“Cold-Trapping” in Retractable Roof Structures To Avoid Spring Frost Damage of Container-grown Nursery Crops

Abstract

Compared to film-covered quonset structures typically used for unheated cold protection of nursery crops in Oregon, unheated retractable roof structures can provide cooler day temperatures (lower average daily temperature, ADT) in late winter and early spring. Late winter and spring growth and flowering of six nursery genera (12 replicate blocks), and air temperatures, were studied when crops were cold-protected in a film-covered quonset structure compared to a retractable roof cold protection structure operated using predawn “cold-trapping.” Compared the quonset structures, retractable roof structures had lower daily maximum temperatures, lower ADT, and similar daily minimum temperatures. Shoot growth of all crops was delayed by 2 to 4 weeks, and flowering of Rhododendron `Hino Crimson' was delayed by 4 weeks, in the retractable roof structures. Erica `Irish Bells' had symptoms of cold-damage and did not flower when protected in the quonset structure, compared to no damage and full-flowering when protected in the retractable roof structure. Arctostaphylos had more leaf spots, and less growth in the quonset structures compared to the retractable roof structures. Our hypothesis is that the lower ADT in the retractable roof structures delays de-acclimation to cold temperatures (delaying vegetative and flower budbreak), providing extended protection to frost damage. Higher relative humidity and higher daylight temperatures in the quonset structures may leave the earlier new growth and flowering of nursery crops more susceptible to frost damage and disease infestations compared to the daily vented and cooler retractable roof cold-protection structures.