Pruning juvenile seedlings

New April 2018

Pruning affects the growth, shape and productivity of apple trees, whether young or old, grafted or grown from seed. Whilst many proven pruning systems have been developed, it remains the greatest single challenge to commercial growers aiming to maintain regular high yields of optimal quality fruit (Worraker and Withnail, 2013).

 

Seedling apple trees should not require pruning during the juvenile phase of growth. This is because seedling vigour and the duration of the juvenile period are inversely related (Visser, 1964). Consequently, any action restricting shoot growth lengthens rather than shortens the duration of the juvenile phase, and delays the onset of flowering (Janick et al. 1996). Most seedlings begin life with a single leading shoot. To encourage rapid and continuous extension growth it is advisable not to cut this shoot back until flowering and fruiting is achieved (Zimmerman, 1971). This advice makes sense given that the apical (youngest) shoot growth is the first to emerge from the juvenile phase (van Nocker and Gardiner, 2014). However, after the transition to the ‘adult vegetative phase’ (77 nodes), factors that reduce apical dominance of the leading shoot can promote lateral flower-bud formation (Hanke et al. 2007).

Pruning at Ystwyth Valley Apple Breeders

Fig. 1 Closely spaced seedling trees require pruning of lateral shoots
Fig. 1 Closely spaced seedling trees require pruning of lateral shoots

We grow our seedlings with a single leading shoot, and don’t prune this until the seedling flowers. The only exception to this rule is when the shoot is damaged, or stunted, by disease (e.g. canker) or pests (e.g. aphids and thrips). If this happens, we cut back the shoot to the youngest healthy leaf bud, whatever the time of year.

 

In contrast, we prune many of the lateral shoots arising from the single leader. This is necessary because our seedlings are grown closely spaced (23 cm apart) in pots (Fig. 1). Lateral shoots are usually produced during the first summer of growth, or early in the second year. Unchecked, they can soon lead to overcrowding and shading between adjacent seedlings. To prevent this, we prune laterals in late summer and again in winter. Viewing a row of seedlings running left to right (see Fig. 1), we cut off entirely most (75%) of the lateral shoots growing out forwards or backwards from the leading shoot. Lateral shoots growing out either side of the leader are pruned back to 3-5 leaves (summer) or 3-5 leaf buds (winter), if they are likely to interfere with adjacent seedlings. However, we leave the remainder unpruned, in case the seedling turns out to bear flowers on the tips of new shoots.

 

In terms of tree structure, our ultimate aim is to achieve a columnar growth form, with shortened laterals developing into fruiting spurs and dards (short laterals <7-8 cm, ending in a fruiting bud), once the juvenile phase is over. In this respect the pruning regime is similar to that recommended for minarette trees. Some of the unpruned laterals should develop into ‘brindles’. These are thin lateral shoots 8-30 cm in length which usually end in a fruit bud (Worraker and Withnail, 2013).

Pruning trial: effects of pruning lateral shoots

Fig. 2 Pruning trial: does pruning back lateral shoots shorten the juvenile phase?
Fig. 2 Pruning trial: does pruning back lateral shoots shorten the juvenile phase?

We began a small trial in 2016 to measure how pruning lateral shoots affects (a) the production of leaf nodes along the single leading shoot, and (b) seedling height. We expected that cutting back lateral shoots might promote extension growth of the leading shoot, due to the removal of competing sinks for water and nutrients. If so, this might increase the annual production of leaf nodes along the leader and shorten the juvenile phase.

Methods

The seedlings for the trial were grown from pips planted in January 2016, produced from crosses made in 2015. Following our usual procedure, they were transplanted into individual 5 litre pots during June 2016. The pots were partially sunk into the soil and arranged in closely spaced rows in a nursery bed outdoors (Fig.1). Seedlings from the same cross were adjacent to each other. The trial began on 5th August 2016, during the first year of seedling growth. Twenty-four matched pairs of seedlings were identified in a single nursery bed made up of four parallel rows of seedlings. To make a pair the two seedlings had to be (a) growing immediately adjacent to each other, (b) progeny from the same cross, and (c) of similar height and lateral shoot development.

Fig.3 A wide range of starting heights in the twenty four pairs of matched seedlings
Fig.3 A wide range of starting heights in the twenty four pairs of matched seedlings

The 24 pairs covered a wide range of starting heights and parental varieties. (Fig.3).

