Although root pruning has been shown, in trials at East Malling, to increase the abundance of flowering on young apple trees (Webster, et al., 2000), the effects on mature trees are much more variable.
- Trials in the USA on semi-mature and mature apple trees have occasionally increased floral abundance but more often have had no effect on flowering.
- The effects of root pruning on the quality of flowers produced are also variable, irrespective of tree age.
- Severe root pruning of five-year-old Braeburn, Royal Gala, Oregon Red Delicious, Splendour, Granny Smith and Fuji apples on MM.106 rootstock planted in New Zealand trials also increased flowering abundance (as in the trials at East Malling) but reduced yields (Kahn, et al.,1998).
- The authors suggest that this yield reduction is brought about by reduced bourse shoot growth following the treatment and the reduced photosynthesis and carbohydrate supply to the roots resulting from this.
- The theory is that it is these root carbohydrates that are so important to fruit set and retention in the subsequent spring.
- Root pruning may be resorted to when other measures have failed to bring over-vigorous apple trees in a balance of shoot growth and fruiting.
- Root pruning on one side of the tree should be tried initially and the effects observed.
- To be effective the treatment should be applied approximately one third to one half of the distance from the trunk to the edge of the tree canopy but no closer than 50cm from the trunk.
- Tree anchorage may be a problem following severe root pruning.