Last year we had a trial with long term storage of cut peonies under Ultra Low Oxygen (ULO) conditions. It might be interesting to read it again, but the conclusion was that there was room for improvement. After two months, there was too much botrytis on the flowers and the foliage due to the high relative humidity (RH) within the boxes. We therefore concluded the experiment last year with the following main recommendations:

  1. Don’t place the peonies in water before putting them in the ULO boxes
  2. Keep temperatures close to and just above freezing
  3. Put some dehydrating solution inside the boxes
At the research centre the ULO experiment was again executed with 'dry' peonies this time and some cardboard paper at the bottom to lower the humidity. The Fawn and Sarah Bernhardt were the varieties here, the same ones as last year. The picture shows The Fawn in the flowering test room after two months of storage.

As it turned out, this year would be one to remember. First there was, and still is, the Covid-19 virus, which also distracted flower markets. Second, it was a very dry and hot year. During the cutting season it never rained, thus we refrained from spraying against botrytis as that usually only occurs during humid weather. That may have been a mistake as it might have helped keep botrytis longer away in those ULO boxes. Demand was high for peonies and although we do have 12 ULO boxes, we were only able to fill about three, not even completely full, because clients kept asking for peonies during the season itself. The research centre also filled two boxes (half Sarah Bernhardt, half The Fawn). July 4th we filled our boxes. One box with part ‘dry’ peonies (cut and put in the boxes immediately) and ‘wet’ peonies (cut, placed in water, taken out to dry, then put into the boxes). Another box only ‘dry’ ones, the last one ‘wet’ ones. As we had a shortage of flowers during the season, it turned out that we had to use the only cultivar left at the end of the season, which was Elsa Sass.

Differences with last year?

  • This time we closed all lids (6 of them) of the boxes from the beginning so the oxygen contents would decrease more rapidly compared to last year
  • Temperature was kept lower, closer to zero degree Celsius (32°F) so the buds would change less
  • Six 500 gr silicagel boxes were placed into the box to keep humidity below saturation (100%) and avoid condensation
  • Peonies were cut and placed directly into an ULO box, thus they were not placed in water beforehand (‘dry’ storage)
  • Only one cultivar available this year, the late double white Elsa Sass
  • No spraying with fungicides was done during the harvesting season as it was a very dry year
The 'wet' peonies box showing botrytis damage to some buds after 3 months of storage, remark also the silica gel boxes on top

The silicagel boxes shown here can contain up to 500 ml of water each. These are from the box with the ‘wet’ peonies after 3 months. Four of them are fully saturated, two still have some spare silicagel (the two uppermost ones). In the box with the ‘dry’ peonies, all silicagel boxes were saturated after this time, but there were somewhat more stems in that box.

Peonies were stored for two months (the ‘mixed’ box) or three months (‘dry’ and ‘wet’). The mixed box at two months already showed that the ‘wet’ peony box would only contain even more botrytis infected ones, but we kept it closed anyway for comparison. The ‘dry’ peonies after two months looked fine, but there were some ‘grey’ petals underneath the outer petals (see further).

The graph illustrates how the temperature, O2 and CO2 changed over time in the ‘dry’ peony box. As can be seen, temperature was kept quite low and constant. Halfway the experiment we needed our cooling facilities for our other flowers and we had to raise it slightly. If we’d put it full of peony boxes, we could keep it lower of course.

Oxygen decreased rapidly in the ‘dry’ peony box with all lids closed, but strangely it never went below 5%, thus we never opened a lid.

The ‘wet’ peony box did go below 5% oxygen and there we opened 2 lids, after which O2 rose again above 5% and we decided upon closing 1 lid again. After that it remained constant around 4%.

CO2 rose very high in the ‘dry’ peony box, to just below 18%. This is not recommended by Janny MT, who supplied the boxes, but a higher CO2 level is used to protect ULO grapes against botrytis with good effect. Thus we considered it worthwhile exploring this as well for peonies. The ‘wet’ peony box also experienced such high CO2 levels, but when the lids were opened CO2 decreased rapidly to some 8% again.

Temperature (blue), oxygen (yellow) and carbondioxide CO2 (red) over time in the ULO box with the 'dry' peonies.

The ‘raw’ data for the ‘dry’ and ‘wet’ peony ULO storage boxes can be downloaded here as excel files:

Dry ULO box

Wet ULO box

What about the Relative Humidity (RH)? As it turned out, the silicagel boxes did what they were supposed to do. The graph shows the ‘dry’ peony ULO box. It remained more or less constant around 95%, whereas last year it quickly rose to 100%. The ‘wet’ peony box showed the same results.

After three months, the six silicagel boxes in the ‘dry’ peony box were fully saturated. The ones in the ‘wet’ peony box not yet, though not far from it. The ‘dry’ peony box had some more peonies in it, which can explain the difference.

There was nearly no condensation on the peonies or the inside of the box this time, which was a huge difference compared to last year.

Relative Humidity (RH) within the 'dry' peony ULO box

What can we deduct from these results?

