Five structureless overwintering systems were evaluated for temperature moderation and protection of 18 container-grown herbaceous perennials from low-temperature injury. Two light-excluding treatments; 30 cm (1 ft) of straw between two layers of 4-mil white polyethylene and 18 cm (7 in) deep, in-ground beds protected with one layer of 4-mil white polyethylene and 30 cm (1 ft) of woodchips, provided the greatest moderation of winter low and early spring high temperatures, but also resulted in severe etiolation. A bonded white polyethylene/microfoam overwintering blanket (thermoblanket) with translucent properties provided comparable plant survival percentages despite dramatic temperature extremes recorded beneath this cover and, in late winter, created an environment conducive to moderate plant growth without formation of etiolated tissue.

etiolation, temperture monitoring, thermocouple, winter protection
Yarrow (Achillea taygetea x millefolium L. ‘The Beacon’), Lance coreopsis (Coreopsis lanceolata L. ‘Goldfink’), Threadleaf coreopsis (Coreopsis verticillata L. ‘Moonbeam’), Threadleaf coreopsis (Coreopsis verticillata L. ‘Zagreb’), Delphinium (Delphinium elatum L. ‘Giant Pacific Hybrid’), Blanket flower (Gaillardia x gradiflora Van Houtte. ‘Goblin’), Geum (Geum quellyon Sweet. ‘Mrs. Bradshaw’), Coralbells (Heuchera sanguinea Engelm. ‘Bressingham Hybrids’ and ‘Splendens’), False dragonhead (Physostegia virginiana (L.) Benth. ‘Pink Bouquet’ and ‘Summer Snow’), Balloon flower (Platycodon grandiflorus (Jacq.) A. DC. ‘Fuji Blue’), Salvia (Salvia x superba Stapf. ‘Stratford Blue’), Sedum (Sedum spectabile x telephium L. ‘Autumn Joy’), Sedum (Sedum spectabile Boreau. ‘Brilliant’), Painted daisy (Tanacetum coccineum Willd. ‘Robinsons Mix’), Spike speedwell (Veronica spicata L. ‘Icicle’), Creeping veronica (Veronica repens Loisel.)
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Author notes

Journal Paper No. J-15142 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 2909. We thank G. Pearston (computer specialist) for computer graphics assistance. This research was supported in part by a grant from Iowa Nurserymen's Association Research Corporation.

2Graduate student, Associate Professor, Professor, and Professor, resp.

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1993