Growing a quality crop with limited waterResearch & Extension
With growers looking to set up their crop for the coming season with the possibility of limited water, AgFirst’s John Wilton explores ways to grow marketable crops in a dry year; methods that are also relevant if water is not an issue.
Matching crop load to tree canopy size and growing conditions determines fruit quality. In the perfect world, which incidentally does not exist, tree vigour should be uniform at around 20 to 25cm of individual shoot length, which will terminate in early summer with a strong terminal fruit bud and the crop uniformly distributed throughout the canopy, largely in singles.
Present indicators are that, for many Australian fruit-growing districts, irrigation water supply will be scarce. By hand thinning time the irrigation water supply situation may become clearer. The amount of water you have will determine how much crop load the trees support.
There is good data available to show that with careful management it is possible to grow marketable crops in dry years.
The first decision to make, then, when setting your thinning and crop loading policy, will revolve around the quantity of irrigation water available. If it is likely to be limiting, careful thought will need to be given to where best to use it. It will be necessary to set some priorities for water use.
Key areas to think about are allocating enough water to maintain young tree health because they are your future. Also, with an efficient water irrigation system, they do not need as much water as older, less intensive producing blocks, but will be much more sensitive to drought stress because their root systems are not fully established.
If planting decisions have been rational, newer plantings will have better fruit value than older orchards. Our data also shows that modern intensive plantings reach district average orchard production levels relatively quickly, usually within three or four years of planting so, as well as having high fruit value, they are capable of matching older orchard revenue well before they reach their full production potential.
Once water requirements necessary to maintain high value newer plantings have been established, it’s now time to attend to the remainder of the orchard. Most established orchards have a range of blocks, some producing good returns, and others very marginzal. If there is insufficient water to irrigate the lot, it may be time to pull out the more marginal ones so available resources can be directed to those blocks with the best chance of success.
The thinning and crop load strategy
Where water supply is adequate to support the crop, a normal crop should be the goal – assuming trees are relatively uniform and there is no entrenched biennial bearing.
Because most of the varieties grown are red or partially coloured and because markets demand high colour, fruit needs to be well spaced, mainly in singles with twos to build numbers, if necessary, on favoured sites.
I tend to favour large, calm canopies, in which interfruit competition reduces the fruit clusters down to ones and twos, usually with the help of an effective chemical thinning program. This makes setting crop loads relatively easy, and hand thinning manageable.
A calm canopy maximises the proportion of the tree’s carbohydrate supply going into the fruit, rather than excess vegetative growth.
Generally, the crop load determines fruit size range as there is a strong relationship between fruit sizing and crop load. Understand your market requirements regarding fruit size range.
When setting up crop loads, analyse your previous crop history as a good starting point for well established orchards with relatively mature canopies.
In younger trees, canopy volume determines their cropping capacity. Assuming that excessive pruning has not disrupted normal tree growth, trunk cross sectional area (TCA) is a reliable guide to fruit canopy volume until the tree fills its allotted space. The number of fruit per cm2 TCA is a very useful method for setting young trees’ cropping levels. Optimal crop levels will depend on variety and growing conditions. Under ideal growing conditions, with no serious limiting factors, optimum crops for two, three and four-year-old trees are from 10 to 12 fruit per cm2 TCA.
Incidentally, by the time the fruit reaches hand thinning stage, most of the damage to tree growth and development has already occurred, it’s the number of growing points at the beginning of tree growth, and flowering density, that determine young tree vigour. Branch selection at pruning time and controlling flower bud density are what determines tree vigour.
Generally, trees growing on precocious dwarfing rootstocks perform better with some crop than without it. This is because their shoot growth terminates earlier than trees growing on more vigorous root stocks and, once this happens, leaves become overloaded with photosynthate and shut down. Carrying some crop prevents this happening. Also, gibberellin from developing seed in the fruit stimulates shoot growth.
Once trees approach full canopy, TCA begins to lose its relevance to cropping capacity because TCA continues to increase while canopy volume remains constant. In older trees it’s better to set crop levels based on individual branch or sub-branch cross sectional area (BCA). As branch diameter increases there is a rapid reduction in optimal crop load per cm2 BCA. Break the tree canopy down to small fruiting branch units up to three centimetres in diameter. Optimal crop loads on branches in this size range are 3.5 to 4.5 fruit per cm2 BCA depending on variety. These branches should be relatively settled regarding vigour, will require minimal pruning because they are small and will produce low fruit numbers. Checking crop load level is relatively simple and it’s a very efficient way to establish a hand thinning policy and then check that it is being correctly implemented.
