The transplanting of deciduous trees, shrubs, and vines is commonly carried out during their dormant season. It is possible in the spring, however, to carry on planting of deciduous woody plants, at a time when the local plants are too far advanced to be moved, by the simple expedient of bringing plants from a storage cellar or from a more northerly nursery where they are still dormant. Again in the autumn, these same northerly grown plants may be used to start planting work before the local plants are matured and safe to move. Transplanting seasons are not so much governed by north and south latitude as they are by the condition of the plants, as explained in another paragraph under discussion of life-zones.
The beginning of the dormant period for woody deciduous plants comes in the autumn when their wood is matured and ripened and the leaves start to fall or to take on their autumn colouration. This occurs early in such plants as lilacs, lindens, flowering currants, and horse-chestnuts, and it will usually be found to occur late in some of the plants which are said to be hard to move in the autumn, such as poplars and silver maples. From the beginning of the dormant period in the fall until the beginning of physiological activity in the spring, deciduous plants may be moved at any time that the ground is in proper condition and the temperature favourable. As a matter of practice, in the northern states this work is suspended entirely during a normal winter, for about four months, except where large plants are moved with a frozen ball of earth about their roots, because frozen ground and snow make the operation of transplanting smaller plants entirely impracticable. This period, longest in Maine and in the section surrounding Minneapolis, lessens as one goes farther south, until in Virginia and Georgia a continuous planting season extends without interruption through the dormant period.
It was early learned that the whole of North America could be conveniently divided into seven transcontinental belts or life-zones, based upon the length of the growing season, which has been defined as the period between the date in the spring when the normal mean daily temperature rises to 43 degrees Fahr., or above, and the date in the autumn when it falls to below that figure. (The reader should consult Bulletin No. 10 of the U. S. Department of Agriculture, Division of Biological Survey, entitled "Life Zones and Crop Zones." Part III of that Bulletin is especially interesting). These life-zones are, as noted above, adapted to plants requiring growing seasons of similar length and temperature range. Thus, if soil conditions, exposure, and amount of annual rainfall are alike in two distant portions of a zone plants which succeed in one portion may be expected to succeed in the other portion. They may in any event be tried out with considerable confidence when all the conditions are known to be the same, as described above.
It now seems evident, from recently gathered data, that these zones correspond very closely to belts of country which have similar planting seasons for dormant woody plants, at least throughout the humid regions east of the 100th parallel of latitude. By consulting the accompanying Plate II, which has been adapted from the one in the above-mentioned Bulletin, and also the chart (Plate III), which shows the reported length of planting seasons, it will be seen that the stations reporting fall into groups which lie in respective life-zones as shown on the map.
Thus stations 2, 3,4, and 6, all of which lie in the so-called Transition Zone, including most of New England, New York State, Pennsylvania, northeastern Ohio, the Alleghanies from Pennsylvania to Georgia, southeastern Ontario, Michigan, Wisconsin, southern Minnesota, North Dakota, and northern South Dakota, all report a short fall and spring planting season, divided by a long winter season, during which ordinary planting work is impracticable. Stations 7, 8, and 10 lie in the northern limits of the upper Austral Zone where they are influenced by the Great Lakes. Stations 13,14,15, and 18 lie outside of the Great Lakes influence in the same zone, which includes a great territory stretching from the Coastal Plain westward to the Great Plains, and from the Transition Zone on the north to central Georgia and northern Texas on the south, except for an arm of the Lower Austral which extends northward along the Mississippi River to Cairo, 111. The first three stations, 7, 8, and 10, show the influence of the Great Lakes in that their spring planting season is delayed, while stations 13, 14, and 15 show a markedly later date for stopping transplanting in winter and an earlier closing date in the spring, due to the greater length of the growing season in this zone. Station 18 has such short winter interruptions that it practically offers a continuous working period from fall to spring. Stations 17 and 20 lie in the Lower Austral Zone, which includes the Atlantic Coastal Plain from the Piedmont Region to the ocean and all the southern states south of the Upper Austral Zone. The Sacramento-Fresno Valley in California also is included in this zone. These stations, together with station 19, which is probably influenced by the Japanese Current, and station 21, which lies in the Gulf strip of the Lower Austral, all report a continuous planting season of about the same length, which is uninterrupted by any cold weather. Station 16 reports a long fall season with a short interruption and a short spring season, while station 22, which lies just north of the Tropical Zone, reports a short, uninterrupted season which closes early.
In the extreme northernmost area, except that of Camden, Maine, the fall planting season does not open early enough, nor does the spring season extend long enough to offset the long winter period of frozen ground, which may extend to five months in the Northern Zone. Thus the total number of working planting days in the Northern Zone may be only 70 or 80 in an average year of not unusual severity of winter, while in the great central portion of the country each of the two seasons may be as much as 50 days long, giving a combined planting season of about 100 working days. In the Southern Zone, where there is no interruption during the winter, the season may be from 115 to 150 or even 160 days long, except that the beginning of growth in the spring curtails the season at that end when the Tropical Zone is approached. It should be noted that the farther south one goes the more abruptly the spring growing season opens and the harder it becomes to prolong the planting season by any of the expedients mentioned above. The growing seasons of the southern sections of the United States open rapidly and there is greater danger in the operation of transplanting after leaf growth has started than in the cooler northerly sections of the country. It is also inadvisable to import cold storage plants into such southerly sections much after the time when local stock is in full leaf.
It is probable that as time goes on much more detailed and complete data will be published regarding safe planting seasons for the different life-zones of the country, thus enabling planters to eliminate nearly all of the guesswork which now exists, when one is called upon to execute work in an unfamiliar territory.
Evergreen Plants (Coniferous and Broad-leaved). The planting seasons for evergreens follow somewhat closely those for deciduous plants. It is probable that as our knowledge of broad-leaved evergreens increases, their planting season can be shown to do this also. There are now about fifty known species of broad-leaved evergreens which are hardy in our northern climate if handled properly. It is important to know what are the requirements surrounding successful transplanting of evergreens. Probably the most important seasonal requirement for transplanting of evergreens is that the soil moisture shall be plentiful just prior to the time of transplanting in the location from which they are taken. In other words, in spite of all the old notions to the contrary, fall planting of evergreens should not start till the fall rains have adequately moistened the soil. Fall planting of evergreens can be successfully done as late as any other planting, provided the ground is moist when it freezes. It is better to wait for the fall rains than to plant too early and subject the plants to a hot, dry spell immediately afterward. In the spring, while it is possible to delay the planting of evergreens past the safe date for deciduous stock, due to the fact that they are practically always moved with a ball of earth, yet the best season is the earliest possible one. In both spring and fall planting, early planting has the advantage of allowing the plant to start root growth before the advent of the very hot weather of summer and the drying winds of winter, which sap the moisture content of the plant from the pores of the persistent leaves. Such sorts as biota, thuja, and taxus, which seem to establish themselves readily, can probably be successfully planted later in the season than others like pice a, abies, tsuga, pinus - except nigra (austriaca), montana (Mughus), and Strobus - and chamęcyparis . Evergreens of the first type will be subjected to much loss if not transplanted in a fully dormant condition immediately prior to the beginning of growth in the spring or if transplanted at a time in the fall when root growth sufficient to fix the plant in its new soil surroundings cannot be developed because of the lateness of the season.