Why do we prune our trees? First things first. Let us go back some paces and get to the basics. For unless you understand some of the theory behind the operation, you will, after I have finished, still be at a loss when you look at the next new species and ask ''Will I prune it ?" "When?" and "How will I go about the job?" Still, there are a lot of errors you and I don't need to make. Or having made the error once, we don't need to do it again.
If I ask you the question, 'As growers of plants, what base business do you share in common with all kind of farmers - wheat, fruit, cattle, sheep, prawn, fish farmers and all gardeners?' What would you answer? You are in the business of harvesting sunlight. How? With leaves of course.
My talk today is about canopy management. The whys and wherefores. Right from the time you plant the trees, your whole operation depends on how well you can coax your trees to harvest sunlight and then convert that sunlight, firstly into a vibrant canopy, and then a few years down the track, into fruit. You, my audience today, are fruit growers but it matters little which branch of this growing business you are in - the same principles apply.
I will be talking mostly about the first six to eight years in the life of the tree or vine as it may be. Trees are like children. If you want them to do their best in their teenage and adult years, you must look after them in their formative years. Trees are no different. It can take years to undo the results of poor initial culture.
Murphy's First Law of Botany: For every good theory there is an equally good exception. Still, if you have a good exception, try it in a small way, and if it works, use it. It is from people following their hunches that we often get new techniques.
In the big scheme of things, it all happens with the meeting of Sunlight and the Biosphere (The Leaf Canopy). The Botany of that system is a story on its own which I have no intention of going into other than to say the obvious - the best results will come from a healthy canopy.
If we go commercial, we grow species that suit the local sunlight intensities, converting sunlight to useable and tradeable commodities. There is, however, a long way between harvesting the sun and the tradeable commodity.
Maximum foliage is the immediate concept that comes to mind. Grow the largest canopy possible would seem obvious, but this is not true. Your methods must be tempered by many other considerations. If the answer was just the largest canopy, well dish up the nitrogen fertilizer - but that is the worst possible scenario - all leaf and no fruit.
Many factors go into the consideration of how to grow and use the best canopy. Soil and soil drainage, Plant space and tractor space (tractor rows), irrigation and nutrition, harvestability, pest and disease control, wind movement, fruit set, flavour/texture/and cosmetic value. Reduce height - control density. Where does it all end? All these factors have some effect on how and why we prune.
So you see some of the inputs and calculations that go into the computer (my brain) when you ask me the question 'How do I prune my trees? Some to be answered and some which should have been answered a long time ago and now just have to be put up with. There are some big difference between the management of a home garden with its one of each and a commercial orchard with hectares of one variety of one crop. But the same principles apply.
Pruning is only one of many inputs to an end. IT CAN HELP BUT NOT OVERCOME:
Depending on our planting layout, we can theoretically manipulate the canopy and leaf areas by factors of 100%, 200% or 300%. Light = fruit (Canopy Height X Canopy Spread X Tree Spacing to get Optimum Sunlight Interception (Sunlight Harvest) and finally Optimum Fruit Harvest).
The canopy and root system ratio: 1/3 above ground to 2/3 below ground. The effect of severe pruning. When we give a plant a severe pruning, the plant will rebalance the Leaf/Root system, i.e. it will shed part of the root system to make up for the heavy leaf loss. LAIs explain -pasture and jungle. LAIs = Leaf Area Index = The number of square metres of leaf area covering a square metre of soil.
CAUTION: It may seem from my talk that leaf growth is the be all and end all of cropping. That is not so. It just happens to be the subject under discussion. It does not sit above water, nutrients, other cultural techniques, pest and disease control etc. It sits along side these, and plays its share in the matrix that is healthy crops.
So what are we doing in those first years? The first operation is to grow a canopy to harvest the sunlight. (On a crop like Meyer lemon it is very easy to relent and grow some fruit, but at the expense of the canopy and what should be the first commercial crop.) Then somewhere between 3-6 years later, with the canopy growing well, we are ready to look at fruit production. If we put the cart (early fruit set), before the horse (the vigorous canopy) it may jump away to an early start, fruit wise, but its later progress is slow and will, after a few years, be rapidly overtaken by the horse with the cart behind in its proper place.
How much do we prune - Almost every variety and species has its own needs. It is fairly easy to control the tree size, but the shape of the canopy tends to have a mind of its own. The tree keeps trying to come back to its basic shape encoded in its DNA in the genes.
