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. Then, they were randomly assigned to three treatments: transplanted into 7.57-L containers, planted in the field at the Horticulture Research Center in Chanhassen, MN (44.859, -93.634) or in the field at the Otis L. Floyd Nursery Research Center in McMinnville, TN (35.709, -85.744) using a completely randomized experimental design. Natural rainfall was supplemented with drip irrigation at both locations to prevent soil moisture deficits. A minimum of 100 random seedlings from each population were evaluated after terminal buds had set at the end of the growing season. Because plants were completely randomized within the container nursery, random sampling was achieved by measuring the first 100 seedlings located in each population. Plants were measured from the top of the soil to the top of the apical bud on the tallest shoot, and nodes were counted on the same shoots. Internode length was estimated by dividing the total height by the number of nodes. Number of primary branches was also measured; because of the lack of secondary branches, branch counts were unambiguous. Seedlings were measured in 7.57-L pots and in both field locations (Minnesota and Tennessee) to assess genotype × environment interactions. Additionally, the canopy width was measured in the field by averaging the width in the widest dimension and the width perpendicular to the widest dimension. See citations for additional details.