Some plants, particularly trees, may be transplanted successfully while young but are more difficult to move as they get older. Examples of this are the hickory and oaks, which, with the exception of the pin oak, form deep tap roots. The tulips and magnolias are hard to move because they have few fibrous roots; while maples and elms, on the contrary, have many fibrous roots and are moved more easily. Junipers transplant more readily when older, for they then have a more extensive lateral root system.
Large trees should be transplanted when they are dormant. It may be necessary to move other material before the growing period has stopped, but this should seldom be attempted with older trees. Large trees are moved in winter, and it is preferable to move them when the ground is frozen. The ball of earth on the roots will then remain fairly intact and there will be a minimum of root loss when moving. Holes for the trees should be dug with straight sides and with bottom rather convex or slightly rounded. This is much to be preferred to making holes bowlshaped and it permits an opportunity to spread the roots more naturally than in a hole where the middle is deeper than the sides. A common error in transplanting large trees is that of providing a hole not sufficiently large to receive the roots of the tree without cramping. A hole for a large tree greater than five to six inches in diameter should not be less than eight to ten feet in diameter, and never less than three feet in depth. The most common method of transplanting large trees is the method of cutting the roots down to a ball approximating eight to ten feet in diameter. Trees transplanted in this way are frequently subjected to a root-pruning process during the previous summer, or preferably during the previous year.
It has been stated by authorities, who are in a position to know, that the ideal method of transplanting trees is by saving all of the root system, if possible. This process is known as "combing" out the root system. Under this method all of the roots are traced down to their fine ends and then the roots are tied up in burlap in order to prevent excessive drying out of the fine fibrous roots. Transplanting in this way requires much more care, but it assures less loss than the other method, which is a violent process and requires strong recuperative powers in the tree which is transplanted.
Ample drainage must be provided for large trees, especially when set in clayey soil. The soil with which trees are transplanted is normally a medium loam, not too compact in character. This soil is much more porous than the heavy clay soil in which the hole for the tree may be excavated. Consequently, the natural tendency during wet seasons is for the water to drain toward the tree pit and to "water soak" the loose topsoil in which the tree has been set. This really places the tree in a reservoir. A tree will survive such treatment if it can withstand extreme moisture conditions, or if the water drains slowly away. However, the tree is generally killed during the first season, or may survive in a much weakened condition. The normal method of draining trees is to provide a four-inch tile connected with some outlet in the form of existing tile drains, or lower ground, so that the water may be taken away. In the event that there is no opportunity to provide this type of drainage it is desirable to excavate a hole to a greater depth - approximately three to five feet - and thereby provide below the tree a space of at least twelve inches which should be filled with broken stone or other porous material and in which water resulting from normal rainfall may be collected. In this way the root growth may be kept from drowning.