Comparative Analysis of Laboratory Freezing Methods to Establish Cold Tolerance of Detached Rhizomes and Intact Crowns in Garden Chrysanthemums

Since 1924, the Univ. of Minnesota herbaceous perennial breeding program has released n = 84 garden chrysanthemums ( Dendranthema grandiflora ). Recent breeding objectives have focused on development of non-destructive phenotypic markers and laboratory freezing tests for continued selection of cold-tolerant Dendranthema, Gaura, and other herbaceous perennial flowers. Such methods have become critical to flower breeding programs during periods of above-average winter temperatures and minimal snow cover. Two different laboratory freezing tests were evaluated for their effectiveness in determining cold tolerance. Acclimated crowns of n=6 hardy and non-hardy garden chrysanthemum genotypes were used in Omega Block (detached, emergent rhizomes) and chamber (intact crowns with emergent/non-emergent rhizomes) freezing test methods. Comparative winter survival in the field was monitored over locations and years. Cold tolerance was assessed at 0 °C to -12 °C with varying ramp and soak time periods. LT 50 temperatures and number of living emergent rhizomes were determined. Rhizome quality at 1 cm, 3 cm, and 5 cm depths was rated on a 0 (dead) to 5 (undamaged) scale. The chamber freezing method was the most powerful to discern LT 50 values. Cold tolerant genotypes included `Duluth' and 98-89-7 (LT 50 = -12 °C). Three genotypes had intermediate cold tolerance (LT 50 = -10 °C) and one genotype was not cold tolerant (LT 50 = -6 °C). Cold-tolerant genotypes also had significantly higher regrowth ratings for rhizomes at 1cm and 3cm depths. Future research will use the chamber freezing method to assay the inheritance of winter hardiness in intact crowns of segregating populations.