FLOWERING AND FRUIT SET IN LYCHEES

SCIENTIFIC NAME: Litchi chinensis
FAMILY: Sapindaceae

Recently I attended a meeting of the North Queensland Lychee Growers Association, where Dr Don Batten gave an excellent talk on this subject. Don was formerly Senior Research Horticulturist with NSW Department of Agriculture and has done some very thorough ground-breaking research on the conditions required to induce lychees to flower.

After taking copious notes and almost getting writers cramp in the process, the following is a condensed version of part of his talk.

LYCHEES ARE UNIQUE
Lychees appear to be unique amongst fruit trees. Flower initiation does not occur in dormant buds as in temperate fruits. Also, a resting period is not necessary to get lychees to flower, as we all previously thought.

FlOWER INITIATION (FLORAL INDUCTION)
Bursting buds
Flowers only form in the bud after the bud commences growth following pruning, but only when the bud is about 2-3 mm long!

Buds longer than 5 mm tend to run to vegetative growth instead of flowering. Low temperatures are also necessary for floral induction to occur.

Low temperatures
Only three days of low temperatures (inductive conditions) are needed to trigger flower initiation once the buds start to grow. The situation when buds are just starting to move before a cold snap is ideal.

It would appear that the growing bud can 'switch over' to form flowers if the temperature is low enough. Dormant buds do not respond to cold weather, they have to be actually moving at the onset of low temperatures.

So what you need is: Mature shoots - lightly prune the tree - bursting buds - cold weather, in that order.

When is the coldest period in Far North Queensland? About the last week in June, according to the pundits.

How long do buds take to burst after pruning? About four to six weeks generally.

THE SEQUENCE OF EVENTS
So what are the the sequence of events one should follow to improve flowering and fruit set in lychees in FNQ?

1. Shape prune
Prune the trees as heavy as necessary after harvest for good shape. The trees will then produce a couple of growth flushes which should all be hardened off by the end of May.

Caterpillar damage to the new flush may cause infested trees to reflush. This could be serious if the reflush of growth occurs too late, as it may not be possible to get the trees to commence growth again to coincide with the cold weather. Be sure to control all caterpillar infestations during the off-season. Don't underestimate this problem.

2. Light prune
About the end of May, lightly prune the trees, taking off about 10 cm from the hardened-off shoot tips.

There should be no stress after this light prune. Irrigate as necessary to ensure bud break coincides with the cold snap (at least you hope it does!).

3. Fertilize
In early June, apply 25-30 g Nitrogen per square metre of canopy area. If using urea (46% N) through the irrigation (fertigation), apply 55-65 g per square metre to stimulate growth.

Cold period - bud burst
It is to be hoped that bud burst will now exactly coincide with the onset of the cold weather about the end of June (in FNQ), thus going some way to ensuring a crop! Once flowering commences, a temperature of around 19°C is ideal for ovule fertilization.

TRIALS
Economic data apparently does not show up lychees to be all that profitable. Therefore it is absolutely vital that you don't miss out on a crop and that the above sequence of events, if correctly timed, will hopefully ensure that alternate bearing does not occur.

To try to time the pruning operation more accurately, the DPI have commenced pruning trials and weather data recordings to see if it is possible to work out a model, the better to synchronise bud burst with the expected onset of cold weather.

SELECTED CLONES
It is my firm belief that the planting of selected lychee clones, of known regular cropping capacity for their particular district, will go a long way towards overcoming erratic bearing. The vagaries of our climate would have much less effect on clonal trees.

Clones are inherently better able to benefit from improved management practices such as described in this article. The capacity for increased yields and therefore better returns on capital and labour is assured. Such is the case with temperate fruit trees and I see no reason to doubt the same applies to tropical fruits.

For home gardeners, who comprise a large proportion of RFCA membership, it is all the more important to have regular cropping trees, as most of them, if they have any lychees at all, have only one or two trees in the backyard.

Dave Hodge, Cairns Branch

DATE: November 1995

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