noted as unusually warm - and there was still climbable ice even at worst times, you just had to go higher for it. The high crags above Highway 45 seemed the best bet if the rest of the area was thawing.