It has been an interesting winter. Given the warm temperatures this year, growers are advised to open up overwintering greenhouses on warm days to ventilate. Either roll up sides, open doors, use fans, depending on the type of overwintering greenhouse. Ventilating will lower the relative humidity. Greenhouses that are closed up will result in moisture buildup and encourage plant diseases to develop. Also, take time to inspect plants in overwintering houses, particularly for disease and injury caused by rodents.
The added protection of thermoblankets works best only during the very coldest of weather, so they should be rolled on over the plants and rolled off as needed. High - low thermometers placed at plant height can be useful to provide the temperature range of the air and soil thermometers are also useful.
Fact Sheet: Overwintering Container-Grown Ornamentals
Tina Smith, UMass Extension