Coverings for Overwintering Container Grown Plants in Northern Regions

Several methods were compared for moderating temperatures and protecting container-grown nursery plants from winter injury. Three layers of microfoam covered by 1 layer of 4 mil white copolymer (poly) and 30 cm (12 in) straw between 2 layers of white poly greatly moderated minimum temperatures of growing medium. The straw-between-white poly treatment prevented rapid rise of air temperatures under the cover in early spring. All covers resulted in minimal winter injury to most plant species, but growing medium temperatures in containers under 1 layer of microfoam or 1 layer white poly dropped below −10°C (14°F), the temperature shown to be injurious to root systems of some plants.