(40) FORMATION AND MANAGEMENT OF PLANTATIONS - DISTANCE APART OF TREES; PRUNING; THINNING
Two important points connected with the formation of plantations are the distances at which trees should be planted, and the use of nurse-trees. As the strength of a plant depends on the number of its leaves, and their full exposure to light, it follows that the strongest young trees will be those which are clothed with branches ad leaves from the ground upwards, and which have their leaves fully exposed on every side to light. The distance from each other at which trees should be placed in a plantation depends on the size and nature of the plants, and the soil, situation. To have tall and straight stems, the trees are planted thickly (conifers more so than other kinds), about 4 to 6 feet apart; but when the lower branches of the plants interfere with one another, thinning should be commenced and continued from time to time. When the lower tier of branches shows symptoms of decay they should be removed by cutting close to the stem. This process of pruning has been condemned by many, particularly in soft-wood trees; but if practiced early and judiciously it may be attended with benefits, especially when the part removed does not exceed an inch in diameter, or as long as the operation can be performed with an ordinary pocket-knife; the wound will heal quickly, and leaves no mark. If branches are allowed to remain till they are 3 or 4 inches in diameter, and are then cut off at some distance from the bole (snag pruning) the timber deteriorates, the wound heals over in time, but the timber is either knotty or unsound. Close-pruning is performed by sawing off a branch close to the trunk or a leading branch. When the wound is large, a dressing should be applied to exclude air and moisture. For this purpose linseed oil, or three parts cow-dung and one part powdered lime, will be found useful. In the Royal Forests if England the branch of an oak is never removed unless special circumstances require it. Long experience has justified this system. Lateral branches which are growing with over luxuriance, and attracting too much of the sap of the plant, should be foreshortened, just as the lateral branches of a hedge are clipped when they extend too far. The only objection to this method of pruning is the amount of labour it entails. Mr McNab of the Royal Botanic Garden, Edinburgh, has recently urged the advantage of stem-pruning of conifers, on the grounds that it encourages a free growth, and tends to make the trees hardy by exposing the bark of the stems to the freer action of the air. Stem-pruned trees also are less likely to be broken by a weight of snow lying on the branches.
For pruning, the following tools are required, - a pocket knife, hand-saw, chisel, and pruning shears; the two last are fixed on long poles. Thinning, carried out with care from time to time, is of the greatest consequence. The removal of weak and crooked supernumeraries prevents unnecessary exhaustion of the soil, while it admits the essential agents, air and light, which favour the expansion of lateral branches. If, however, the trees be thinned out too widely, the side branches become robust and the stem is not drawn up. the rule in thinning should be, to keep the trees clear of each other, so that the branches do not interlace, and the air circulates all round. The time when thinning should commence depends on local circumstances, and the species under cultivation. In a well-managed plantation the proceeds of thinnings in twenty years should go far to cover the expense of culture and interest of capital.
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