Hello! Tell me, did someone plant cereals in concrete containers? How do they winter? The size of the concrete niche: width 0. 4m, depth 0. 8m, can be done deeper. Question: is it necessary to insulate for cereals and there is still an idea, which shrub can fit into such a niche?
From my own experience I will say that cereal grasses cannot winter without snow every year (only about once every 3 winters.). If 0, 8 m sticks out above the ground, then it will freeze from the sides for sure, since a layer of snow at 0, 8 m usually sweeps only in February. In pots not buried and only 30 cm high, they perished only in this way. And here more than 80 cm. Therefore, I can advise: a) to make a snowdrift and hide a container in it. Just snow would be at least 30-50 cm.
b) make a wooden box without a bottom at the rate of + 20-30 cm from the sides and cover it with dry leaves, and cover with a film or something waterproof on top.