Bloom Date Project
In the past peony bloom dates have been collected by many people. To my knowledge, previous to this project, there have been two major attempts at collecting/consolidating and publishing data over a large number of cultivars. To this we can add the body of information published in nursery catalogues.
The Miller Data
Rev Floyd Miller collected bloom dates for 200 cultivars at Fergus Falls, Minnesota from 1963 to 1975. The location is 140 miles West and 80 miles North of Minneapolis.
The Miller data has two strengths. The first is the use of 13 years of data. The second is the evidence on many older American lactiflora cultivars. It is unlikely that we will find new evidence on these older American cultivars. The Nichols Arboretum has many early American lactifloras but they do not have the staff to collect bloom data.
As presented on pages 191-2 of the APS, Best of 75 Years, the Miller data is a single date for each cultivar. The data is not a simple average of the actual observations but appears to be a median rather than a mean. A median date has an equal number of bloom dates before and after. It will differ from a mean, or simple average, date but probably not by more than one or two days.
The Heartland Peony Society Data
Leon Presnell compiled bloom sequence data and placed it on the Heartland Peony Society web site. This is the most comprehensive list of bloom dates available. There are about 440 cultivars and 1300 observations in the Pesnell data.
Nursery catalogues provide information on the bloom time for cultivars that they sell. They all do this using variants of a classification system that designates a cultivar as an early, middle or late bloomer. Many use a more complex system that includes very early, very late and some intermediate cases such as early-middle or mid-late. Don Hollingsworth of Hollingsworth Nursery has one of the more complex systems while Klehm's Song Sparrow has a relatively simple one.
I constructed some tables that compared the blooming period category given in catalogues from Klehm's Song Sparrow, Wilds, Hollingsworth Nursery and La Pivoinerie D'Aoust with the bloom data. It is not surprising that there is rough general agreement between the nurseries and the bloom data. There are however some definite disagreements which need further investigation.
Al Rogers in his book "Peonies" is very careful to exclude the bloom period for the cultivars in his longer lists because he recognizes that there is no common source of information.
Lindsay D'Aoust of La Pivoinerie D'Aoust and I have discussed trying to create a common classification system. Personally, I favour a system with five groups or categories, VE, E, M, L and VL. A new common classification system is important because it is not likely that nurseries will ever use a system such as the Red Charm relative dates. The basic weakness of any classification scheme is that some cultivars will bloom near the boundaries of the groups.
Recent Canadian and American Data
New bloom data has been collected and organized by myself, Michael Denny from observations in 1999 to 2003. This adds about 1200 new observations. Bloom data was collected by Brian Porter, Carlos Beca, Lindsay D'Aoust, Leo Smit, Julia Dicks, Val Ames and Irene Tolomeo. I added my own data and organized the information. Some of the data are for years prior to 1999. In particular, Julia and Brian had observations over a number of prior years.
I am hoping that everyone who contributed data in the past will continue to collect bloom dates in 2011 and beyond.
In our current data, we are measuring the bloom dates relative to the bloom dates of Red Charm. This is an attempt to adjust for different bloom dates for a given cultivar at the same location in different years and at different locations in the same and in different years. It is a relatively crude adjustment but will remain in use until we devise a better one.
Our objective is to provide reliable information on the bloom sequence of peonies. The simplest method would be to construct a ranked list of cultivars starting with the earliest bloomers and moving on to the later ones. Such a ranked list is simpler than our current methods because no information is provided about the period of time between the blooming of each cultivar.
We have evidence that even a ranked list is not totally reliable. We have data over a number of years from several gardens and the ranking by bloom time is not identical from one year to the next. This should not deter us from our task but it should warn us that the bloom sequence data that we create has to be used as a valuable guide and it should not be interpreted too literally.
Climate variations are likely the major source of variations in bloom dates. However there is a mystery because some cultivars seem to be affected to a different extent than others by the same weather variation. If this were not true, we would not see the rankings change in the same garden in different years. This is not a simple story of hybrids versus lactifloras but seems to occur for both groups.
Heat is probably the most important variable in explaining the variation in bloom time. In agriculture and the building industry, there has been wide- spread use of degree days. Degree days are a simple transformation of the average daily temperature. For example suppose we believe that it is the temperature above 40F that is important. Then the number of degree days for a day with an average of 60F is 20 ( = 60 - 40) degree days. There are many variants of degree days because one can choose the base temperature (40F in my example) to suit the problem of interest. We will probably want to create our own degree day measure from data on daily maximum and minimum temperature.
When this evidence is available, it would allow us to assess the influence of heat on the bloom sequence. One of the open questions is whether degree days is enough information to explain most of what is happening. If it is, there is less of a role for moisture and soil conditions. The latter may be important but I am hoping that it is temperature that is the major source of variations in the bloom dates.
What is Missing?
We have very limited information on the bloom times for tree peonies and for the peony species. In both cases, far fewer peony enthusiasts grow the plants. Over time, I hope that we can improve this situation.
For the lactifloras and hybrids the current data is more extensive. What we need is more years of observations from more locations and additional cultivars.
Finally, there are many common cultivars for which we have no bloom date observations.