Where tree canopies have excessive branch numbers, over-cropping with this method of assessing crop load is possible, so it’s necessary to do some whole tree fruit counts to make sure that the trees are not carrying too many fruit.
Also, when making thinning-time fruit counts, it’s necessary to build in an allowance for the natural drop which occurs between hand thinning and harvest. Our data indicates that around 15 to 20 per cent of the thinning-time fruit load will vanish by harvest.
Where irrigation water supply is short, crop loads will need to be reduced to maintain fruit size in the marketable range.
At around 50 per cent of irrigation requirement, fruit numbers will need to be halved. If it’s not going to be possible to maintain irrigation application at the 50 per cent level it may be necessary to abandon the lower-value performing blocks to grow marketable crops on the better blocks.
A good thick organic mulch markedly reduces water needs and has the added advantage of lowering soil temperatures in the root zone which will also improve apple tree performance during hot weather. There is good data to show that mulching can double the irrigation interval compared to bare soil.
Where sprinkler irrigation is used, evaporation losses can be very high, both during application and from the wet soil surface following application. Increase the irrigation interval and use longer runs to drive irrigation water well into the soil to minimise these evaporation losses.
In hot climates, fruit needs transient shading to avoid sunburn injury. Thin off excessively exposed fruit in positions likely to suffer injury. We have also seen increasing problems with both sunburn and fruit colour development with increasing crop loads.
The first pass of hand thinning needs to be done before the really hot weather arrives, and before the bunches close on a bunchy crop.
To grow a high-quality crop, two hand thinning passes may be necessary. The first to single the fruit, or at least bring them down to ones and twos. The second, to fine tune the crop which will involve targeting those areas where crop loads are still too high, remove russet and damaged fruit, and space fruit to give it enough room to grow without being shaded by nearby fruit. As this second crop-grooming thin has to be done later, when temperatures are high, there is a high sunburn risk unless the first thinning brought fruit down to mainly singles. If you are forced to do late hand thinning in a bunchy crop to reduce crop load, it’s best to take out whole bunches rather than try to break them down into ones and twos.
In terms of growing a high-quality crop getting the hand thinning right is critical and one of the most important crop husbandry practices.
Apples and pears are naturally predisposed to alternate cropping. The warmer the climate, the greater the biennial bearing tendency.
There is also a link between high tree vigour, vigorous rootstocks and increasing biennial bearing. With appropriate management, targeting excessive tree vigour and excessive ‘on’ crop fruit numbers, biennial bearing is manageable.
While precocious dwarfing rootstocks are generally less prone to alternate cropping, they can go biennial and once they start this habit are very difficult to bring back to regular cropping. The message here is ‘don’t risk letting them go biennial’.
As we have been pushing up yields in recent years there has been a lot of effort go toward understanding and managing the biennial bearing problem.
An early thinning program is critical to minimising biennial bearing. The foundation to this program should be a sound chemical thinning strategy. I have found it necessary to commence the chemical thinning program on young orchards planted on dwarfing rootstocks in year two with an aggressive post-bloom thinning strategy, e.g. benzyladenine (BA) plus NAA, or BA plus carbaryl. In general, this strategy has been superior to using bloom thinners as it carries less overthinning risk. Incidentally, overthinning young trees can also predispose them to biennial bearing, as can completely stripping the crop off in the second or third growing season. It’s better to build up the crop gradually as the tree canopy builds, rather than grow the canopy first then shock it into crop with a heavy crop.
Where biennial bearing patterns are established, in the aggressive ‘on’ crop year, thinning is necessary, usually in conjunction with other cultural practices known to stimulate flower initiation and bud development to bring the trees back into regular cropping the following year.
Where vigour is high, reducing it helps. Trunk girdling, once fruit shedding induced by chemical thinning becomes apparent, will do a good job of reducing annual shoot growth as well as stimulating flower bud development for the next year. The optimal window for treatment is mid to late November. If trunk girdling is done too soon fruit does not shed and the hand thinning task becomes colossal. Where young tree vigour is very high causing fruit to shed rather than set, girdling at petal fall is a very useful tool to solve this problem.
Summer sprays of either Ethephon or NAA applied at low rates from late November into December as a three-spray program with cover sprays have been found in North America, and in New Zealand, to be very beneficial for return bloom. Particularly, NAA is now widely used to minimise biennial bearing in susceptible varieties.
Maintaining good leaf health is essential for good fruit bud development. Early season water stress adversely affects bud development. Low nitrogen status, as well as adversely affecting fruit set and retention, also reduces flower bud development.
