My impression is that S. palmetto prefers sandy soil to clay. Sure, it'll grow in clay, but if you look at them in North and South Carolina, those in the sandy soil outperform those in clay. Certainly Columbia SC and New Bern NC are colder than Gainesville and the S. palmettos look great in both places. If you want to see some nice pix of S. palmettos in cold places, go to www.garysnursery.com and look at his photos. Snow and freezing rain does cause problems. If the fronds bend under the weight, it's important to remove the snow or the fronds are likely to remain bent down. Sabal palmetto around Charlotte and Raleigh suffer this fate. Regardless, I don't think the observed lack of vigor is related to the temps. S. palmetto is pretty solid to at least 5-10F. In Columbia SC and Augusta GA S. palmetto withstood temps below zero in the 1980s.