I grow purpurea along with militaris, morio, Dact. sambucina, Gymnadenia, Anacamptis and Himantoglossum in clay pots plunged in sand in a 4x4 Access frame which has a subsoil heating element that comes on when the air temperature in the frame drops to -1C. The sides of the frame are usually partially opened for ventilation (too wide and the cat gets in!). Because of its aspect the frame gets no direct sun between November and early February. I shade with 50% shade netting from mid-April.
The potting mix is about 40% coarse (concreting) sand with 30% granite grit ( 4-5mm), 10% baked clay granules (cat litter), 10% perlite and 10% Melcourt fine bark ( the only organic); top-dressed with a good layer of the granite grit.
With this mix I can water freely without too much worry. I try to keep water off the rosette at all times. Despite the lack of organics this mix doesn't dry out as long as the plunge is kept moist. I water freely from September through until the first flowers are opening (usually early-mid May). After this the compost is kept just moist. Try to keep the plant as cool as possible at this time to extend the growing period. Dry off when the leaves start to brown ( I lift the pot out of the plunge -it dries off fairly quickly when free-standing). I store the dried-off pot under my greenhouse staging where it remains bone dry but not too hot through the summer. I re-pot in a hand-moist mix in late August.
I feed using half-strength proprietary orchid mix (the higher nitrogen prep in the first half of the growing season and the high potash feed later on).
This all works for me. The first season my modest tuber increased quite markedly. The second season the tuber was huge. This summer it became one big and one small tuber (initially I misplaced the pot under the greenhouse benching and thought I had lost it so I bought another from Jeff Hutchings last month). It has not set seed for me but perhaps the addition of a second clone may sort this.
