Mar 14, 2010
How cold was this winter?
Finally elms have flowered. Last year I took a photograph of elm flowers emitting pollen on February 21. They bloomed three weeks later this year.
Elms are wind pollinated. So are maples, oaks and other trees. Wind-pollinated trees typically flower before they leaf out. Insect-pollinated trees do the opposite, and their flowers are large, even ornate. Wind-pollinated flowers are tiny. The plants up their odds of fertilization by saturating the air with pollen, sparing the cost of growing larger flowers. Before insects evolved, flowering plants relied on wind to spread pollen. The most primitive plants require water for pollen transport.
Because elms need not synchronize their flowering with the emergence of some bee or beetle or butterfly, they have more flexibility in their timing. If winter is colder and pollen production takes longer, they just postpone flowering.
I have seen elms flower as early as February 11 after a mild winter. The day their flowers open is probably as good a measure of winter's severity as a statistician could manage. The winter of 2010 was 21 elm-days harsher than winter 2009.