West Midlands,
United Kingdom 
Dry greenhouse for arid plants?
I want to make my greenhouse much drier for semi-desert plants. I can easily manage the ventilation and low watering but want to prevent moisture rising form the earth floor. Fortunately the plants I want to grow (New World Buddlejas) can take extreme cold but not damp. My idea is to put down a semi=permeable membrane (not impermeable as that wouldn't allow excess water to drain away) covered by about three inches of limestone chippings. That way little ground moisture should be able to evaporate into the atmosphere. Is this the best way or is there another recommended way of keeping the atmosphere dry? Advice gratefully received.
29 Jan, 2013
Answers
How little dampness can these plants take?
29 Jan, 2013
Some damp is okay and probably more than the average cactus but botrytis is a big problem.
29 Jan, 2013
I only ask as I grow a fair number of Primula allionii and they too are martyrs to damp induced Botrytis. They can take as much cold as any other alpine and I find that a really good through draught keeps them dry, leaf wise.
My greenhouse is in a fairly windy spot so a through breeze is easy, but a friend whose garden is much more sheltered from the wind has to use an electric fan to keep the air moving. I was wondering if ventilation is the answer to your question.
Let the wind blow through!
30 Jan, 2013
Like Owdboggy we grow allionii. The greenhouse has reasonable air circulation and we do not need fans. However, they are not what I would call a plant from an 'arid' region as the air in their natural habitat, the Maritime Alps, is often misty and damp, unlike the desert. Some alpine growers do have fan ventillation in their greenhouses for some of the more difficult high alpines. In nature these get rained on regularly but it is very windy and excess water is quickly dried off.
30 Jan, 2013
Like Dionysias you mean!
30 Jan, 2013
Dionysia would be a good example, Ob. I don't try to grow them, in fact very few growers in Scotland do.
31 Jan, 2013
Your idea seems to be basically sound, Bg, but I would see a problem with the air that you draw in from the outdoors. If the greenhouse is sealed then you could possibly obtain an arid environment but with no ventilation! The outside air that you draw in for ventilation will be moist. All I can think of is a de-humidifier installed in the air inlet to the greenhouse, but I expect that this would be running all the time and so be expensive.
29 Jan, 2013