How can you determine whether your peonies will bloom in time for a June wedding? With peonies there can be as much as 7 weeks difference in bloom time depending on the species or cultivar. Weather, especially temperature, also has a big impact.

There’s no fool-proof way, but here’s what I have found is a pretty good indicator. I’ve been tracking peony bloom times here on our farm Fina Gardens in northern Wisconsin in order to better predict when a given variety will come into bloom. I know that many factors can play a role in when a peony will bloom, but I’m finding a high degree of correlation between bloom time and the accumulation of degree days. Degree days are a mathematical way of measuring the amount of heat available above a threshold temperature on a particular day. The number of degree days accumulated over some period of time is often related to the seasonal development of plants and insects. So many in agriculture use degree days for integrated pest management regimens to determine the most effective times to spray pesticides, put out insect traps or release beneficial insects. There are a number of Internet sites that provide degree day calculators. For my area of the country I use www.soils.wisc.edu.asigServlets/asos/SelectDailyGridDD.jsp

Using the approximate longitude and latitude for our farm (several Internet sites can provide that information just by entering your address), I began looking at the accumulation of degree days in the range of 40 degrees F (when peonies an begin breaking dormancy) through 90 degrees F (when it’s too hot for good growth) during a period starting March 1. In 2007, for example, there was an accumulation of 1,018 degree days between March 1 and May 31—well above average. The converse was true in 2008, when just 591 degree days accumulated during the March 1 to May 31 period—much less than average.That difference resulted in peony varieties coming into bloom 18-20 days later in 2008 than was the case in 2007.

For instance Roselette opened on May 21 in 2007, but didn’t open until June 9 in 2008. When I calculated the degree days for both years I found that Roselette had opened after 795 degree days in 2007 and 794 degree days in 2008.Other varieties showed this same variation in bloom time bloom.