By Michael Denny
In the last CPS Newsletter, an article described the history of the Bloom Date Project and the method used to collect data. In this issue, I will discuss recent developments and the interpretation of the bloom date evidence.
The project has a web site ( http://peonybloomdate.com ) that contains several articles about the project and the data. There are two data files that contain 11,000 observations on approximately 1100 cultivars. These files contain the same evidence. However, the difference lies in that one file sorts by the cultivar name, while the other sorts by the bloom offset. For each cultivar, there are columns showing the description, the observations and the offset. The description shows the colour but this is very elemental at the moment and so almost the only colours cited are red, white and pink. (Better cultivar descriptions are found in other sources.)
The crucial data in this project are the observations and the offset. The observations indicate how many inspections have been collected on the cultivar’s bloom date. This data is an indication of the reliability of the evidence. The offset measures how many days the cultivar first blooms before or after the peony Red Charm. The offset data for any cultivar varies and I report the average offset. If there are only a few observations, the average offset may be unreliable. As more observations are collected, the average offset may change considerably. Currently there are five or more observations on over 600 cultivars. This is encouraging; but it indicates that for about 500 cultivars, there are only four or fewer observations.
One of the reasons to continue to collect data is to increase the number of observations on those cultivars, which currently have relatively few observations. At the moment, I still have another 1100 observations on 500 cultivars that have not been included in the web site data. These should be added this coming winter and hopefully will push some additional cultivars beyond the five observations threshold. Please note that you should not assume that the information on cultivars, which have only a few observations, is wrong. The information may be correct. However, it is a safer indication when there have been many observations.
How to Use the Data
Individuals should use the offset evidence with some caution. Here is how I think about this data. Suppose I am considering three cultivars, peony A, peony B and peony C. The offsets are 7, 9 and 2 for peony A, peony B and peony C respectively. The offsets for cultivars A and B are close. So, I would consider them to bloom in my garden at the same time. It may even be that A will bloom after B in some years. However, they should bloom at about the same time.
Cultivar C has a lower offset and the difference is large enough that I would assume C would bloom before the other two cultivars. It may not bloom precisely five days before the others in my garden but it should bloom earlier.
The basic lesson is that the detailed data can be misleading; but when used with caution, it can be very helpful. One can easily choose cultivars that will bloom early in their garden as well as those which will bloom very late.
To my way of thinking, the data provides a ranked list of blooming for peony cultivars. At any given location and in different years the time between the blooms for any given pair of cultivars can vary. If the weather is very warm, the time shortens; if it is mild, the time expands.
There are definite limitations to the data in our project. First, we are dependent on many volunteers to observe and report data. As a result, the following problems may arise: the identification of the cultivar may be wrong; local soil, rain and the number of hours of sunshine can alter the information. These I cannot control. The major limitation arises from temperature variations both between sites and across years.
At any site, the calendar dates on which a cultivar blooms and the time between blooming for different cultivars is altered by temperature patterns. The use of offsets from Red Charm represents an attempt to address this. The offset adjustment is very useful; but it is far too simple to completely adjust for temperature variations across sites and across the years, even at the same site. There is no simple alternative to our adjustment. In my judgement, the project provides us with a large volume of useful information but we just need to be careful in its use.
In recent years, there have been discussions about the creation of a new system of bloom information based on weeks. This would build on the detailed project information but create six categories that would cover all the cultivars. These efforts will be put forward in a final article in the next issue.