Evaluation location: Minnesota, United States
In 2018, seed from 17 populations which represented the latitudinal range of the species from Tennessee, Mississippi, and Florida were germinated. Seed from each maternal parent was surface-sown onto a soilless germination mix (Sungrow Horticulture, Agawam, MA). In the greenhouse, flats were placed on heat mats (Hydrofarm Horticulture, Petaluma, CA) and covered with clear plastic domes to maintain high relative humidity. Greenhouse temperatures were maintained between 25 and 30°C with natural daylength. After germination, seedlings were transplanted into 7.62-cm square pots holding a soilless potting medium (Sungrow Horticulture, Agawam, MA) and grown in the greenhouse until May. Seedlings were transplanted into 186.7-cm3 square pots with peat:pine bark (1:1) potting mix and transferred to an outdoor container nursery with overhead irrigation. Plants were watered as needed throughout the growing season. During Spring 2019, plants were cut back to three nodes above the soil to induce branching in their second year of growth. Cutting the plants back also served to equalize plant height at the beginning of the second growing season. They were then transplanted into the field at the Horticulture Research Center in Chanhassen, MN (44.859, -93.634) using a completely randomized experimental design. Natural rainfall was supplemented with drip irrigation to prevent soil moisture deficits. In May 2020, the seedlings were evaluated for winter damage using a scale from 1 to 5. The damage scores were as follows: 1 = 81% to 100% of the above ground tissue received winter damage; 2 = 61% to 80% damage; 3 = 41% to 60% damage; 4 =21% to 40% damage; and 5 = 0% to 20% damage. One hundred plants per population were scored. See citations for additional details.