Seedfork said: Does anyone have any other sources for USDA zones applying only to woody type plants?
"A BRIEF HISTORY OF HARDINESS ZONES AND ZONE MAPS. A relatively
simple method used to visualize geographic patterns of the biological severity of low-temperature events is to map a climatological variable that closely
correlates with patterns of plant survival. Rehder (1927) developed the first such map for the United States, with a mapped zonation system that related winter minimum temperatures to the survival of
specific woody plants. He roughly divided the temperate portion of the conterminous United States and southern Canada into eight zones based on the mean temperature of the coldest month, each zone spanning 2.8 degrees C (5 degrees F)."
The above quoted from Horticultural Applications of a Newly Revised
USDA Plant Hardiness Zone Map, 2012, Widrlechner et al. You can get the PDF by searching on Google, a direct link would be too long to include here.
Other factors do come into play with woody plant hardiness as well as herbaceous plant hardiness but the USDA zone system is based on air temperature minimums. The survival part of a daylily is the crown which is below ground, and not exposed to air temperature. If there's mulch or snow cover, then there is even more protection from air temperature. Where I am there is no freeze-thaw during winter, the ground stays frozen right through, so some daylilies can be hardy here but not so much in milder zones if they keep trying to grow in warmer spells.
Instead of using a zone map, one can instead use indicator plants that have been established for each zone. These are all woody plants. I can't get the USDA list to work right now, but here is the Cornell page for New York:
http://www.gardening.cornell.e...
