Planting density effects on growth, yield and quality

This article originally appeared in the Spring 2021 edition of AFG, available online here.

Dr Gordon Brown reviews research on the impact of planting density on yield and fruit quality.

Take home messages

• The yield advantage of higher density plantings observed in young orchards continues into orchard maturity.
• The increased yield leads to smaller fruit in the lower portion of the canopy only which carried 50 per cent of the fruit.
• The smaller fruit in the lower half of the canopy indicates that treatments to improve fruit size in the lower canopy will be needed in mature high density orchards in order to produce a crop of consistently large fruit.
• The reduction of fruit colour with higher densities indicates that it will be necessary to perform fruit colouration treatments to the lower canopy for low colour apples such as the original Cripps Pink and Royal Gala.

The Future Orchards project aims at establishing high density orchards as the norm when planting new orchards.

High density orchards have many advantages including high yields early in the crop life, vigour control by crop competition and labour efficiency.

There have been many research studies to develop and support these claims and the results from these research studies have been promoted to industry through a variety of means.

This has led to widespread adoption of high density orchards in the Australian apple industry in recent years. Only a handful of growers have been planting at high densities for the past 20 years so
experience with impacts of high-density systems in mature orchards is limited.

Research results for planting and training systems in apple orchards rarely study orchards well into their maturity so the impact of high-density planting on long term orchard productivity are
poorly documented. In this edition of AFG we look at Lithuanian research on the impact of apple tree density on yield and fruit quality over three years in an 18-year-old apple orchard.

Lithuania has an extremely cold winter and as a result this trial was planted on P60 rootstocks. The main difference between P60 and M9 rootstocks is their cold hardiness. In terms of tree vigour and fruit productivity they are very similar. In addition this trial used the cold tolerant cultivar Auksis. In this trial trees were planted in 3m wide rows at spacings of 1.5m, 1.25m, 1.0m or 0.75m (2222, 2667, 3333 or 4444 trees/ha respectively). The trees were trained as slender spindles. There were five trees per plant spacing and this was repeated three times. Over the three years of the study, tree growth, fruit productivity and fruit quality were studied. Fruit characteristics were measured in the lower (below 1.5m) and the higher portions of the canopy.

While many growth characteristics were measured, the easiest way of visualising the impact of planting density on tree vigour is the annual weight of prunings. Here it was found that the weight of
prunings per tree decreased with increasing tree density indicating increased tree to tree competition was still restricting growth in the denser plantings.

On a per hectare basis, however, the weight of prunings increased with increasing tree density (figure 1) indicating better utilisation of available light.

Figure 1. Yield of fruit (green) and weight of prunings (orange) from mature apple
trees with planting density averaged over three seasons.

It was also found that, as for newly established orchards, the yield of fruit increased with tree density such that the yield at 4444 trees/ha was 15 tonnes/ha greater than the yield at 2222t/ha (figure 1). Hence the benefits of increased tree density on yield early in the crop life persist into the mature orchard with a major impact on orchard accumulated income.

Of interest is that the vigour of trees at 1m spacing was greater than expected while the yield was lower suggesting that this treatment could carry a greater fruit load. Converting the yield per tree to yield per trunk cross sectional area this 1m treatment had a low crop load compared to all the other treatments supporting this observation on this treatment. An explanation of method of crop load control is not provided in this publication.

While yield is important, premium prices are paid by consumers for larger, well coloured fruit. High yields are normally associated with smaller fruit which have to be sold at a discounted price. The impact of tree density on fruit size was found to be most pronounced in the lower half of the canopy which carried about 50 per cent of the fruit across all tree densities (figure 2). While there was little impact of density on fruit size in the upper canopy in the lower canopy doubling the tree density to 4444 trees/ha reduced the yield of large fruit from 33 per cent to only 19 per cent of the harvested fruit. This has the impact of reducing the commercial yield of large fruit at the higher densities.

Figure 2. The impact of planting density and position in the canopy on percentage of large fruit.

Adjusting the yields for fruit size identified that all densities yielded about 40 tonnes/ha of large fruit. This response will be cultivar specific with the impact being less severe on large fruited
cultivars and more severe on cultivars that struggle to produce large fruit. This highlights that a market for small fruit will be needed for mature extremely high density orchards. Alternatively additional fruit thinning should be considered in the lower half of the canopy in these orchards to encourage the development of larger fruit in this zone.

In addition to fruit size fruit colour is also commercially important. Fruit in the lower half of the canopy had approximately 15 per cent less colour than those in the upper canopy across all
planting densities (figure 3).

Figure 3. The impact of planting density and position in the canopy on percentage of fruit colouration.

Increasing the planting density also caused a reduction in fruit colour with a 10 per cent reduction in colour in the entire canopy when moving from 2222 trees/ha to
4444/ha. These results emphasise that at high densities it will be necessary to perform fruit colouration treatments to the lower canopy for low colour bicolour apples such as the original Cripps Pink and Royal Gala varieties.

Dr Gordon Brown, Scientific Horticulture
E: [email protected]
T: 03 6239 6411

Further reading:

Planting distance affects apple tree growth, fruit yield and quality.

Research updates are commissioned by APAL as part of the delivery of the Australian Apple and Pear Industry Communications Program funded by Hort Innovation using the apple and pear levy.