Time to describe our ongoing quest for a perfect long term storage procedure for peonies. If you have read last year’s report, you will know that we were rather hopeful to see many good peonies several months after cutting them. Dry storage instead of wet storage (thus no placing them in water), cooling down fast and lowering the oxygen fast in combination with lots of drying silicagel to keep relative humidity in check should do the trick we thought. So, you may be curious as to whether this all materialised?
Well, the short of it: out of 9,000 stems stored this way, 6,000 had to be thrown away after lots and lots of grading during an already busy period in late Summer. To say we’re a bit disappointed would be quite an understatement. The average result for our ULO boxes is shown in the image below.
But I’m sure you’re interested in the long story, after all, even from failures something can always be learned, Rome wasn’t built in a day.
Let’s first start with the season itself because it was an exceptional one. The main reason for this is that it was the coldest and wettest in decades. Temperatures remained far below longtime averages right up until flowering time, which was some 2-3 weeks later than what we are used to. Of course when that time finally arrived, a heat wave struck, complicating matters even more with everything flowering in a very short time span. This all resulted in very high disease pressure on the plants, with in the general market numerous complaints about botrytis on the foliage and flowerbuds. We ourselves had it more or less under control, although some peony rows that were hard to reach when spraying were also severely affected, but these were wisely not cut. Still, without a doubt, this year we had to start with peony stems that will have had some more latent infections of botrytis, which is not a good starting point, but there’s nothing to choose here. On the positive side however: the cultivar we use most, The Fawn, usually has ‘open’ buds which may be more sensitive to botrytis infection on the inner petals compared to Sarah Bernhard which has closed buds and where the outer petals better protect the inner ones. This year, probably due to the long cold spell, The Fawn also had perfectly closed buds, something we’ve never before experienced.
The Fawn peonies we used for our ULO storage boxes were cut halfway June, during dry weather, put in the boxes dry (thus no hydrating beforehand), cooled down and then the boxes were closed after some 24 hours with lots of silicagel in it. Into the boxes we also placed an average of 3 containers silicagel / month intended. Each could suck up some 500 ml of water and given that we aimed for 2 – 4 months, some 6 to 12 such containers were placed in the ULO boxes. The number of stems in it was between 700 and 1,100, always leaving free some 10 cm at least from the top cover of the box. The time of closing those boxes was June 12-14 and temperature was kept below 1°C. The advised number of stems in an ULO box is some 800, we also had some with those numbers. The 1,100 was because we could simply but so many in and it would not be surprising to find that other growers would want to do that as well for it obviously restults in a lower cost per stem. The 700 stems box was only half full, this was the very last one we filled, June 14 and it contained some smaller stems that had been laying in the cold storage for over a week but hadn’t been prepared due to time constraints. Given that at that time wholesale prices were at a season’s low and we had one ULO box left but no other peonies to fill it with we simply decided to use them. Even though they were somewhat dried out and although we normally only use larger fresh ones.
As you may know, I’m not alone in this endeavour, because I get some help from a research project named Atmo-Flor which brings together industry stakeholders, Ghent University and the Ornamental Research Centre (PCS). At the PCS, they also closed their ULO boxes at around the same time, with their own grown Sarah Bernhardt and my The Fawn, next to a very few stems I delivered of Amalia Olson, Don Richardson, Madame Claude Tain and Elsa Sass just to have some additional comparisons (not enough to run statistics on them). A difference with me is that they also placed cardboard paper on the sides of the box to suck up some of the excess humidity.
As usual the PCS also placed some stems in a simple cardboard box with plastic in it where the oxygen level would not be decreased but remain at ambient levels. The plastic is rather important here as it means humidity also remains rather high. To avoid excess humidity on the flowers in this plastic they were additionally wrapped into some normal paper as well within the plastic.
Another difference from what I did was that they wanted to test a new fungicide that is often used for long term storage of fruits like apples and pears. In the results it is mentioned as Pr296, because it has no offical approvement for regular use in peonies, but fruit growers here will know it as ‘deccopyr’ by BASF (active ingredient pyrimethanil). This is a fungicide that has to be burned in a closed room; the smoke will then fill this room and cover the peonies which will protect against botrytis and other fungi. So, as they have two ULO boxes at the PCS, they filled both with all cultivars and the only difference between the two boxes was that the contents of one of them had been treated with this experimental fungicide. Mention also that all peonies had received preventive spraying in the field before cutting. My own peonies had been sprayed with the biological/organic fungicide Serenade (active ingredient bacillus amyloliquefaciens strain QST713) and a more conventional one Luna Sensation (fluopyram and trifloxystrobine). The PCS had sprayed them with Signum (boscalid and pyraclostrobin). Those are some of the most efficient fungicides nowadays available.
