by Bas van den Ende and Judy Tisdall.
In a fruit tree, the yield of fruit results from a balance between vegetative growth and associated fruiting. The production of fruit is tied to management which affects how the tree’s resources (e.g. photosynthates, water, nutrients and hormones) are distributed throughout the tree. Economics force us to consider how and to what degree those resources need to be distributed toward vegetative or reproductive growth, both in the current season and in the long term.
In this article we report how root growth and fruit production can be controlled through managing the soil, the tree and through managing the supply of irrigation water in orchards, using the north-central region of Victoria, Australia as an example.
The general objective of orchard management is to maximise fruit production while minimising growth of unproductive wood (branches that do not produce fruit). With any combination of scion and rootstock, vegetative and reproductive growth are genetically balanced.
It is well known that the tree maintains a constant ratio between roots and shoots. That is, the more vigorous the roots, the more vigorous the shoots are. So you need to control the roots in order to control the shoots. This can be done by a) natural and artificial barriers in the soil, b) competition with roots from neighbouring fruit trees, or c) controlled spread and penetration of irrigation water. These effects can be manipulated to greatly enhance yield and manageability of high-density plantings.
You can use several methods to control root activity in spring, thereby decreasing vegetative vigour and increasing fruit growth.
It therefore follows that if you can control root growth in spring you can decrease the vegetative vigour of shoots, and hence increase fruit growth, especially on young fruit trees. We realise the importance of putting this in practice, since the climate and growth-controlling soils of the Goulburn Valley in Australia, enable orchardists to grow a wide range of temperate fruit crops during a season lasting 8 months. To manage root growth of fruit trees, a good knowledge of roots and root growth is essential.
Facts about roots and root growth
• There is a direct relationship between the size of the root system and the size of the tree above-ground. That is, roots regulate the growth of shoots.
• Roots also absorb and supply water and nutrients to the tree and anchor the tree.
• Before leaf fall, substantial concentrations of carbohydrates and nutrients are moved and stored in the trunk and roots to be used during the next growing season. Roots also produce growth substances such as cytokinins and gibberellins that are transported to the shoots to control shoot growth.
• In a growth cycle, root cytokinin stimulates terminal buds and use of stored carbohydrates and nutrients. At first, there is vigorous vegetative growth with consequent production of auxin and gibberellin in the shoot tips. Photosynthates produced by the fully mature leaves then move through phloem to the shoots, roots and fruit.
• A lack of hormones produced in the roots decreases shoot growth. Thus, roots play a major role in the maintenance and balance of the whole tree.
• Fruit trees speed up or slow down their growth in response to signals about the soil that are sent by the roots to the shoots. Root tips launch these signals as hormones, to inform shoots and leaves about the soil’s water and nutrients, hardness, water availability and temperature. All of the root’s primary growth (cell division) is concentrated at about 5 mm from the root tip. These effects may increase or decrease the volume of soil accessible to the roots for taking up water and nutrients.
• Management of the soil definitely effects root development. It is well known that roots of mature deciduous fruit trees grow in flushes. The end of the initial peak in root growth usually corresponds with the beginning of active shoot growth. The root system is customarily inactive during summer. After harvest in autumn or cessation of shoot growth, root growth starts again, but is not as strong as in early spring. The seasonal periodicity in the production of new (white, unsuberised) roots has not only been attributed principally to fluctuations in temperature and water content in the soil when fruit trees grow under natural rainfall, the periodicity of new root growth also occur even under frequent irrigation and apparently favourable soil temperatures.
• The flush of white unsuberised roots in the autumn depends on the presence of leaves, because the new roots need photosynthates for energy and to produce new cells. Hence, defoliation, even 4-6 weeks before natural leaf-fall, can greatly decrease the concentration of white roots.
• Fruit trees generally develop fairly shallow root systems with many horizontal roots and a few vertical roots.
• Rooting depth is very variable and affected primarily by soil conditions. Soil conditions also have a big effect on root growth and root distribution within the soil. The parts of the soil profile that are rich in water, oxygen and nutrients inevitably encourage more active root growth than the less fertile, less oxygen areas.
• High-density plantings result in increased root densities, but less root weight, length and volume per tree.
