Please pay attention to the attached picture of mulching film : drip irrigition under the film. The film can decrease the evaporation of the ground, you can lessen the times of irrigation, and so the humidity will not increase quickly.
Just a suggestion,Ok?
Thanks for your kind and detailed explanation. I’m going to try some plants at once. Sarah blooms in the middle of May in my nursery, perhaps it’s a little late to use GA3 now.

I see. The mulching film is thus the white plastic? I suppose drip irrigation under that plastic will indeed be necessary, otherwise the only water would come from the openings around the plants. Are you planning on using those same ‘growing bags’ or do you want to plant the peonies in soil and then place such white plastic above it, with openings for the plants?

Some growers use anti-root-cloth, because this keeps weeds out (at least as long as there is no soil above the cloth), it lets water through and decreases evaporation.

There’s a research station I cooperate with that has done some experiments with different mulches in peonies. I should write an article about it, but there are also many other things to be done and I don’t always have enough time.

But the mulch is not really the issue. I’ve grown peonies in polytunnels with mulches also, and whilst it does what you’d expect, the soil evaporates less and is moist for a longer time, the humidity in the polytunnel is hardly influenced by it.

That is because most humidity simply comes from evaporation of the plants themselves (you could grow them drier of course, but water shortage tends to show up in less flowering stems (or relatively more flowerless stems)). When the peonies are just peeking through the ground, there is not much of a humidity problem. Relative humidity is not particularly high on average then, and if it does (because of very humid and grey weather outside) there’s no damage. It’s when the plants are actively growing and have a lot of leaflets open that they evaporate that you see the relative humidity rising fast. The plants are most prone to damage when the flower buds are almost about to appear from the upper leaflets but are still encircled by them, making something of a ‘depression’ where the water can remain standing. That period is usually short, perhaps a week or so, but if humidity at that time is high… A mulch of compost or something might take up the droplets of water falling on it, but most droplets from the polytunnel plastic will fall on the foliage and flower buds. A plastic mulch will do nothing at all there, the water will simply remain on it. Anti-root-cloth will be somewhat comparable to the compost mulch. Anyway, what you need at that time is adequate ventilation. If temperature is higher inside that outside (as it normally is during daytime with the slightest bit of sun), the Relative Humidity can be lowered through the exchange of inside and outside air where the outside air will have a lower absolute water content (warmer air can contain more water in absolute number at the same relative humidity). If the colder air enters the warmer polytunnel, the absolute amount of water in it remains but relative humidity drops because that air is now becoming warmer. If there is also some wind flow it can dry the leaflets and flowers. A fan might also help somewhat. Spacing the plants further from one another is another possibility. Drip irrigation, as mentioned before, is absolutely necessary (unless you’re growing them in some very dry area like they may be doing in Israel at the Golan Heights). But in fact what I wanted to say: that plastic mulch will be of no help to keep relative humidity low.