Root-Pruning has been thus considered by Dr. Lindley, in his excellent Theory of Horticulture: -

"In the nurseries, it is a universal practice to prune the roots of transplanted trees: in gardens, this is as seldom performed - which is right. If a wounded or bruised root is allowed to remain upon a transplanted tree, it is apt to decay, and this disease may spread to neighbouring parts, which would otherwise be healthy: to remove the wounded parts of roots is, therefore, desirable. But the case is different with healthy roots. We must remember that every healthy and unmutilated root which is removed, is a loss of nutriment to the plant, and that, too, at a time when it is least able to spare it; and there cannot be any advantage in the removal. The nursery practice is probably intended to render the operation of transplanting large numbers of plants less troublesome: and, as it is chiefly applied to seedlings and young plants with a superabundance of roots, the loss, in their case, is not so much felt. If performed at all, it should take place in the autumn; for, at that time, the root, like the other parts of a plant, are comparatively empty of fluid ; but if deferred till the spring, then the roots are all distended with fluid, which has been collecting in them during winter; and every part taken away carries with it a portion of that nurture which the plant had been laying up as the store upon which to commence its renewed growth.

"It must now be obvious that, although root-pruning may be prejudicial in transplanting trees, it may be of the greatest service to such established trees as are too prone to produce branches and leaves, instead of flowers and fruit. In these cases, the excessive vigour is at once stopped, by removal of some of the stronger roots, and consequently, of a part of the superfluous food to which their 'rankness' is owing.

"The operation has been successfully performed on the wall trees at Oulton, by Mr. Errington, one of our best English gardeners, and by many others, and, I believe, has never proved an objectionable practice under judicious management. Its effect is, pro tanto, to cut off the supply of food, and thus to arrest the rapid growth of the branches.

"Under all ordinary circumstances, the roots must necessarily be injured more or less by removal: in that case, all the larger wounds should be cut to a clean smooth face, and not in long ragged slivers, as is often the case, and which is only substituting one kind of mutilation for another; but at an angle of about 45°, or less.

"If the ends of small roots are bruised, they generally die back a little way, and then emit fresh spongioles; but the larger roots, when bruised, lose the vitality of their broken extremity ; their ragged tissue remains open to the uncontrolled introduction of water ; decays in consequence of being in contact with an excess of this fluid; and often becomes the seat of disease which spreads to parts that would be healthy.

"When, however, the wound is made clean by a skilful pruner, the vessels all contract, and prevent the introduction of an excess of water into the interior; the wound heals by granulations formed by the living tissue; and the readiness with which this takes place is in proportion to the smallness of the wound. It may be sometimes advantageous to remove large parts of the coarser roots of a tree, even if they are not accidentally wounded when taken up, the object being to compel the plant to throw out, in room of those comparatively inactive subterranean limbs, a supply of young active fibres.

"This is a common practice in the nurseries in transplanting young oaks and other tap-rooted trees, and is one of the means employed by the Lancashire growers of gooseberries, in order to increase the vigour of their branches; in the last case, however, the operation is not confined to the time when transplantation takes place, but is practised annually upon digging the gooseberry borders. The reason why cutting off portions of the principal roots causes a production of fibres appears to be this; the roots are produced by organizable matter sent downwards from the stem; that matter, if uninterrupted, will flow along the main branches of the roots, until it reaches the extremities, adding largely to the wood and horizontal growth of the root, but increasing in a very slight degree the absorbent powers: but if a large limb of the roots is amputated, the powers of the stem remaining the same, all that descending organizable matter which would have been expended in adding to the thickness of the amputated part, is arrested at the time of amputation; and, unable to pass further on, rapidly produces granulations to heal the wound, and immediately afterward young spongioles, which soon establish themselves in the surrounding soil, and become the point of new active fibres." - Theory of Hort.