How to prune understock of grafted plants

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When grafting is done (either scion or bud) it usually is necessary to prune the understock after the graft has "taken" (united with the understock). For example: In the case of Rhododendron, when a side graft is used, the union takes place in three or four weeks and the top half of the understock is then cut off. A few weeks later, when the callus has changed its color to a deep brown, another inch or so of the understock is cut off and a few weeks later the remainder of the understock can be removed, making the cut just above the union. Blue Spruce, Picea pungens Koster, is grafted on an under-stock of P. abies (Norway Spruce). These are grafted in a propagating frame in a greenhouse beginning late in February or early in March.

The potted understock, just prior to grafting, is cut back to leave it about 8 to 10 inches tall. When the scion starts growing, 2 or 3 inches more of the understock are cut off. Toward the end of May a further reduction of the understock is made. Part of it is left until the end of the growing season, when the remainder is removed.

Grafting is usually done on one- or two-year-old understocks, but if a scion of a different variety is grafted in a mature tree—as, for example, when a Pink-flowering Dogwood is grafted in a white-flowering form or when a male shoot is grafted on a female plant of Holly to insure pollination—it is necessary to make sure that the grafted portion is not smothered by the growth of the host plant. This is done by cutting off any shoots that are encroaching, and when the grafted portion has made enough growth to encourage the sap to flow in its direction, the understock is cut off just above the point of union.

In the case of the Dogwood, neighboring portions of the white-flowering understock are cut back so that the graft has a chance to become dominant.

Suckers from grafted plants

We are constantly getting letters from beginning gardeners who complain that their beautiful red Rose has changed to one which has only small white flowers in large bunches. The explanation of this is that the understock on which the red Rose was grafted, usually by budding, has grown at the expense of the grafted Rose. This means that you should be suspicious of any shoots which originate below the ground.

Usually these shoots have leaves which possess seven or more leaflets as contrasted with the five leaflets that are usual on Hybrid Tea Roses. If it is noticed soon enough it may be possible to pull it up in its entirety, but all too often it is not seen early enough, with the result that you have to cut it off with pruning shears and that means that it will start growing again. Then it may be a case of digging up the entire plant, separating the understock, and replanting the rose that you want to Florida rubra (Pink-flowering Dogwood). Close watch must be kept on these, and if the understock is growing it should be eliminated immediately.