• Dry soils slow down root growth. Irrigation increases root density, especially in the surface layers of the soil beneath the fruit trees. Targeted irrigation can affect root growth and distribution of roots. Drip irrigation has given variable effects on root growth possibly due to differences in soil type. Soils differ significantly in their hydraulic conductivity and, therefore, in the area of soil wetted beneath the irrigation dripper. Often the roots of fruit trees are concentrated within 600 mm of the drippers.
• Soil compaction which decreases aeration when soil is wet and physically impedes root penetration, limits root growth.
• Roots are sensitive to high soil temperatures, especially feeding roots. Roots grow best in the range of 20°C to 25°C. Above 25˚C root growth slows down. Tree roots do not grow when temperatures rise above the lethal 35˚C in the top 70 mm of soil wherever the surface receives direct radiation during summer. This means that with long hot summers, root growth of fruit trees is decreased, especially in the upper depths of soil where most of the feeder roots grow.
• Mulching increases root growth in the soil surface, because soil water content and soil temperature are kept ideal for root growth. Together with the shade that the canopies cast upon the soil surface, the soil temperature under the mulch can be kept between 22˚C and 25˚C.
• Roots grow mostly during the night.
• The root system cannot spread beyond the available soil volume when the roots are restricted. Root restriction can result when fruit trees are planted in ridges over heavy clay, which restricts downward penetration, or planted closely in a high-density planting. In high-density plantings the weight, length, volume and surface area of roots of individual fruit trees decreases in comparison to similar fruit trees planted at wider within-row distances.
• Root restriction is a successful method to manipulate root growth, because root restriction can dramatically increase precocity and yield efficiency of fruit trees. Root restriction can also decrease canopy volume and growth control can increase linearly with decreasing soil volume.
• Root growth is greatly affected by the general nutritional status of the tree. The effect is most likely indirect and acts through influencing the above-ground part of the tree first.
• Interference with the proper functioning of leavers, or with the transport of materials from the leaves, decreases root development of fruit crops.
• When root growth slows down as fruit trees age, there will be less need to control tree vigour. The unused portion of photosynthates is then channelled into fruit growth. This phenomenon causes higher yields obtained from mature fruit trees and also probably accounts for the similarly high yields obtained from young fruit trees when grown on size-controlling rootstocks.
Climate and soils of the Goulburn Valley
The fruit-growing area in the Goulburn Valley (GV) consists of some 12,000 ha of irrigated orchards of stone and pome fruits. The GV is also the centre of pear production in Australia. The climate is semi-arid (Mediterranean type) characterised by cool wet winters and hot dry summers. Rainfall is sparse (about 440 mm annually). Most rainfall is in winter and spring, with November being the wettest (average about 50 mm). The irrigation season usually extends to 7 months (mid-October to Mid-May), with an average of 280 mm of rain.
Many orchard soils are fragile and need careful management to sustain the production of fruit for a long time.
The red duplex soils which are commercially used for irrigated fruit trees in the GV are mainly shallow loams (0-150mm) overlying heavy red-brown clays. The surface soils (A horizons) have high silt contents and contain up to 80% fine sand plus clay, and less than 2% organic carbon, making them prone to ‘slaking’. Slaking is the breakdown in water of soil aggregates (crumbs) to smaller aggregates. It is a quick process which occurs mainly within the first few minutes of wetting. The other is ‘dispersion’ which is the breakdown in water of aggregates to individual sand, silt and clay particles. This is a slower process than slaking, often taking hours to complete. Clay dispersion is particularly undesirable for fruit trees. It is the main cause of several problems in clay loam and clay soils, including low water intake, poor drainage, low aeration, surface crusting and cloddiness. Clay dispersion at the surface typically results in hardsetting or crusting surface soil.
The subsoils have low saturated hydraulic conductivities, low air-filled porosities when the soil is wet. In a clay subsoil swelling and clay dispersion can result in low permeability, poor aeration and when the soil is wet high soil strength, leading to restricted root development.
Tree roots grow mainly in the surface soil, but even there they are usually sparse. Because these soil properties are so often adverse and difficult to control, soil management can affect the performance of the fruit trees.
Experience in the GV has shown that depth of penetration of irrigation water, volume of available water, drainage, impedance and hard soil that restrict roots are the most important soil physical factors that affect tree growth.
In these shallow soils the striking feature is that almost all the roots of the highest concentration of densely-planted fruit trees is found in the surface soil. This high concentration of fine roots near the surface is the most important feature of the root distribution of fruit trees in GV orchards. This is in marked contrast to the root distribution found in deep soils, where the highest concentrations of roots are not necessarily at the surface.
