This section is from the book "How To Make A Flower Garden", by Wilhelm Miller. Also available from Amazon: The Well-Tended Perennial Garden: Planting and Pruning Techniques.
One is often told that it is not practical to raise violets in New England in coldframes, but from experience I can affirm the contrary. I bought six ready-made coldframes, and they are so well made and of such excellent material that they can withstand the coldest of weather. It is best to be on the safe side, however, and in severe weather straw matting should be placed on the glass, and then boards. The mats can be made at home with burlap and of straw made into mattresses, or they may be bought for one dollar and a half apiece.
My violets are a source of great pleasure to me. They are a delightful family, and on cold days, when all the ground around is frozen, they alone are warm and fragrant. As I spent two winters in trying to get my frames "mounted," so to speak, it may be of some help to others to hear of my trials. First, ground-moles attacked them and plowed and replowed the roots until the violets were almost dead. Had I but known it, "rough on rats" kills them. I always sunk the frames, which proved a poor plan in my case.
Coldframes of brick and iron.
Select a high and dry place near a fence or hedge of evergreen trees for shelter from the north. It may even pay to build a fence along the north side. The frames must have as much sun as possible. The soil should be banked up around the frames to keep out cold and dampness.
In my six frames are planted one hundred and eight Lady Hume Campbell violets. Every two weeks or so I pick at least two hundred blossoms, and they are of an unexcelled fragrance and colour. Next year I intend to double the number, as I believe my coldframes produce finer flowers than any I have seen grown in a greenhouse.
There are very few days when it is too cold to pick violets in the middle of the day. Take a small covered basket, lined with something; open the frames a little at a time, and drop the blossoms into the basket. Of course, there are some days when the matting cannot be taken off, but there are not many of them. Each day the frames must be aired, if for only ten minutes at a time. It is best to have a small thermometer inside; and seventy-five degrees is the highest temperature that should be allowed.