 

One seedling in each pair was designated as ‘unpruned’, and the other as ‘pruned’ (Fig. 2). The ‘unpruned’ treatment consisted of leaving the leading shoot and all lateral shoots unpruned for the duration of the trial. The ‘pruned’ treatment comprised no pruning of the leading shoot and our standard pruning regime for lateral shoots. The majority (75%) of lateral shoots growing out forwards or backwards from the leading shoot, when viewed from the front, were cut back flush with the leader. Half the lateral shoots growing out either side of the leader were pruned back to 3-5 leaves, or leaf buds. The rest were left unpruned. Seedlings were pruned in late summer and again during winter, when they were dormant.

 

Seedling height and the number of leaf nodes on the single leader were measured at the start of the trial and at the end of each successive year of growth. The statistical significance of differences in mean heights and numbers of nodes between pruned and unpruned treatments was assessed using paired t-tests. We plan to continue the trial for five years (to winter 2020/2021). The results will be updated here each year.

Results (Year 2)

Mean seedling heights and numbers of leaf nodes on the single leading shoot of 'unpruned' and 'pruned' seedlings are compared in the bar charts (Figs 4 & 5) and accompanying summary table.

Fig.4 Seedling heights
Fig.4 Seedling heights
Fig. 5 Number of leaf nodes
Fig. 5 Number of leaf nodes

effect of pruning lateral shoots on seedling heights and leaf nodes on main stem

Mean seedling heights showed a small but statistically significant (p=0.002) difference between unpruned (55 cm) and pruned (58 cm) seedlings at the start of the trial. The difference in heights increased during Year 1, and further during Year 2. However, it was statistically insignificant (p = 0.07) in both years. The high standard deviations for the Year 2 seedling heights reflect the wide variation in height between the different pairs of seedlings (Fig.6). Several pairs showed big differences in height between unpruned and pruned seedlings by the end of Year 2, although the differences remained modest for the majority of pairs. 

Fig.6 Variation in seedling heights after 2 years of growth
Fig.6 Variation in seedling heights after 2 years of growth
Fig.7 Variation in numbers of leaf nodes after 2 years of growth
Fig.7 Variation in numbers of leaf nodes after 2 years of growth

Graph showing proportion of seedlings attaining 77 internodes after successive years of growth

The effect of pruning on the mean number of leaf nodes along the leading shoot was statistically insignificant, after Year 1 (p=0.85) and Year 2 (p=0.44) of growth. The results were remarkably similar after two years of growth for unpruned (90 nodes) and pruned (92 nodes) seedlings. Furthermore, with one or two exceptions, the differences in number of leaf nodes within pairs of seedlings remained modest after 2 years of growth (Fig. 7). The photograph opposite shows the differences in lateral shoots between pruned (LHS) and unpruned (RHS) members of a seedling pair typical after Year 2.

 

Conclusions (Year 2)

  • Seedling height is not significantly influenced by pruning lateral shoots.
  • The number of leaf nodes along the leading stem is not significantly affected by pruning lateral shoots.
  • It is unlikely that the juvenile phase is significantly shortened by summer and winter pruning of lateral shoots.

References

Hanke M-V, Flachowsky H, Peil A, Hättasch C. 2007. No Flower no Fruit – Genetic Potentials to Trigger Flowering in Fruit Trees. Genes, Genomes and Genomics 1(1), 1-20.

Janick J, Cummins JN, Brown SK and Hemmat M. 1996. Apples. In: Janick J, Moore JN, eds, Fruit Breeding, Volume I: Tree and Tropical Fruits. John Wiley & Sons, Inc. 1-77.

Van Nocker S and Gardiner SE. 2014. Breeding better cultivars, faster: applications of new technologies for the rapid deployment of superior horticultural tree crops/ Horticultural Research 1, 14022; doi:10.1038/hortres.2014.22.

Visser T. 1964 Juvenile phase and growth of apple and pear seedlings. Euphytica 13, 119-129.

Worraker R and Withnail M . 2013. The Apple Pruning Manual. Esquire Print, Nettlestead, Kent, UK. 117p. ISBN No. 978-0-9531850-1-6.

Zimmerman RH. 1971. Flowering in crab apple seedlings: methods of shortening the juvenile phase. Journal of American Society of Horticultural Science 96, 404-411.