ekoboat

I have two basjoo in a cage with straw around them and plastic over the top. We have been in the single digits multiply nights, offically 6 deg F low although I have only personally recorded 9 deg F. Anyway, I have to keep reaching up under the plastic to cut off new growth that has pushed up out of the straw and is pushing up against the plastic. These two were cut off at about 3 ft. I have three other basjoo that I cut to the ground and put staw over, but no plastic, and all of these stopped growing. I did mix some leaves in with the straw nearer to the outside of the cage. I figured if some moisture got to the leaves they would compost and put off some heat - could it be working too good?
Day temps have ranged from the low 20's F to upper 60's F this past month. Had several inches of sleet/ice/snow two weeks ago and today cut off another 8 inches of plant.