Four-year-old Packham trees on Winter Nelis rootstock planted 22.5 cm (9 inches) apart on Open Tatura (9,876 trees per hectare, or 4,000 trees per acre). Although tree density is excessively high, this simple experiment clearly shows that tree vigor can be controlled, because growth of roots has been restricted. Much of the energy of the tree goes into producing fruit, instead of wood. You do not need many pears per tree to get a high production per hectare—40 pears per tree equates to 80 tonnes per hectare or 32 tons per acre.
Many orchardists still believe that fruit trees need room to grow, and that planting trees closely will shorten their productive lives.
Giving trees more room to grow than is necessary will ultimately mean that your trees are going to produce more wood than fruit.
Size-controlling or dwarfing rootstocks for apples have been bred to restrict the size of the roots, and by doing so restrict the size of the tree and make the tree more fruitful than are seedling rootstocks. Apple trees on size-controlling rootstocks, such as Ottawa 3, Malling 9, M.26, M.7, and MM.106, allow orchardists to plant trees densely and get early and high yields per acre. Unfortunately, universal size-controlling rootstocks for most stone fruit species are not yet available, or have not yet proved to be of commercial value. Yet, orchardists must move towards dense orchards of small trees for all fruit crops, in order to improve orchard efficiency and to survive.
I can report that some orchardists in Australia are able to control vigor successfully, improve fruitfulness, and increase yields per acre of pear and stone fruit on vigorous rootstocks. These orchardists planted trees closely, trained them on a trellis, pruned more in summer and less in winter, and cropped their trees at an early age.
The part of the tree that is mostly affected by these methods is the roots. Since the roots control the tops of trees, these methods are actually used to control root growth in order to control tree vigor and increase fruitfulness.
But what about the importance of roots as a reservoir for stored carbohydrates and nutrients? Should trees not have large root systems?
In an experiment with peach trees at the Tatura Research Institute, Australia, we found that trees, which were conventionally planted 5.5 by 5.5 meters apart with 330 trees per hectare (equivalent to 18 by 18 feet apart with 134 trees per acre), had tops four times larger (heavier) than their roots. Tatura Trellis trees at 1,666 trees per hectare (675 trees per acre) had tops that were only twice as heavy as their roots. The difference in the top-to-root ratios meant that there were more stored carbohydrates and nutrients available to the Tatura Trellis trees than to the trees that were conventionally planted. Conventionally planted trees were freestanding, and had six to eight leaders shaped like an open vase.
In the absence of size-controlling rootstocks, it is important to have management systems in place that take advantage of the benefits of controlled root growth. One such management system is to plant trees closely in the row, so that roots have to compete for space.
If you give your trees plenty of space, or remove some trees to give others more space, you will end up with more vigor, more wood, more shading, and less fruit. This was demonstrated in another experiment that we did with peach trees on Tatura Trellis. We measured growth of trees during the first eight years of the planting, and compared this with trees that were planted conventionally. The Tatura Trellis trees were planted 1, 0.75, or 0.5 meters (3, 2.5, or 1.5 feet) apart.
Once a year, we measured the circumferences of 25 trunks at marked spots. By calculating the annual increments over eight years, we could follow the growth of the root systems, because research has shown that the size of the trunk (known as the butt) is statistically correlated with the size of the root system. We also changed the distance between trees in the narrow spacing from 0.5 meters (1.5 feet) to 1.5 meters (5 feet) when trees were four years old, by cutting two trees out at ground level between the first and fourth tree.
Results in Figure 1 show that it took the conventionally planted trees five years before vigor began to decrease. With Tatura Trellis trees, vigor decreased after the first year. Conventionally planted and Tatura Trellis trees were cropped in the fourth and second year respectively. We could not separate what effect cropping and root competition had on reducing vigor. However, when we cut the trees off at ground level to increase the tree spacing from 0.5 to 1.5 meters, we could measure an immediate growth response (see Figure 1). The remaining trees became more vigorous and the roots invaded the vacant areas. An examination of some root systems revealed that “cannibalism” took place, where the live roots of the remaining trees fed on the dead roots left after the tops had been cut off at ground level.
Planting trees closely does not affect their productive lives, provided the trees are managed well. Seventeen-year-old Golden Queen peach trees, a clingstone canning variety, planted with 2,222 trees per hectare (890 trees per acre) produced a total canning-grade yield of 747 tonnes per hectare (302 tons per acre) from 16 harvests. In comparison, when trees were planted at 1,668 trees per hectare (675 trees per acre) they produced a total canning-grade yield of 774 tonnes per hectare (313 tons per acre).
Where tree density was reduced from 3,334 to 1,112 trees per hectare (1,350 to 450 trees per acre) the yields totalled only 576 tonnes per hectare (233 tons per acre) and yields never caught up with the other densities. The conventionally planted trees produced 361 tonnes per hectare (146 tons per acre) from 12 harvests.
Similar results of vigor control were obtained with pear trees on seedling rootstock. Packham trees on Winter Nelis rootstock planted on Open Tatura at 9,876, 4,444, and 2,222 trees per hectare (4,000, 1,800, and 900 trees per acre) had average butt circumferences of 126, 153, and 182 millimeters (5, 6.125, and 7.25 inches) respectively when five years old (Figure 2, photo at top of page). Trees were irrigated with double drip line.
The duplex soils of the Goulburn Valley, Australia, where these experiments were conducted, might have helped to achieve root competition. However, the low-flow irrigation systems and soil-monitoring equipment currently available should enable orchardists anywhere to control growth of roots. Soil modification, such as deep ripping, should be questioned in relation to creating a soil environment that encourages roots to explore larger areas than are necessary. Instead, laser levelling the orchard and ridging the rows to ensure good drainage, no tillage, and improving structure of the topsoil with organic matter, better fit the philosophy of controlling tree vigor and increasing fruitfulness by containing growth of roots.Van den Ende is a tree fruit consultant in Australia’s Goulburn Valley.