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Girdling and cincturing treatments have potential for managing tree vigour, enhancing fruit set in vigorous trees, stimulating return bloom, enhancing fruit colour and advancing fruit maturity. Effect varies with timing of the girdling cut.
Girdling at petal fall or soon after, increases fruit retention and offers strong vigour control, giving reductions of 50 % or more in annual extension growth. As girdling is delayed, the reduction in extension growth diminishes. Even so, with very vigourous trees December girdling can still result in a 20 to 30 % reduction in extension growth.
Stimulation of return bloom is strongest around petal fall, and again just after the natural fruit drop has occurred. In cropping trees, this later girdling is preferred because it allows fruit drop to occur, thereby reducing the hand thinning job. December girdling also allows shoot growth to give sunburn protection, and a more favourable leaf to shoot ratio.
Fruit colour enhancement and advance in fruit maturity can be obtained from girdling as close as three or four weeks from harvest.
Girdling treatments interrupt the translocation of photosynthates from the leaves to the roots. This increases carbohydrate levels in the upper tree at the expense of the carbohydrate supply to the roots. This gives a similar effect to planting on a dwarf rootstock. Girdling offers an alternative to dwarfing rootstocks for higher density plantings and there are a number of examples of intensively planted blocks on vigourous rootstocks being successfully held in check by regular girdling each year.
Because some root growth is necessary to keep the roots healthy and allow them to carry out the function of supplying the tree with nutrients and water girdling treatments should not completely cut off the sap flow to the roots. However, to be effective, girdling cuts must sever most of the phloem tissue which carries photosynthates down to the roots.
Girdling Techniques
Two girdling techniques are in general use. Double "C" overlapping cuts, one each side of the trunk, and spiral cuts which overlap. Experience has shown the Double "C" technique to be more reliable, however, the overlapping ring or spiral cut is useful for young trees.
For general purposes, the Double "C" technique using cuts 50 mm apart, with 50 mm cut overlap has been shown to give about the right amount of effect. The effect of girdling can be increased by either bringing the cuts closer together or increasing the amount of overlap. Conversely, the effect can be weakened by widening the gap between the cuts, or reducing overlap.
Girdling is a fairly precise technique, so needs to be carefully carried out and well supervised. Unsatisfactory results and failure have frequently occurred when girdling treatments have been poorly implemented.
For young trees up to about 50 to 60 mm trunk diameter, an ordinary knife or saw cut with bark removal done as an overlapping ring with 10 mm overlap and 5 mm separation between the cuts does a very good job.
Older trees need the double bladed girdling knife which removes a strip of bark. These knives come in a range of sizes, taking out bark strips from 3 mm to about 7 mm in width. The 3 mm knife is satisfactory for trees up to about 150 mm trunk diameter.
Larger trees need more aggressive treatment and because of tough, thick bark, may need a powered saw such as a chainsaw. On these bigger trees, some cutting into the wood beyond the phloem does not seem to matter too much.
Trees girdled correctly will show mild signs of leaf paling two to four weeks after treatment, then recover their normal leaf colour. Excessively severe treatments result in trees sulking for much longer periods, and sometimes the entire growing season.
Disease Problems
Three potential disease problems can develop as a result of girdling.
One of these is collar rot, Phytophthera cactorum which can enter the wound from soil splash. In general, little if any infection of this disease has occurred following girdling. It is considered a risk with highly susceptible varieties such as Cox's Orange Pippin.
Another disease is fireblight, Erwinia amylovara, which poses a serious risk in pears and certain very susceptible apple cultivars. Trees infected with active fireblight may carry systemic infection in their sap stream, so the infection can be transferred from one tree to the next on the girdling tool. In this situation, it is necessary to sterilize the knife between trees.
Also, in trees with active fireblight, we believe that rain or irrigation may wash fireblight bacteria down the trunk into the girdling wound. Great care needs to be taken in regard to managing the fireblight problem.
European canker is another disease which has potential to enter girdling wounds.
It is possible that wound treatments containing low concentration of copper fungicides may give protection against these diseases.
Girdling is a commonplace treatment in stonefruits to enhance fruit size and advance maturity. Although gumming often occurs following girdling, there is not a lot of evidence to suggest that blast infection is a big problem where trees are girdled.
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