Bloom Date Project, Part 1

By Michael Denny

One of the important characteristics of peonies is the difference in the bloom dates of different cultivars. This variation extends the peony season and assists in the identification of cultivars. Peony catalogues, books and registration lists provide rough guidelines to the time at which a cultivar will bloom. There is no single source for this information and different sources often provide conflicting information.

When lactifloras were the dominant peonies the problem was not severe. Most lactifloras bloom over a very short period of time. As hybrids became more important, the bloom period lengthened from five to seven weeks and the value of correct information increased. Each nursery chooses its own
method of describing the bloom period. Most are variants of Early (E), Middle (M) and Late (L).

However, a cultivar that is listed as Early in one catalogue is often listed as Middle in another. The situation is even more confusedwhen some, but not all, nurseries expand their categories to include Very Early (VE) and Very Late (VL).

One could not be sure which cultivars listed as (E) in one catalogue belonged with those listed as (VE) in another catalogue.  The Bloom Date Project is an effort to improve the current situation for all peony lovers.

History From 1963-75, the Reverend F. Miller recorded the date of the first bloom for about 200 peony cultivars in Fergus Falls, Minnesota. For a given cultivar, the date varied each year depending on the spring weather. Rev. Miller chose the most common date from the 13 years, which is similar to choosing the median date. The results are published on pages 191-2 of the American Peony Society publication, APS, The Best of 75 Years.

In 1999, I entered the Miller data into a spreadsheet and posted the file on the peony group web site. Currently, this is the Yahoo peony group; but it had earlier names. The posting of the Miller data led to an extended discussion on the usefulness of this type of data. A group composed of Jim Langhammer from Michigan, Leon Pestell from Kansas, Bob Johnson from Oregon, Tom La Bron of  Oklahoma and I began investigating the possibilities of extending the Miller data to a larger number of cultivars.

Thanks to Jim Langhammer, over 20 years of observations on different cultivars at a small number of locations had been found and transformed into spreadsheets. Leon and Jim did the transformation from paper to files. At that time, we had information on the date of first bloom for about 450 cultivars. This original data can still be found as part of the Heartland Peony Society web site.  In the last decade, I have headed the effort to collect more data. Members of the Canadian Peony Society have been important partners in this task. By the end of 2012, we will have over 10,000 observations on the bloom dates of about 1000 different peony cultivars.

Method The data is collected from many individuals in different locations. At each location, the  calendar date on which the first bloom opens is recorded for each cultivar. This is not a scientific collection process.  We all know that weather impacts the time at which peonies bloom.

For example in my garden, ‘Red Charm’ has bloomed as early as May 26th and as late as June 13th in the past thirteen years.  Most of the bloom dates are clustered around June 3rd to 6th.  At any location, there will be variations in the bloom dates based on the weather at that location in that year.  Gardens that are located in warmer climates than mine will have their dates for ‘Red Charm’ shifted to an earlier period. We are confronted with variations in bloom date of ‘Red Charm’ at different locations and at the same location over time. Two methods have been used to reduce these problems. First we have converted the calendar dates into the number of days before or after ‘Red Charm’ blooms. This is called the ‘offset’. As the calendar date for ‘Red Charm’ changes, the offset for all the other peonies will change.

Second, we take averages of the offsets. This is an important step because it will tend to eliminate the extreme observations. To make averaging work, we would like to have at least five observations for each cultivar.  These two adjustments are not perfect solutions to the variation in bloom times. It is difficult to imagine a better system that is as simple. The issues are not unique to our efforts. Anyone who wants to establish accurate data on the bloom periods will confront the same problems.