Grafted woody plants – trees, conifers, roses, etc. – are more and more common in nurseries. In such plants, a desirable variety is multiplied by grafting it onto a wild seedling of the same or related species. This gives faster results that growing plants from cuttings, plus many plants that are very difficult to grow from cuttings are easy to graft. Grafting can be done at various heights, but in most cases is done just at the base of the plant, near the ground. Normally you would never plant a woody plant deeper than it was in its original pot, but here may be worthwhile making an exception.
That is because the bud union, that is the usually swollen node at the base of the plant that indicates the junction between the scion (desirable plant) and the rootstock (wild plant), will remain a weak point throughout life of the plant, subject to frost, fractures, cracks, insect infestations, diseases, etc. Just wind rock alone (the force of the wind pushing the top growth back and forth) can do damage. However, if you bury the bud union, not only will you reduce wind damage, there is an excellent chance that the scion will begin to produce its own roots and thus “free itself” from the rootstock over time, becoming an “own-root” plant and overcoming many of the problems mentioned. It is not necessary to bury the bud union very deeply: 1 to 2 inches (2 to 5 cm) is usually enough. Burying the bud union is even more important in cold climates than warmer ones, as alternating periods of freezing and thawing are even more likely to damage the bud union.
A secondary advantage of covering the graft with the soil is that it tends to reduce suckering at the base of the plant, a common problem with grafted plants.
Exception to the Rule
Of course, there are exceptions to every rule and here is one. When you buy dwarf and semi-dwarf fruit trees, you’ll notice the bud union is not at ground level, but about 2-4″ (5 to 10 cm) higher. There’s a reason for that. Here the rootstock is not a wild seedling, but a specially chosen variety designed to reduce the growth of the upper part of the tree, making it easier to harvest. If you bury the bud union of a dwarf or semi-dwarf fruit tree, the scion will eventually put out its own roots, begin to grow normally… and the dwarfing effect will be lost.
So, bury the bud union on most woody plants, but leave the bun upon of dwarf fruit trees exposed.