Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Nancy Howard Agnew

Second Advisor

Nick E. Christians


A series of experiments was conducted to identify appropriate winter protection systems for container-grown herbaceous perennials and to investigate the low-temperature tolerance of several species of herbaceous perennials. Results from a field study indicate polyethylene/woodchip and polyethylene/straw/polyethylene structureless covers effectively moderated temperature extremes and should be favored when overwintering known cold-sensitive species. However, they must be removed before the onset of warm, sunny weather to prevent development of etiolated tissue. A translucent thermoblanket provided comparable protection while prohibiting tissue etiolation. During subsequent laboratory freezing studies with containerized plants, -10C was identified as the critical low temperature for overwintering growing points of Gaillardia 'Goblin', Physostegia 'Summer Snow', Salvia 'Stratford Blue', Tanacetum 'Robinson's Mix', and Veronica repens. Exposure to lower temperatures caused significant injury, however, regrowth indices were erratic using containerized plant materials. Dormant crowns of Heuchera 'Chatterbox', wrapped in damp cheesecloth to prevent supercooling of tissue, had a killing temperature of -12C. This freezing protocol reduced experimental variability and allowed for more experimental units per replication. Finally, cold acclimation patterns were established for Sedum 'Autumn Joy' and 'Brilliant'. 'Autumn Joy' began cold-acclimating before the advent of freezing temperatures, gradually reaching maximum hardiness by January (-24C killing temperature). 'Brilliant' initiated cold-acclimation well after 'Autumn Joy', achieving the majority of its cold hardiness by November, and attaining maximum hardiness by December (-21C killing temperature). The electrolyte leakage viability test was accurate in predicting killing temperatures for 'Autumn Joy', but was less reliable for 'Brilliant'.



Digital Repository @ Iowa State University,

Copyright Owner

Jeffery Kenneth Iles



Proquest ID


File Format


File Size

131 pages