Arisumi's observations are important because they indicate that daylilies respond to temperature in the same manner as all other plant species. They have optimum temperatures. They suffer from heat stress. Each daylily cultivar will have its own optimum temperature for growth. Some daylilies will have lower optimum temperatures than others. Arisumi used only one cultivar; its specific optimum is not the important point. {Using first flower opening dates as a measure of growth, I calculated that 'Barbara Mitchell' would have an optimum temperature around 78F.} The natural habitats of daylilies are varied. For example, many natural populations of Hemerocallis dumortierii are found in mountainous regions at altitudes of 4000 to 8000 ft. Others are found at sea level. The optimum temperatures for growth of the different populations will differ. The optimum temperatures for growth of northern-bred daylilies will on average differ from the optimum temperatures for southern-bred daylilies.
Daylily hybridizers cannot select for daylilies that will grow well in other locations and growing conditions unless they grow their plants in those locations and conditions. They would have to choose only plants that do well in all their locations to use as parents. Otherwise, over time, each hybridizer will inadvertently produce daylilies that grow best in their own location and conditions. Those will be the optimum conditions for their introductions. Thus, hybridizers who produce daylilies in locations with frequent high summer temperatures will produce plants that can grow acceptably well in those conditions. Hybridizers who produce daylilies in locations with infrequent high summer temperatures will not be able to produce plants that will grow as well in those locations with frequent high summer temperatures.
Moldovan [Ohio] learned about this. He said, "When I started daylilies, I grew northern varieties because I'd heard that southern evergreens would die for us. So I bred dormant with dormant. I remember sending some of my initial seedlings down to southern Florida to be grown and tested to see how they'd grow, and they didn't! I was crushed, of course. I went down there and said, "What am I going to do?" I started a program of crossing northern things with southern
things."
From the Daylily Journal about Steve Moldovan and Bill Munson [Florida], "Because Steve became a close-working daylily friend, Bill's stock traveled north.
Every year over the course of three decades the two of them spent considerable time at peak bloom season in each other's gardens talking, photographing, evaluating, and sharing thousands of seedlings."
What they were doing was moving from having introductions that grew very well in their own conditions and poorly in the other conditions to having introductions that grew at least well in both conditions. There will be many differences in growing conditions between northern locations and southern locations - temperature is the most obvious and plants suffer from heat stress at high temperatures.