Shallow rooting of fruit trees on GV soils is caused by a natural barrier. There is little doubt, that the hard, heavy subsoil is responsible for the shallow rooting.
Some 40 years ago, the Tatura System of Soil Management was developed at the Tatura Research Institute. The permeability of the hard subsoil and the depth and structure of the surface soil were improved before fruit trees were planted.
The aim of modifying the previously hard impermeable subsoil was to make it more attractive to roots because drainage and aeration were improved. It has been shown that the greater the friability and the more strongly developed the structure of the subsoil, the bigger the trees grew, as measured by trunk size (circumference) and tree height. Furthermore, it was found that the greater the trunk size, the deeper is the penetration of roots into the subsoil depths, and the higher is the root concentration in the subsoil. Subsequent experience has shown that most shallow fragile soils in the GV are capable of successfully growing a wide range of fruit crops.
However, it was not realised at the time that an increased root growth led to a substantial increase in tree vigour and did not allow roots to be controlled as needed in high-density plantings.
We now propose changing parts of the Tatura System of Soil Management, which would use a sufficiently vigorous rootstock, or sufficiently high tree density, to fill the allotted tree spaces during the mandatory phase of vegetative growth as quickly as possible, and then slow down vegetative growth and initiate full cropping.
Ideally, full cropping fruit trees should be managed so that only minimum concentrations of photosynthates are used for growth and maintenance of shoots and roots, with most directed towards fruit production. As we gain a better understanding of root physiology in the field, it should be feasible to manage fruit tree roots more directly as we routinely manage tree canopies.
Our proposal for a permanent soil management system is based on firstly allowing close-planted fruit trees, with or without a size-controlling rootstock, to fill their allotted spaces both in the soil and above-ground, within less than 3 years. The strategy to achieve this starts with the preparation of the soil before planting the fruit trees.
Preparing the soil before planting
Establishing a high-density orchard is costly. It is important to do it the right way, because you get only one chance. Once the orchard is established, it is difficult and costly to correct soil problems in later years.
To produce early and high yields of good quality fruit, fruit trees need lots of feeder roots in the surface soil so they can take up plenty of water and nutrients. To enable this, the surface soil should be deep, soft, stable, well-structured, well-drained, fertile, and cool in summer. Here are steps to achieve that.
The following steps will help you to plan any new planting of fruit trees.
1. Have your soils tested.
Whether it is new land or old orchard land, have the surface soil tested to see if you need to add lime, gypsum, and/or phosphorus, and to what depths. Methods for collecting, preparing and submitting soil samples vary in different regions, states and countries. These methods are described on various websites, so follow the methods appropriate for you.
As you sample the soil, you will also see how deep the surface soil is, and whether there are any hard layers that restrict water, air and roots from deeper layers.
Lime will be needed if the soil pH is acidic (5.7 or less). Gypsum will be needed if the soil is hard due to dispersion. Phosphorus will be needed unless superphosphate has previously been applied each year to the soil and a soil test shows that there is an adequate amount of soluble phosphorus available to the young fruit trees.
2. Rip the surface soil lightly to break up any hard parts and grade your block (if necessary).
Before applying lime, gypsum and/or phosphorus (if soil test results indicate these are needed), laser-grade the block to make sure that it has a slope of at least 1: 80 along the traffic lanes so that excess water will drain from the surface of the soil. This will help avoid waterlogging of the surface soil.
The aim is keep the subsoil not only drainable but also impermeable to discourage roots growing into this layer.
3. Apply lime, gypsum and phosphorus (if necessary), and rip and lightly till the soil
Some surface soils are naturally hard and dense, while others have a plow sole or shallow hard pan due to excessive tillage and traffic. All these hard layers need to be broken up. After spreading lime, gypsum and/or phosphorus (if soil test indicated these are needed), rip the soil both ways to a depth of about 300 to 400 mm. Be careful not to mix heavy subsoil with the surface soil. After ripping the soil, slightly till the moist, but well-drained, surface soil to form small clods. Do not pulverise the soil, which would happen if the soil was tilled when too dry. This is also the time to put in the mains and sub-mains of a new irrigation system.
