This past winter was the best test winter yet for hardiness, plants experiencing some of the coldest soil temperatures in at least the last three decades (as long as I’ve been monitoring soil temperatures). In my northern Vermont trials site, we once again were zone 4a, with 4 days in Feb, -20F (air) or below, with -27F (air) on Feb. 24. In colder years of the past, soil temperatures on average reached 28F, perhaps a few days to 25-26F in colder winters. In several recent winters, soil temperatures seldom dropped below 32F. This past winter, soils were 28F or below 21 days in January, and 16 in February. Of these, soils reached 25F or below 17 days in January and one day in February. Of these latter, soils reached 22F or below 6 days in January, the coldest being 17F on Jan. 18.
It was a good test “spring” for plants too. As they were getting unhardened we had 6 days the last couple weeks of MARCH with air temperatures 10F or below, reaching 0F on Mar. 24! Four days the end of March we had SOIL temperatures 28F or below, with 22F (as in January, the lowest in over 25 years) on Mar. 23.
So it was amazing that as many grasses survived as did. Several switchgrass (Panicum) cultivars in which new plants had been replaced fall 2014 had several die. Of those living, overall results were similar to the previous year, only with slightly more growth (a bit higher and wider), and a bit better floral display on several cultivars. Most cultivars that had several plants were quite variable in growth, as in the past, among the plants. Most uniform among all plants of a cultivar were Dewey Blue and Prairie Sky. Overall best, s imilar to 2014, were Northwind, Prairie Sky, and Shenandoah. Also good this year overall was Rotstrahlbusch. one plant each were outstanding too of Rehbraun, Thundercloud, and Trailblazer (not officially in trials, and only one plant).
Little Bluestem (Schizachyrium) cultivars survived much better. Best overall as in 2014 was Blue Heaven, but joining it among the best this year was Blaze and Standing Ovation (added in 2014, not officially part of the trials). All plants of each of these three cultivars not only survived, but were uniform among plants. Standing ovation at about 50cm high was half the height of the other two. Prairie Blues fared the worst, only one of the four plants surviving and rating only average.
Hopefully one final winter of hardiness data can be collected before these VT trials end spring 2016.– Leonard Perry, UVM Horticulture Professor
Little bluestem, Sept. 2o15, Vermont