The influence of a polyethylene film covered house on the cold hardiness and subsequent plant condition of container-grown nursery stock

Polyhouses provide winter protection for nursery stock in northern climates. Selected container-grown nursery stock was subjected to three overwintering treatments. Plants were placed in a polyhouse in October (early treatment), and in December (late treatment), plus stored pot-in-pot under natural conditions. Controlled temperature freezing studies were conducted to determine shoot hardiness during the acclimation, mid-winter, and deacclimation phases of the 1995/1996 and 1996/1997 test periods. Acclimation was interrupted for plants placed in the polyhouse in October which prevented maximum hardiness from being achieved. The pot-in-pot treatment was the hardiest and also deacclimated slower than the two polyhouse treatments. Periods of warm temperatures during the 'midwinter thaw' had an impact on the hardiness of each cultivar. Plants overwintered in the polyhouse were affected more by the warm temperatures. Whole plant performance was used as an indication of root injury when subjected to controlled warming temperatures and natural deacclimation. In general, plant performance ratings declined as days of warming increased and as freezing temperatures decreased. Performance for plants exposed to controlled warming increased from 30 to 60 days during the 1995/1996 test period. Ratings declined from 30 to 60 days during the 1996/1997 test period. Ratings for plants subjected to natural deacclimation did not change from 30 to 60 days.