Lime and phosphorus are not very soluble and move very slowly in soil, so they need to be tilled into the surface soil. Apply agricultural limestone (calcium carbonate) over the whole block, but apply superphosphate along the future planting lines about 1 to 2 m wide and rototill it in. Phosphorus is important for root growth, and young fruit trees will benefit from phosphorus if it is nearby, i.e. if mixed into the surface oil.
Gypsum is moderately soluble, so, if applied to the soil surface, it might eventually be washed down the profile to the subsoil.
Why do some soils need lime?
The feeder roots in the surface soil need soft, stable, well-drained soil, with a pH of between 5.8 and 6.5. In acidic soils (with a pH below 5.8) excess aluminium and possibly manganese become available and are directly toxic to roots. The roots become stunted and unable to take up sufficient water and nutrients. Other nutrients such as calcium and magnesium may be present in acidic soil but become unavailable to roots. Also phosphorus and sulphur may be present in an acidic soil, but combine with aluminium to form aluminium phosphate and aluminium sulphate compounds, which cannot be taken up by roots.
Why do some soils need gypsum?
Gypsum (calcium sulphate) is sometimes needed to soften surface soils and to improve their structure. Soil stability is improved and pores can be created chemically by sticking together (flocculation) of clay particles by the addition of gypsum.
With gypsum, the soluble calcium swaps with some of the exchangeable cations, such as sodium and magnesium. Gypsum does this better than lime does, because gypsum is more soluble than lime. Gypsum will not neutralize acidic soils or effectively raise pH does, but lime does.
Cations (positive ions) such as sodium and calcium, exist in soil as either exchangeable cations (loosely bound to clay particles), or soluble cations (dissolved in soil water). The soluble cations often swap with exchangeable cations in soil. When exchangeable sodium makes up more than about 5 per cent of the total exchangeable cations, and there are low concentrations of soluble cations, the soil is sodic and unstable. Sodic soils are very dense and hard, so it is very difficult for feeder roots to grow through them.
When sodic soils are wetted, the clay particles push each other apart. First the aggregates swell and decrease the size of the pores. On further swelling, small groups of clay particles separate from the larger aggregates and become suspended in the water until the clay particles block the small pores. This is called soil dispersion. Adding calcium in the form of gypsum causes flocculation (sticking together into clumps) to form a building block for improved soil structure.
4. Hill-up the surface soil
Make good use of the shallow surface soil by hilling (ridging) it up along the rows so that the depth of good quality free-draining soil available to the tree roots is increased. Hilling-up can increase up to 4 times the volume of surface soil in the row as compared with no hilling-up.
Most feeder roots grow in the surface soil, so when the surface soil is shallow, these roots are severely restricted. Few roots grow in the compacted surface soil in the traffic lanes between rows of fruit trees. If the land is also flat, the soil can easily be waterlogged in wet conditions.
To solve these problems, use a road grader to take the wasted surface soil from the traffic lanes, and hill-up the surface soil before you plant the fruit trees. This increases the volume of soil for the feeder roots to explore, and the sloping hills also allow excess rain water to run off. Surface drainage is as important as irrigation.
5. Sow ryegrass onto the beds or let voluntary weeds develop to decrease slumping and to create a good environment within this area of topsoil, because tree roots are confined to a relatively small area of soil.
This step must be carried out in early autumn to ensure that the ryegrass or weeds become established before the winter sets in. Use irrigation water to germinate and establish ryegrass or weeds.
6. Spray out ryegrass or weeds before you plant fruit trees
Ryegrass or weeds are needed to keep the hilled-up surface soil covered to avoid impact from heavy rain, avoid impermeable crusts from forming and to stabilise the soil. Kill the ryegrass or weeds in spring, because they compete with fruit trees for water and nutrients during the growing season. The dead roots of ryegrass or weeds and associated fungal hyphae create pores and stabilise the surface soil.
Do not walk on the hills.
If possible, 1 to 5 are best done a year before planting.
This is then followed by restricting root growth to control tree vigour and impose cropping. Root restriction provide orchardists with a useful tool for tree growth manipulation
Strategies to achieve root restriction include close planting of fruit trees, regulating the supply of irrigation water, applying growth regulators, using rootstocks which induce precocity (if available), pruning, and keeping the soil in optimal conditions to implement the long-term effects of root restriction on controlling root growth, tree vigour and fruit production.
Close planting restricts roots
Fruit trees with confined root systems do not grow much and if properly cared for in regard to water and nutrients, can remain healthy for a very long time.