There are general principles that need to be followed:
a/ - How to cut branches.
b/- The general shape of the canopy - the top diameter should not be larger than the lower canopy diameter or the lower areas will be shaded out.
c/ - Cutting back the top growth.
d/- Weak growth generally promotes weaker growth. There are too many growing points for the vigour of the tree. Pruning shortens the stems, reducing the number of growing points, producing less stems but more growth per stem. This may or may not equal increased vigour. Increased vigour is more an effect of good all round husbandry of which pruning is only one part.
e/- Keeping the stem below the graft free of sprouts. This should be done regularly. Every six months if needed in years 1 and 2. A good healthy canopy usually suppresses basal sprouting. This basal pruning should never be neglected to the extent that large cuts at ground level are needed to remove neglected basal rootstock sprouts. (eg citrus.) Such pruning (while necessary) leaves large cuts open to infection and the trees are prone to resprout from this basal area. It will require consistent and regular removal of sprouts to suppress the basal sprouting instinct in citrus and cashews if this operation has been neglected while the trees were young.
f/- It is generally the best option to keep the first 3-500 mm of trunk as a single stem. (With the range of species now being grown there are plenty of exceptions to this rule). This clean stem makes trunk hygiene easy ( - trunk diseases, weed control, leaf litter accumulation.)
g/- Where two stems cross and rub, abrading the bark, one stem should be removed. Left as they are, the abraded areas suffer fungal infection in the wet season necessitating removal of both branches.
h/- After pruning a bearing-aged tree, a minimum of 0.5m free of canopy is needed to keep fruit clear of the ground as the fruit matures and the weight drags the branches down. (Scarred fruit, dirty fruit, scale-infested fruit - ant-borne.)
Stems of many species need to be protected from the Sun, particularly in summer. If in its natural habitat the tree has developed as part of a closed canopy environment (even if that canopy is only light-to-dappled shade), the potential exists for sunburn on the trunk when it is planted out in the open, ie the orchard.
Many young trees, when put out into full sun, run the risk of sunburn unless the trunk is shaded. Macadamia - young trees very prone to sunburn of the trunk ( as are Avocados, Durian and Mangosteen).
Sunburn - long term effects: Litchi, Longans. Mangosteen and Durian. Durian propagation. Sunburn protection will always be needed following a severe pruning. White paint, bentonite, eg.
Winter sun can also cause damage. If the ground is cold, root action is reduced, water uptake slowed, and may lead to water stress if warm, windy air conditions exist above cold ground.
Young citrus: Severe overpruning of young citrus delays fruiting. Minimum pruning to ensure a good skeleton is formed, then leave alone for 3-4 years with the exception of removing watershoots or shortening any very vigourous branches. And keeping below the graft zone free of sprouts. Commence removal of excess inner branches at 3.5-4 years. Undercut to lift the canopy, prune canopy to at least 600mm above ground to keep developing fruit above the soil.
Water shoots (usually on citrus) are triggered by strong light source reaching the inner branches. Essentially a reaction to gaps in the canopy. Most should be removed, but the canopy must be refilled. If necessary, keep some water shoots to thicken the canopy.
The aim in all trees is to have a total canopy with no large gaps. Gaps = loss of canopy = loss of crop. (I did say there were always exceptions. Some trees never make a dense canopy.)
The inner part of the tree is usually pruned to just the skeletal branches. Leaf canopy inside encourages pests, branch and leaf diseases, and any internal fruiting is difficult to harvest and usually later ripening than the fruit in the outer sunny areas. This may be an advantage in the home orchard, but is not a part of good commercial management.
Custard apples have the buds under the leaf base and new growth will not sprout till old leaves fall. At pruning, old leaves are either rubbed off by hand or 3-4 leaves are cut back to the leaf stalk which will then die and fall.
Coffee - total pruning (stumping) every fourth year of cropping - lose one year of production while tree regrows.
For slow-growing crops such as jaboticaba: "Why grow masses of foliage to be pruned off?" Prune lightly each year while small, and push that nutrient into the branches and leaves you want to keep. Suggest pruning to a fan about 1.5 metres width. Should allow picking while protecting your eyes.
Litchi and Longan with tight crotch angles and their propensity to split in storm/cyclone conditions: How to prune. If you must have a litchi/longan tree of one of those varieties that have narrow crotch angles, careful regular pruning in the first 2-4 years can form a strong basal trunk structure. The tips are pruned back to just below the narrow crotch and flatter branches are forced. This pruning is repeated till 1.5-2.0 metre of strong trunk and lower main branches is formed (I did say for special cases only - labour costs could not be justified for a commercial plot.) In theory the basal structure should be storm proof even if the upper branches split.
Seedling Annonas (Custard Apples, Soursops, and Rollinias) tend to make double trunks with very tight crotch angles. Neatly cut one trunk away while the tree is still small. ( This usually means before the seedling is 1 metre high).
Grapes: Where are the flower buds? Different varieties have the flower buds at different position along the canes. You must know which nodes will have the flower buds. This decides pruning length.
Differing tree species have different reaction to major leaf loss. Cashews do not like to lose their leaves. If allowed to defoliate from causes like insect damage, eg red banded thrips, or by strong wind, it may be 12-18 months before a good canopy is regrown. This may mean the loss of a year's cropping.
Staking young trees: Never tight. Stem movement promotes ethylene build-up, triggering stem thickening. (I don't know just what is too tight. At a guesstimation, I would say if the trunk at the point of tying, can move 100mm from centre in each of two directions then there will be enough stem movement to promote stem thickening. Make some observations of your own if in doubt)
DATE: November 1990
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