At Valentinfleurs the three Valentin brothers grow peonies. My Peony went there to check it out which resulted in some great insight into this originally Dutch horticultural company now turned fully French.
Since 2006 they have been growing peonies in as many all natural ways as possible. In the open ground underneath a strong French sun. Since then they have been expanding their field of growth by a little each year. At the moment they have around 9,000 square meters of greenhouses and 4 hectares of open fields because of this. A large part of the product is being grown in tunnels giving the plants enough space to grow according to the brothers. Doing it this way makes it so that the sunlight reaches better, you will get more stems and the quality improves.
They now have 15 species but already have plans for expansion.
source: My Peony