Temperature treatments for peony seed germination

It’s a well-known fact that for good peony seed germination there’s a sequence of temperatures required. The usual procedure is to give an extended period of warm temperatures as soon as the seeds are harvested in late Summer. This usually means temperatures of 20°C for a few months. During this time the embryo within the seed will grow and when large enough will send a first root (called a radicle) through the seed coat. When this root is long enough (some 5 cm usually) a period of cold is needed to break dormancy. Normally somewhere between 1 to 3 months of cold (below 5°C) is needed. After that period, the rooted seeds can be placed in warmer conditions again and only then will they send up their first leaf (called the epicotyl). There are some differences between what is advised obviously. At the end of the first warm period, a slightly lower temperature of some 10-12°C  is sometimes suggested as this will invoke root growth.

When you plant seeds outside it is common knowledge that most seeds will only send up their first leaf the second year of growth. When hybridizing peonies you obviously want to go as fast as you can because this opens the possibility to have more generations of seedlings.

Round here I usually place the seeds in ziplock bags in my heated and vented greenhouse, which is kept at some 20°C during daytime and 16°C during night. After a few months I check the seeds for roots and then they are moved into cold storage. I was however wondering whether alternating daily temperatures, with more difference between day and night time might be better. After all, that is how nature works and in general it is safe to assume that nature always knows best.

So we collected a large number of open pollinated seeds from several plants of PVBM005. This selection number is known here to give large quantities of seeds which germinate easily. We divided them randomly and counted 100 normal looking seeds, which sank when placed in water, for each treatment. Three treatments were tried and started around August 25th.

1. Ambient conditions outside. Placed in slightly moist vermiculite in a ziplock bag and protected from sun so that air temperatures would be more or less equal to the temperatures in the ziplock bags. Natural conditions mean that there will usually be quite some difference between day and night.
2. Same thing, but this time placed in an unheated greenhouse with no automatic venting. Also protected from the sun inside the ziplock bag. This means that temperatures will generally be higher than the previous treatment. During daytime on sunny days, temperatures might even rise very high because the vents don’t open further automatically. During nighttime it can be expected that temperatures will be slightly higher than outside and will fluctuate more slowly. As in the previous treatment, there will be large differences here between day and night temperatures, but thus with a higher average temperature.
3. Same thing again, but this time in a heated greenhouse with automatic venting. Daytime temperatures are set around 20°C and nighttime temperatures at around 16°C. During sunny days a screen and some extra venting will keep temperatures lower than in the unheated greenhouse but still higher than outside. During night times, temperatures here will be highest of the three treatments. Thus here the difference between day and night will be lowest and we have more constant warmer temperatures.

We had temperature loggers for each treatment and within the heated greenhouse we even had two, one within the ziplock bags and one just outside of it. Just to see if there would be a difference. Unfortunately the logger for the first treatment malfunctioned due to excess rain and had to be thrown away. So I don’t have any data for it, but an average colder temperature during both day and night is obvious.

What did we expect? The outside treatment would be worst as this is the ‘natural’ conditions in which peony seeds grow and thus we assumed most would only germinate in their second temperature cycle. Whether the unheated or heated greenhouse would be best, we didn’t really know. The higher daytime temperatures in the unheated greenhouse, combined with the alternating lower temperatures during night, might be more beneficial than the less fluctuating temperatures in the heated greenhouse.

Now what do the data show? There is in general not too much difference between air temperatures outside of the ziplock bags and inside it. If you have a look at the data, you can see this. The table below shows temperatures measured from noon Sept 28 through noon Sept 29. You can see that in the heated greenhouse the temperatures in the ziplock bags and outside of it follow the same pattern, although slighly higher in the ziplock bags. Anyway when you set temperatures in the greenhouse or anywhere else it will be the air temperature which is monitored thus the two last columns should be compared as those show air temperatures in both the heated greenhouse and unheated one. As you can see in the unheated one they rise higher during day when it is sunny, but they also go much lower during night. All data can be downloaded in pdf or as an excel file.. Most days show this pattern, but of course later in October the unheated greenhouse had lower temperatures when the sun was not shining as strong and long anymore.

Temperature controlled greenhouse air temperatures (outside of ziplock bags)

Now what about the results? We graded the seeds into three groups on November 6th, thus after some 2 1/2 months. Large roots (some 5 cm or more), small roots (shorter than 5 cm, and also the ones with roots just peaking through the seed coats) and finally the seeds that seemed still unchanged. The results are quite clear:

A constant warm temperature for a longer period is much better than fluctuating temperatures slowly decreasing over that same period of time. What’s more: the seeds from the heated greenhouse generally had much longer roots and some even had initiated growth of the epicotyl already. Often it is advised to place the seeds in a ziplock bag and place that bag upon a refrigerator as such an appliance tends to give heat outside. It seems to be sound advice from the results here and clearly it would be wasted time trying to replicate natural fluctuating conditions.

Some images will say more than a thousand words:

The 10 largest roots of each treatment. Left to right: outside; unheated greenhouse; heated greenhouse.

All seeds graded. Above to below: largest roots; starting roots; no germination. Left to right: outside; unheated greenhouse; heated greenhouse

Some seeds starting their first leaf already November 6th!