Some apple varieties, such as Spartan and Bramley’s Seedling, are inherently poor in branching and often produce no feathers in the nursery when grown as ‘maiden trees’ but with the use of special techniques to promote branching can produce a well-feathered 2 yr old nursery tree. Feathering on trees of Bramley, Spartan and other varieties may be improved by:
- Growing trees on for a second year in the nursery.
- Producing the trees using the ‘knip boem’ or ‘snip tree’ technique.
- Producing trees using interstems.
- Sequential removal of the lamina (blades) of the leaves at the shoot tip of the scion central leader.
- Spraying trees with ‘branching agents’.
Growing trees on for a second year in the nursery
As might be expected, growing ‘maiden’ trees on in the nursery for a second season greatly improves the number and quality of feathers produced. The height of these feathers above ground level can also be controlled such that fewer of the low and unwanted feathers are produced.
- Two-year-old trees are, inevitably, more expensive and growers can expect to pay perhaps 50 pence more for a two-year-old tree.
- Usually, nurseries undercut the root systems of the trees at the end of the maiden year, even when the trees are left for a further year in the nursery.
- This aids lifting at the end of the subsequent season and encourages more fibrous root systems.
- Two-year-old trees are larger at the time of transplanting to the orchard and often receive a slight check to growth following transplanting.
- This can be advantageous in reducing shoot growth and increasing the number and quality of the flowers produced.
- However, excessive checks to growth and severe tree stress must be avoided.
Producing the trees using the ‘knip boem’ or ‘snip tree’ technique
Trees with much improved feathering are produced using the ‘knip boem’ or ‘snip tree’ technique which has been common practice amongst the best Dutch nurseries for many years. The benefit of this technique is that a tree of approximate two-year quality is produced within the normal time span needed for producing a maiden tree. Knip boem trees are generally less expensive than two-year old trees produced from maidens grown on for a second nursery season. The methods are as follows:
- Rootstocks are either grafted or budded and grown in the nursery at normal spacings.
- At the end of the first season of growth, a single-stemmed ‘whip’ is produced. This is headed back to 70 cm height by removing all but the top bud, strong growth is encouraged, with abundant feathers between 70 and 100cm from the ground.
Producing trees using interstems
Trees raised using other scion varieties or rootstocks as interstocks or interstems are generally produced using either the knip boem technique (see above) or as two-year-old trees.
- For the reasons outlined above, such trees are usually better in terms of their feather number and position than maiden trees.
- Often trees purchased from a continental nursery will have an interstem of another scion variety.
- Golden Delicious is a commonly used interstem.
- Such trees should give no problems and will usually come into cropping earlier than similar maiden trees without interstems.
Sequential removal of the lamina of the leaves at the tip of the scion central leader
Research has shown that the removal of the top half of the leaf blades of the unfolding leaves at the tip of the scion central leader can help the induction of feathering.
- These leaves are thought to produce plant hormones, such as auxins, which suppress the growth and development of the lateral buds into feathers.
- By removing them, the buds immediately below the tip are freed from this inhibition and can develop into feathers.
- The process may be carried out three to five times during the growing season, when leader growth is stong.
- It is vital that the apical meristem of the central leader is not removed during this operation.
- The objective is not to pinch the leader, only to remove the unfolding leaf blades immediately below it.
Spraying with chemical branching agents
Research trials in many parts of the world have shown that feathering of apple trees may be improved using sprays of various plant growth regulating chemicals. Three types of chemical branching agents have been shown to have beneficial effects on young apple trees.
- Auxin transport inhibitors: Chemicals known as auxin transport inhibitors slow down the movement of auxins from the shoot tip. These auxins inhibit the growth of feather-forming lateral buds on the main stem. Reduction in the auxin flow allows these buds to break and grow into feathers.
- Gibberellins and cytokinins: Research conducted principally in the USA has shown that sprays of Promalin can greatly improve feather production on apple trees in their maiden year. Promalin is a mixture of the gibberellins A4 and A7(GA4+7) and the cytokinin, benzyl adenine (BA). Sprays at high volume (at the USA label recommended concentrations) are applied when the scion central leader is growing actively. Unfortunately, Promalin is not approved for use in the UK.
- Surfactants: Research conducted in Canada and the USA has shown that, occasionally, certain types of surfactant can, when sprayed onto actively growing apple trees in the nursery, induce feathering. None of the surfactants shown to have this beneficial activity are available for use in the UK.