This section is from the book "The Gardener's Monthly And Horticulturist V21", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
I have often wondered why our nurserymen or horticultural societies have not supplied us with complete lists as to hardiness of the different varieties of fruit and ornamental trees, shrubs, etc. I am well aware that there is a great variation of climate and soil in the same latitude, but this can be taken into consideration by the tree planter and acted on. Of apple trees we have partial lists, but there are still many varieties of apple that are comparatively hardy that are not given in these lists. Of pear, plum, peach, cherry, and nearly all of our introduced ornamental trees, we can learn little or nothing except by personal experience. Now it appears to me that with a little observation by our nurserymen and horticultural societies, lists could be made out giving the comparative hardiness of all the different varieties cultivated. They could be corrected one with another until they were entirely revised and complete. Until this is done tree planters in the Northern States and Canada will be at a great disadvantage, having little or nothing to guide them except their own experience. We know that trees are killed by frost in different ways, but never when the wood is properly ripened.
There must be a superabundance of sap in the trunk or branches before frost will cause any ill effect. Now it would be an easy matter for nurserymen to make a note of the time of ripening of the wood of the different varieties cultivated, and the percentage killed of each variety by frost.
The high state pf cultivation in a nursery will generally cause trees to be later in ripening their wood than where they only receive an ordinary orchard cultivation; consequently notes taken on nursery rows would be of the greatest value in a season in which all varieties ripen their wood well. The comparative earliness could then be seen. Here in Canada the effects of frost are generally seen in three different forms:
A sudden severe frost setting in early in the season will expand the outside of the trunk while the heart is still unfrozen, and split the tree. The split is always on the side most exposed to the wind.
The tree being expanded by frost, when the heat of the sun becomes great enough, through the course of the winter, a strip on the south or southwest side becomes thawed and contracts, splitting away from the part that remains frozen, or bursting the sap vessels, thereby retarding or entirely stopping the circulation in that part.
The young shoots having more sap in proportion than the trunk or old wood, and being easier and more frequently thawed and frozen, the sap vessels become completely clogged, in which case the sun and air will soon dry them up.
So far as my observations have gone, this action of the sun on frozen sap vessels appears to me to be the true cause of the pear and apple blight, the sap vessels becoming partially clogged. So long as the strong upward flow of the sap continues, no effect will be seen; but as soon as the leaves have attained their full growth and the circulation becomes weaker, then a portion of the sap will remain stationary at the partially clogged point. Warm weather setting in, fermentation takes place, and we have blight.