Many orchardists are not aware of the problem with scion rooting.
It means the scion—the part of the tree above the graft or bud union—has formed roots. Scion rooting can only occur when the graft union (where the rootstock joins the scion) is below ground level.
Rootstocks of apple trees are designed to control tree vigour and increase early fruitfulness. The most common rootstocks for apple trees in Australia are MM106, M7, M26 and M9.
If trees on these rootstocks (and on other less common ones) are planted too deep, the chances are that the scion will form roots. When this happens, the rootstocks will not control vigour.
Roots above the graft
Apple trees with roots above the graft or bud union develop vigour of seedlings.
It is not uncommon to see some trees in a row with many long and strong shoots, and other trees in the same row with less vigour and much calmer.
If you look closely at the lower part of the trunks of the vigorous trees, you will see that the rootstock/scion union is underground.
If you remove the soil you will find that roots have formed just above the union.
Over time, scion rooting will cause the rootstock to waste away, especially dwarfing rootstocks such as M26 and M9.
Removing the soil and cutting off the roots of scion rooted trees is expensive and often causes ‘saucers’ which collect water around the tree.
Prevention is better than cure:
- Inspect your trees at the nursery to see that your trees have been grafted or budded well above the ground
- Plant trees with their unions 100 mm to 150 mm above the ground
- Scion rooting is more prevalent when trees are planted on hills. The soil is often soft and the trees are ‘sucked’ into the soil when the hills consolidate.
See this article and images in Tree Fruit July 2016