Fruit trees planted closely together experience root competition. Root distribution, as well as abundance of roots is influenced by the root growth of other trees planted close by. Roots will in preference always grow into areas of soil free of competition from other fruit trees. It must be assumed that the roots exude or produce substances that are inhibitory to root growth of fruit trees of the same species or variety.
High-density planting suppresses tree growth, and while competition for sunlight may cause shoots to extend and result in taller fruit trees, girth of the trunk, expressed as trunk cross-sectional area (TCA), is decreased. The weight of above ground parts of a tree, excluding the fruit, is proportional to TCA, and thus, root competition decreases vegetative growth above the ground.
Early bearing, high yields and high pack-outs per hectare, generated by high tree density, have the most effect on profitability of orchards and, consequently, have been the aim of much tree management research.
In orchards, yield is the product of the number of fruit trees per hectare, the number of flowers on each tree, the number of flowers which set fruit, and the ultimate size and quality of the fruit. Maximising orchard performance, then, means maximising each of these factors early in the life of the orchard, and each year.
Management becomes a challenge when closely-planted fruit trees have filled their allotted space, because most fruit trees are planted at higher densities or allocated less area than the fruit trees normally occupies when fully grown.
In young orchards there is nothing wrong with strong growth, so long as it is properly directed towards building a full productive canopy. It is once you have filled the allotted space that excess vigour becomes a wasteful problem.
Young orchard performance depends on how fast the new fruit trees develop their canopy. Limiting factors to tree growth are: water stress, weed competition, inadequate control of pests & diseases. Young fruit trees have small root systems, so cannot fully use water reserves during dry periods.
Fruit trees have the capacity to crop when they are young and small and management practices are developed to take advantage of that fact.
High-density plantings frequently become uneconomic because tree size, although decreased by root competition, cannot be controlled sufficiently to prevent competition for sunlight, internal shading, and fruit trees become increasingly difficult to manage.
The secret of managing physiologically balanced fruit trees is to switch over from wood production into fruit production and achieve regular yields, high quality of fruit and extend an orchard’s lifespan.
Management becomes a challenge when closely-planted fruit trees have filled their allotted space, because most fruit trees are planted at higher densities or allocated less area than the fruit trees normally occupies when fully grown.
We are no longer in the canopy development phase. Maintaining an efficient productive fruiting canopy. As we are no longer trying to increase canopy volume, but contain it within its allotted space, we need calm fruit trees that divert most of their photosynthates production into fruit, rather than unnecessary shoot growth that needs to be pruned out annually.
Working with generative wood is the main benefit of high-density planting. As we move towards more intensive plantings with simple tree forms, setting the trees up with the right bud numbers and fruit tree form should become much easier.
The fundamental message is that orchard efficiency is determent by establishing a light-efficient canopy as quickly as possible.
As orchard plantings intensify, quality standards refine, and more stringent market requirements must be met, good tree vigour management becomes critical.
Regulating the supply of water restricts roots
In dry climates low-flow irrigation (drip or microjet) can be managed to restrict root volume simply by controlling the amount of the soil which is wet during irrigation. A smaller root volume under drip irrigation can decrease tree size.
Irrigation management is a useful tool that you have at your disposal as a means of:
a. Maximising growth
b. Controlling tree vigour.
c. Cost savings through less pumping.
d. Decreased need for other vigour control options, i.e. cincturing, root pruning, applications of Regalis, or summer pruning.
Drip irrigation potentially provides you with an extraordinary level of control over the precise amount of water delivered to each tree; far greater than for any other system.
Drip irrigation has the ability to precisely deliver the intended dosage of both water and fertiliser and do it in a way conducive to tree root growth. It can be very reliable and cost effective if well designed and managed and brings levels of efficiency simply not possible with any other system.
A well-designed drip system will allow uniform distribution of water, minimise water losses, provide a healthy root system, be reliable and have both relatively low capital and operating costs.
Fruit trees grown under drip irrigation are less vigorous and crop earlier than fruit trees grown under microjet irrigation. This is attributed to a restricted root volume under drip irrigation.
