Might depend a little on where you are coming from... we come from the frozen tundra, so it's usually shorts and t-shirts for us. But, we always pack fleece jackets, jeans and rain slickers because we've encountered every type of weather. Our first visit they had record breaking lows and had to wrap many of the plants in blankets to protect from frost overnight. Almost felt like home.
Crowd wise it's low season... but, the locals show up on the weekends making them busier. Still 3 or 4 on the crowd predictors, though.