“The Iris are coming on earlier than usual this year!” Sound familiar? Or, “This is the latest bloom season we’ve seen in years!” As we dig the remaining acres of Iris for the season, we are simultaneously planting for next year’s bloom. We ask ourselves each fall, “Will next spring be early or late?” ….Late or early, is global “climate change” to blame for the flip-flopping perennial habits of our beloved Iris?
A recent inquiry from a customer in the Bay Area regarding the unusually early onset of her Iris blooms prompted us to ponder the question further. We consulted with our good friend, and fellow Iris enthusiast, Dr. Benjamin Herman, Climatologist with the Department of Atmospheric Sciences and the Institute of Atmospheric Physics at the University of Arizona, Tucson.
On the question of how much effect “climate change” can have on the bloom time for Iris, Ben Herman writes, “While I have never tried to relate bloom time to climate change, I think we have to define what we really mean by climate change. The warming that has been taking place over the past 30- 40 years that is usually referred to as climate change has been of the order of, at best, a few tenths of a degree per 10 years, or about 1.5 degrees per century. Natural changes from year to year can be considerably more than this, but it will vary from year to year. Over a ten-year period, I think it would be difficult to see any significant change in the bloom time for Iris due to “climate change.” Year to year natural changes however, could and do cause noticeable changes, but in general, it wouldn’t be always in the same direction. By that I mean one year it could be late, another year it could be early, etc. I think all growers that have been at [Iris growing] for many years have experienced this.
There could be other causes also for changes in bloom time. Rainfall amounts could have an effect as could cloud cover. I would guess that more than normal cloud cover might delay bloom time, while the opposite might move it forward. However, temperature variations are probably the largest control over bloom time. Those of us in more southerly regions of the U.S. generally have bloom times from mid- to late April, while more northerly regions are generally from late May into early to mid-June, the latter dates applying to the colder northerly regions. These dates, everywhere, can vary due mostly to temperature changes, but I do not believe that the changes due to what we refer to as “climate change” can have a noticeable effect over a ten year period.”
Thank you to Benjamin M. Herman, PhD, Professor, Atmospheric Sciences (Institute of the Environment, University of Arizona) for his contribution to this month’s post!
The U.S. Department of Agriculture (USDA) issued an updated Plant Hardiness Zone Map in 2012. This map shows the slight changes in zones compared to the previous map issued in 1990. Wondering what your USDA Zone is? Visit the USDA website for more information.
As we plant our Iris around the gardens this month, we trust in the cycles of nature to deliver us our much-anticipated Iris bloom season next May. Have you noticed changes in the bloom habits of the Iris in your garden or your region? We’d love to hear about your observations.