This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
A writer in the London Florist and Pomologist gives an account of his experience in wintering geraniums by burying them in a trench under ground, below the reach of frost The result was quite successful, only two plants out of fifty having decayed when dug out the last of April. The trench should be made in a location where no water can remain at a depth of two feet below the level - that being the depth at which the trench ought to be dug - the plants laid in by the heel in a row, then covered with straight straw, set so as to carry off water, and then covered with earth, according to the climate, to a depth sufficient to keep out frost; finishing off in a ridge or roof shape to carry off water. Covering the plants over the straw first with boards set in a ridge form and then heaping on earth, we think, would render pressure less liable, and tend to keep a more open circulation in the trench, and thus lessen the liability to damp or decay; but with a little care in this way, we see no reason why any half hardy plant may not easily be kept over winter in this man ner.