The stems put in were thus cut and stored dry before closing the boxes. At the PCS they also measured the moisture contents of the leaflets, stems and buds. Thanks to the dry weather at the time of cutting and the dry storage, moisture percentages were comparable to the previous year.
Moisture contents | 2019 start | 2020 start | 2021 start | 2021 after 10 weeks ULO box | 2021 after 10 weeks cardboard box |
Sarah Bernhardt | 77% | 73% | 72% | 73% | 73% |
The Fawn | 78% | 73% | 73% | 74% | 75% |
The moisture contents are calculated simply by weighing the fresh stems, drying them completely out in a special oven until stable of weight, weighing the dry stems; and the difference being the moisture content. The table above shows how they started at some 73% moisture content, both this year and last year, compared to 77% in 2019 when they were first placed in water to hydrate them. Within storage they kept their moisture level more or less, perhaps it even increased very slightly in the conditions they were placed in. In normal storage where they are really kept ‘dry’, thus neither an ULO box nor placed in plastic, they would further dry out (see later).
During the period that the peonies were in those boxes, regular measurements were taken of temperature, oxygen, CO2 and ethylene. As all lids were closed at once, oxygen levels went down very fast. So fast in fact that when I checked a week later, oxygen levels were below 1% in a few boxes. This I had not expected given the results of previous years and I thus opened a few lids to raise oxygen levels more towards 5%. At the same time CO2 reached higher levels, something which might also help deter botrytis, although these high CO2 levels quickly went down when one or more lids were opened. This shows that an O2/CO2 meter, although very expensive, is no wasted money when working with these boxes. At the research centre, measurements were better followed up and they never got this low in oxygen as can be seen from the following graph.
One remarkable find from all measurements were the ethylene levels. Previous years we had already noticed that high levels of ethylene during cold storage don't seem to result in lower quality or damage, even though in literature levels as low as 10 ppb (parts per billion) have shown to inflict damage on plants. Probably the low temperature protects the peony flowers from this. In the cardboard box ethylene could not accumulate as there was free interchange with outside air and there was an average 32 ppb ethylene concentration there. In the untreated ULO box, the ethylene concentration went up to 1,136 ppb. From previous years there was a clear correlation between high ethylene levels and the presence of botrytis, probably because the latter produces ethylene as well. The researchers were quite happy then to find that in the other ULO box, where the peonies had been treated with pyrimethanil, ethylene levels did remain under control, only to 136 ppb. This raised hopes for suppression of botrytis given the correlation between them in earlier years.
Some 10 weeks later we started opening the ULO boxes. The PCS opened everything at once for comparisons, I spread it amongst the whole month of August. Now what did we see?
Despite all the silicagel that should have been enough to keep relative humidity in check, it turned out that not all seasons are equal when it comes to this. From the first box opened in the beginning of August it became already clear that the top cover was dripping wet when I opened it. This was surprising, especially so as the silicagel was not yet fully saturated. The first one I opened only had six containers of silicagel in it as it was meant to be opened after two months. As it turned out, the boxes intended for longer storage, which had 9-12 containers in them, were better (as were the boxes with fewer stems in them), even at the end of August, thus an extra month later. I can only guess to the failure this year, but to me it seems that not the amount of water that could be absorbed is the problem, them being not saturated yet, but rather the fact that it cannot take the water out of the air fast enough. With double the silicagel containers in the ULO boxes for even longer storage the capacity of taking water out of the air in any given time is obviously twice as large.
Long story short, boxes with only 6 silicagel containers were terrible and the peonies full of botrytis. The 12 container versions fared better, botrytis levels were far lower there. But there was another issue. A very large number of them had small transparent to white flecks on the petals. Whilst you had to look good to see this, especially as it often only showed a few petal rows further inside the flower bud, those inconspicuous white marks invariably turned into dead tissue and in turn of course developed botrytis, making them worthless. The PCS did not have this problem, so it may be a consequence of my extremely low oxygen levels the peonies experienced for a short time. On the other hand, not all boxes went so low and that kind of damage showed up in nearly all ULO boxes. The images show the problem pretty clear I think. With the best boxes I had to do careful checking of each and every stem. This was a lot of work and knowing what awaited me each time when I would open another ULO box was quite discouraging. But eventually a few thousand stems could be sold in August to make up for at least some of the losses.