Warmer than usual winters can cause all sorts of problems for plants, including bearded iris. In a recent post at the American Iris Society blog, World of Irises, Bonnie Nichols of zone-8a Dallas explains:
“In December [last year] when the Christmas Day temperature was 82 degrees ... we knew the iris bloom season was in jeopardy. And, it didn’t get better when on January 31 the high was 79 degrees.
“When I saw various bearded irises blooming in December and January, I asked friends if they thought it was rebloom or what would have been our spring bloom. We all had no idea. In April, we knew [it] was the ‘spring’ bloom because we ... had no additional bloom. Maybe 20% of tall bearded irises bloomed....
“We saw more than normal increases on some of the plants because they did not use their energy to bloom. On other plants we noticed something that we had not had much experience with – ‘lightbulb’ rhizomes. Lightbulbs are rhizomes with no increases and the roots wither away.... The rhizome increases in size and twists slightly as if it is pushed out of the ground. [If it blooms] the stalk comes up in the middle of the fan and dies back quickly. The rhizome eventually dries up and dies also....”
Commenting on Bonnie’s post, Phil Williams offered an alternative explanation: “Strong root growth is what produces good bloom here. Makes me wonder if the prolonged heat [in summer and fall] might have created a false dormancy ... and the plants did not root deeply.”
Either way, warmer temperatures are the culprit. Is that global warming? Bonnie says she’s not sure but “I’m beginning to believe it is.”