Drip irrigation is an irrigation method with which small amounts of water are applied at frequent intervals directly to a small surface from a single emission point. Microjet irrigation is a small spray used. The difference in root volume between drip and microjet irrigation are often small and the wetting pattern under microjet irrigation will, with time, also restrict root volume. Microjet irrigation, which spreads water over a wider area of soil surface and allows longer time for drainage between irrigation, is often a safer method of irrigation, especially in heavy soils. At high tree densities, however, the spray pattern should be limited to restrict root growth if it is desired to control tree size. Both methods of irrigation can be finely tuned to manipulate early growth and yield.
Early cropping also checks the growth of young fruit trees and when combined with drip irrigation in high-density plantings, enables tree size to be contained more easily.
Both methods of irrigation save considerable water and are highly suitable for fruit growing in areas with limited water.
Management of root growth is an important method to control vegetative growth and production of fruit trees. Regulation of water supply is a convenient way of achieving this management.
The natural barrier in shallow soils, allow regulating the supply of soil water, and becomes a powerful management strategy and convenient means of manipulating tree growth for greater fruitfulness and less vegetative growth, in much the same manner as dwarfing rootstocks.
Irrigation can be developed into a powerful management strategy to manipulate tree growth for greater fruitfulness and less vegetative growth. The growth rate or vigour of the vegetative portion of the aboveground part of the tree is directly correlated with the growth rate of the roots.
On many heavy orchard soils, low-flow irrigation will not decrease tree vigour to an extent that allows adequate penetration of sunlight in high-density plantings. A method has been developed, called regulated deficit irrigation or RDI, for stone fruit and pear trees with medium and long fruit development periods, to reduce tree vigour and overcome the shading problem.
With RDI, the level of irrigation is decreased during the period of fast shoot growth by cutting back on the quantity of water delivered but maintaining irrigation frequency. At the start of fast fruit growth, optimum irrigation levels are resumed. RDI has not only decreased shoot growth and the need for summer pruning, it has also increased fruit size and yield. The phenomenon associated with RDI is compensatory growth. When fruit trees are temporarily deprived of adequate water, fruit growth slows down. On the assumption of ample irrigation, growth accelerates.
RDI is of significant value where :
(i) Water is limiting.
(ii) Excessive vigour is an issue. An early season irrigation deficit can strongly decrease tree vigour and does not affect fruit size at harvest.
The most difficult question to answer is how much of a deficit should be imposed on trees. To get positive benefits the deficit needs to be such that soil wetness is being held close to levels such that all readily available water is depleted. Fruit trees will still be able to extract water from the soil at these levels, but will be under pressure to do so and hence the positive and negative effects will accrue.
Over watering (above field capacity) can lead to leaching of nitrogen and causes anaerobic soil conditions (water logging), reduces the fruit trees’ normal water uptake, limits root activity or even root death.
RDI is to be avoided for young fruit trees until their canopies have been fully developed.
The high root density obtained with close planting and drip irrigation is designed to allow controlled management of root growth. Because the tree roots are confined to a relatively small area of soil, it is important to create a good environment within this restricted area. Such an area often consists of surface soil only.
Growth regulators restrict roots
Chemical growth regulators are being used extensively in controlling excessive shoot growth and enhancing the production of deciduous fruit trees.
There are three growth regulators or growth retardants for vigour control, Regalis®, Paclobutrazol and ethephon. Care needs to be taken in using these growth retardants. Strict adherence to recommendations is required to avoid damage to fruit trees and crop loss.
Growth retardants decrease growth, but have occasional negative side effects on fruit quality, and may perceive environmental or human health risks, and may not be registered for use on fruit trees in some countries.
Size-controlling rootstocks do not restrict roots
Size-controlling rootstocks, if available, do not control vigour. Grafting or budding to a size-controlling rootstock sacrifices the option for rapid vegetative growth.
However, size-controlling apple rootstocks have been used successfully to decrease vigour of the scion and keep apple trees to a manageable size. To a lesser extent size-controlling rootstocks have been developed for other fruit crops. Many countries have or are currently breeding a range of size-controlling rootstocks which are better suited to a particular country’s needs. A few of these rootstocks may prove to be ideal, but many will either fail or adapt to a small niche location. It will take many years to prove rootstock adaptability to multiple conditions.
The roots of size-controlling rootstocks have slow rates of growth that are genetically determined, and, which limit the rate at which the fruit tree can grow aboveground. Even with vigorous roots, the soil limits the growth of the fruit trees aboveground more than air does. Hence, you should manage your soil, not size-controlling rootstocks, to restrict roots.
