Rock Gardening: Alpine Houses and Frames
As you will no doubt be aware, there are rock plants and alpine plants. In the latter category, there are a number of choice plants known as high alpines that only the most dedicated of alpine gardeners will want to grow. If you intend to get serious about cultivating alpine plants, you will eventually want an alpine house.
This is like a greenhouse, but with various design differences. First, the ventilation is arranged differently, so that as well as roof vents there are vents along the sides of the house at the same level as the top of the staging. The pitch of the roof is quite low so that the plants in their pots are nearer to the light than they would be in a standard greenhouse with a conventionally pitched roof. To maximise the plants’ exposure to sunlight, the alpine house should be aligned on a north-south axis with an entrance on the south side. With the exception of the roof pitch, a standard greenhouse can be modified to make a passable alpine house.
Staging is important and it is best to use metal staging with plunge trays full of clean sand. The pots can have their bases submerged, which helps to keep thecool and moist.
Alpine houses are unheated as their main purpose is to keep the plants dry during autumn and winter when cold, wet conditions prevail. But that is not to say that they should not be frost-free. A thermostatic heating device, which will switch on when temperatures approach freezing, is therefore a worthwhile investment. In their native habitats, high alpines spend the winter under a blanket of snow, which serves as insulation from severe and damaging frosts. In the artificial atmosphere of a greenhouse, the temperatures can vary from daytime warmth to night-time freezing and it is just this sort of fluctuation that alpines cannot withstand.
A frame is a structure built at ground level, usually adjacent to a greenhouse or alpine house, along the north-facing side, made with wooden sides and a glass top. The floor of the frame is made of horticultural fabric mulch, over which is spread a thick layer of clean sand. A frame is used to protect pot- or pan-grown alpines. To raise cuttings or as a holding bay for newly acquired specimens awaiting good planting conditions. On dry, sunny days throughout the year, and when there is no threat of frost during winter months, the cover of the frame can be lifted to allow ventilation. This is also the practice with an alpine house. For alpine plants under glass the rule is: windows open unless it is freezing.
The other area where you might risk killing by kindness is watering. In the gardenmust be perfect; similarly, pot-grown plants under glass must never become . Therefore, the rule is to err on the side of safety and don’t water unless you are convinced it is necessary.
Pot-grown alpines will, in time, outgrow their pots and pans and have to be upgraded to the next size container. It is best to do this in the early spring or when the plant has finished flowering.