The belief is quite general that quite heavy mulching of the roots of orchard trees when the ground is deeply frozen will retard the blossoming period. Repeated trials at the experiment stations and by private orchardists have demonstrated the fact that this belief has no foundation.
The flowers and leafage are started in spring by the temperature of the air. If we draw a branch of the grape or other climber through a hole into a greenhouse or warm room it will soon develop blossoms and leaves when its roots are encased in frozen earth and the temperature is that of winter outside. For this reason the retention of frost in the soil will not retard the period of flowering a single hour. This is true also of shrubs, vines, and all ligneous plants with tops exposed to the air. But strawberry-plants, grapes, climbing roses, and, indeed, any growth laid down and covered with earth or litter, will be retarded in blossoming several days.
The principle involved is that the bursting-time of flowers and leaves depends on local stores of nutriment stored in the cell-structure, and to some extent it is independent of root action. But the roots usually are not frozen at the season of blossoming even when encased in frozen ground. Moisture really ascends to the top when the air-temperature is warm enough to start top-growth; that is, enough moisture to expand the flowers and start leafage. But it often happens where a heavy mulching is covered over the deeply frozen earth in which the feeding-roots are held that the full supply of water and plant-food is not carried to the top in time for healthy growth. In rare cases we have known the imperfectly supplied leaves and starting shoots to he injured by sun and wind and the vitality of the trees and bushes permanently lowered. Indeed, in one case reported by G. P. Peffer, of Wisconsin, bearing apple-trees were killed in this way.