Size-controlling rootstocks are not the panacea we have been led to believe. Comparative size differences between the currently planted rootstocks can also be affected by soil structure, nutrition and tree management.
Pear and peach trees on seedling rootstock, and even self-rooted fruit trees, have outperformed clonal rootstocks.
Another problem with size-controlling rootstocks is that excessive crop loads can stall the growth of young fruit trees.
Hormones, such as auxin, are normally moved from apical shoots to the root system to encourage root development. Pruning removes these apical shoots, and, as a result, decreases root development. With fewer roots, cytokinin production is decreased. In addition, since total leaf area is decreased by pruning, fewer photosynthates are produced. These photosynthates are moved to storage areas of the tree, such as the root system. With less photosynthates produced, root growth is decreased.
Methods of vigour control that minimise the use of summer pruning, through better growth and fruiting balance, lead to more efficient fruiting canopies and therefore higher orchard performance. Limited summer pruning still has a place for vigour control and particularly for rubbing out of vertical shoots on the upper surface of fruiting units.
Pruning fruit trees in winter invigorates the trees and can lead to excessive vegetative growth and shading which may worsen the quality of fruit and subsequent formation of floral buds. Summer pruning of excess regrowth can help avoid shading, but this is costly.
Unwanted shoots use (and therefore waste) photosynthates that could have been used to grow more and better fruit. Instead, you should rub out (not cut out) watershoots in late spring when they are still soft enough.
If a lot of summer pruning is necessary to keep the fruit trees under control there is something fundamentally wrong with management. The photosynthates being wasted in excessive summer pruning is better directed towards more or better fruit through altering the overall management to avoid this unwanted vegetative growth.
Pruning should be used more to direct growth rather than limit growth.
Generally, managing tree growth has focussed on either genetic means or on cultural practices of the tree (aboveground). However, managing the growth of the root system is also possible through root pruning.
Root pruning has been used for centuries in order to decrease vigour of the fruit tree aboveground. This is the principle of bonsai plants. However, the results of root pruning of fruit trees are unpredictable and root pruning can also lead to weak trees, sunburn and small fruit.
In many ways, root pruning restricts the tree rather like hard soil does. The differences are that with root pruning the treatment is imposed once or at most twice in a season, whereas with methods of root restriction the inhibition of root growth persists all of the season and the conditions allow no compensatory new root growth.
High sustained production
When root restriction is combined with management of the fruit trees, the soil and the method of irrigation, high-density plantings can be highly productive and easy to manage for a long time. You can achieve this by:
a) Maintain a good soil structure
Keep the soil in optimal conditions to implement the long-term effects of root restriction on controlling root growth, tree vigour and fruit production.
Gypsum applied every 2 or 3 years plus the weed mulch will keep the surface soil soft, permeable and well-aerated.
In hot dry climates with shallow soils, a thick organic mulch helps to conserve water and cools the soil down. This decreases both irrigation and heat stress and improves performance of the trees. However, some water is lost through deep-drainage.
In grassed orchards, mowers that feed the clippings sideways towards the tree rows, effectively mulch the tree rows.
c) Crop load.
Mature and semi-mature fruit trees with heavy crop loads produce much fewer shoots than fruit trees with light crop loads do. Hence, crop loading (stimulating the set and retention of adequate fruit on the tree) is one of the most economic and environmental sensitive methods of controlling excessive shoot growth.
Cropping has a profound effect on controlling tree size, primarily through decreased root growth, a consequence of roots being the weakest competitor between fruit and roots for leaf photosynthates.
By manipulating crop load, you are able to manipulate the balance between cropping and vegetative growth. Cropping is one of the most practical and effective means of controlling growth. Managing crop load to avoid either under or over cropping in the establishment years of a high-density planting is critical and requires good horticultural skills.
Originally it was believed that the only significant functions of roots was to facilitate the uptake of water and nutrients from the soil and to anchor the tree to one position. However, it is now realised that roots have other important functions.
The scientific study of root growth has lagged behind that of shoots, flowers and fruits, almost certainly because of the difficulties encountered in conducting studies within the soil. The first authoritative root studies were reported in 1939. More research work was done on root systems and the distribution and effectiveness of roots of fruit trees in the 1970s and 1980s.
The information provided in this article is a good example of soil science interacting with tree physiology to gain more knowledge that can then be translated into better management decisions for tree fruit production.