  • The ‘dry‘ peonies use up less oxygen, as although there were more of them in the box (600 to 450), the oxygen levels never went below 5%. The ‘wet’ peony box easily went below, although it had less stems.
  • How much silicagel is needed? Each silicagel box of 500 gr can contain up to 500 ml of water when fully saturated. The numbers are more or less: 600 stems, 3 months, 6 silicagel boxes saturated, thus 3 liters of water. A full box can contain some 800 stems, thus for three months we would need some 8 silicagel boxes. For ‘safety’ we might suggest 9. Thus keeping it simple: 3 silicagel boxes for each month of storage should do the trick…
  • The high CO2 levels were not detrimental to peonies, but it is unsure whether the different botrytis effects are due to this or the ‘dry-wet’ difference of the storage.

What about the flowers themselves?

Well, visually it was clear that the ‘dry’ peonies were perfectly marketable after two months. The ‘wet’ peonies weren’t all, too much botrytis. After three months the results were the same, though more extreme. For the ‘dry’ peonies some of the outer petals were somewhat dried out and were slightly brown in addition. Those outer petals, when not good enough, could easily be pulled off, making them look good again. The ‘wet’ peonies could only be thrown away, unless one would want to go through a lot of sorting out the remaining good ones. Another find is that the ‘wet’ peony buds had evolved more than the ‘dry’ ones. The latter went out as they had gone in, whereas the former were more open.

But there was another, unexpected, issue. Elsa Sass seems to be a peony that can give a peculiar problem. Some of the inner petals can have greyish brown spots even though the outer petals are perfectly fine. It can almost never be seen beforehand, thus good looking buds may show the symptoms after a few petals have unfurled. Those spots will invariably rot after a few days thereby attracking botrytis and destroying the whole flower. Not all buds have this, but as beforehand it cannot be said which ones are good or not, this issue results in all stems being worthless in fact. We thus threw away nearly all of them, except for a few ones which we gave to several florists to try them out. All with the same results: some flowers were good, others had those greyish brown spots and succumbed to botrytis.

Afterwards I’ve heard from several colleagues growing Elsa Sass that it can give this problem even during the harvest itself. It is not botrytis (one colleague sent some to a laboratory), but really a problem specific to this cultivar. Some years it happens, other years not. Not unexpectedly complaints from customer are common in bad years. I myself had never experienced this problem during the harvest thus I did not expect this. I should have known it however as last year there were some Elsa Sass in those boxes as well, as ‘test cultivars’ and they showed the same problem, but I had forgotten about it (remark: the article about ULO storage from last year shows an image of it, although I originally mixed up Mme Claude Tain with Elsa Sass).

An image can be better than a thousand words to describe something, so here are some images to illustrate it (click on them to enlarge and read the description):

I had also taken some samples of Elsa Sass to the research centre (PCS) after two months (not three) so they could measure and compare the differences. Their conclusions were as follows:

  • ‘Wet’ stored peonies were more open at the beginning
  • Both treatments did have some damage at the outer petals, although clearly more for the ‘wet’ peonies
  • ‘Dry’ peonies did have slightly dried out foliage, after recutting and placing on water they became fully turgescent again
  • ‘Dry’ peonies opened more fully and prettier than ‘wet’ peonies
  • ‘Wet’ peonies developed more botrytis than ‘dry’ peonies (15 flowers out of 50 for the ‘wet’ peonies developed botrytis at the end of the vase life but only 8/50 for the ‘dry’ peonies)

The research centre also did a trial themselves with those ULO boxes, their detailed results I haven’t obtained yet. They could compare their results with last year as they used the same number of stems and same cultivars. Only difference with them was that they placed some cardboard paper on the bottom to soak up the excess water and they also cut and stored directly (‘dry’ storage) instead of placing them on water first.

  • Sarah Bernhardt had 17% of the stems that had to be thrown away immediately after storage (meaning more than merely the outer petals affected), last year it was 50%, thus a marked improvement, though far from perfect
  • As last year The Fawn performed best, 73% had no damage at the outer petals from the onset, 25% only at the outer petals and a mere 2% had to be thrown away because of more damage. That 2% compares to 21% in the previous year.
  • The cardboard paper kept the buds dry this year, but humidity was still 100%

When we have the detailed results, we’ll update this article. I’ve seen those peonies in the research centre, but I must say that many, really many, have been thrown away as they developed botrytis later on. I’m pretty much convinced that silicagel is far superior to using cardboard paper.

To conclude:

Unfortunately we are not there yet :-( Fortunately we have made major progress :-) Other people are also using those ULO boxes and they are usually quite secretive about it, thus it might be that I’m the only fool struggling to get it right. But as the research centre had some phone calls from people in Holland trying the same and confronting the same difficulties, this probably isn’t the case. I do think I have a decent chance of success next year if I keep the following extra recommendations in mind:

  • Spraying during the season against botrytis, this may prevent botrytis in the ULO boxes, no matter how dry the season may be.
  • Not using Elsa Sass again. We’ll be trying several different cultivars next year, although we have already gotten rid of hundreds of cultivars, we still have many to try.

A minor remark is also that it’s not exactly easy to ‘dry’ the used silicagel again for use the next season. Supposedly you can place it in the oven after which the water will evaporate. The large silicagel boxes are not particularly suited for this as they are partly made of plastic. They deformed and started to leak in my oven when the heat went up. So I’ll have to throw all used ones away. An alternative may be clay, it cannot contain as much water as silicagel, but at least it’s environmentally clean and by using somewhat more of it, the same results should be attainable.

Feel free to place your remarks below this article, it sure would be nice to hear your opinion!


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