Pests, such as red mite, which reduce foliage efficiency, reduce flowering in the following season.
Excessive crop load carried much beyond four to six weeks after bloom supresses flowering for the following season and may override the effects of the treatments mentioned above.
In orchards with biennial bearing issues, growers often try to compensate for the reduced number of fruiting sites by leaving more fruit on the fruiting sites to maintain fruit numbers. This is not a good strategy for several reasons. Firstly, fruit site distribution is usually irregular in the tree so it’s very easy to end up with excessive crop loads on a portion of the tree. Fruit on these overloaded branches often fails to ripen properly and will tend to behave as if the whole tree was over loaded. Secondly, fruit in bunches often fails to colour well and is difficult to pick. Thirdly, unless your thinning labour supervision is very good, once you allow thinners to leave fruit in multiples there is a risk that normal crop trees will have fruit left in multiples where it should have been singled.
Set hand thinning policies that match the requirements of the majority of trees in a variable block.
In many blocks with biennial bearing problems it’s only a small proportion of the trees with really ‘off’ crops, so their crop does not amount to much volume compared to the crop on the rest of the block. Thinning them correctly therefore does not amount to much reduction in the total crop but will certainly make the harvest run more smoothly and the better fruit quality from doing the thinning correctly will more than compensate for less crop.
Vigour has a huge influence on fruit quality and, where excessive, represents lost crop potential.
High vigour is only necessary in recently planted trees while you are building canopy volume. As full canopy approaches, tree vigour needs to be calmed down to annual shoot growth in the region of 20 to 30cm.
Branch strength determines vigour status. Cropping is the key to vigour control. Branches need to have sufficient crop to utilise most of the photosynthates produced by their leaves so there is little surplus to fuel unwanted growth, or export to elsewhere in the tree such as the trunk and roots.
The relationship between branch diameter and its length has a huge influence on vigour. Calm branches usually have diameters near the main branch or trunk of 2 to 2.5cm per metre of length. They also need good distribution of fruiting laterals and spurs on which to set crop.
Shortening longer branches that have outgrown their allotted space destroys the vigour/cropping balance and will stimulate excessive shoot growth. These branches are better removed in winter pruning rather than shortened to contain them within their allotted space.
In higher density plantings a common mistake is to carry too many branches. Where there are too many branches shading is a problem, as is insufficient space, to allow for adequate fruiting buds to carry crop.
Branch density needs to be around six to eight branches per metre of leader extension and less if these branches need to carry much fruiting lateral development to accommodate the crop load required to control excess vigour.
While establishing the correct branch hierarchy is the long-term answer to vigour management, in the shorter term there are a number of vigour control measures that can be implemented during the growing season to manage vigour. These include:
• trunk girdling
• summer pruning
• deficit irrigation
• growth regulators
• branch training.
Summer pruning is a very useful tool and it can take different forms. It can range from plucking out unwanted water shoot growth prior to hand thinning on high vigour branches, to targeted shortening of the current season’s extension growth to stimulate fruit bud development.
Tipping back shoots at the seven to ten node stage, which is sometimes done mechanically, reduces their growth and forces fruit bud development behind the cut. This is a very useful way to avoid bare wood in varieties such as Nicoter (apples marketed as Kanzi™) and Scifresh (apples marketed as Jazz™), which are prone to this problem.
Summer pruning needs careful supervision, particularly if it’s done as a panic move towards harvest to try and lift fruit colour. Late summer pruning increases sunburn risk and, if poorly understood, often results in large amounts of next year’s fruit buds being lost.
Deficit irrigation is a very useful tool for vigour management in pears and stonefruit. It’s less suited to apples due to high sunburn risk, and it can increase fruit cracking problems. It could be a useful tool in apples with net protection because water requirements under net are lower, and the net gives some sunburn protection.
Growth regulators are useful tools, but their use needs to be well understood and well managed.
Branch training is widely used to control branch vigour, and to stimulate bud development and fruit setting. It is expensive and timing it well is critical. It is an important tool in varieties that tend to give stiff upright growth such as Cripps Pink.
Common mistakes with branch training are tying down far too many branches, tying down too early and spending time and money on weak branches where training is unnecessary.
In recent years reflective mulch has become widely used to improve fruit colour. For this purpose, it’s usually put down around four weeks pre-harvest. There is also an earlier, less practiced application, which is to lay it down over the flower initiation, bud development and cell division period to lift lower tree light levels. In orchards where lower tree light levels are marginal, such as orchards under net, significant improvements in bud strength, fruit size and quality can be obtained by using reflective mulch over this period.