LONGAN ROOTSTOCKS
SCIENTIFIC NAME: Euphoria longana
FAMILY: Sapindaceae
My experience growing Longan is fairly recent in comparison with some growers but here goes!
When purchasing grafted longan trees, there are several things one must be aware of if the problem of 'graft incompatibility' is to be avoided. The symptoms of 'graft incompatibility' range from a slow-growing tree with dull, limp leaves to one that is virtually just staying alive and making no growth over a period of years. The rootstock will also be of a smaller diameter than the scion.
A well-grown longan with compatible rootstock will have very dark green, shiny leaves.
If the variety being purchased is either 'Chompoo', 'Haew' or 'Biew Kiew', it is advisable to ensure that the rootstock is of the same variety seedling as the scion e.g. 'Biew Kiew' seedling rootstock should be used for a grafted 'Biew Kiew' tree.
An interesting twist to this is that the variety 'Haew' in our orchard have been grafted on both 'Chompoo' and 'Haew' rootstock and the 'Chompoo' rootstock appears to make the tree slightly more vigorous. The variety 'Kohala' does not appear to be as exacting in rootstock requirements as any of the aforementioned.
If you do have a longan showing signs of 'graft incompatibility', one solution is to plant a seedling of the same variety beside the problem tree and then approach graft it on to the scion wood. Approach grafting longans is not difficult, and with a reasonable amount of care most people should be able to manage this.
In Thailand the growers mound soil around the trunks of their trees to the extent that the lower parts of the branches are covered. This is done over a period of years and it is claimed that this anchors the tree in strong winds. Mr. Barry Nichols, of Byfield, told me that he thought that if a longan suffering from incompatibility was mounded with soil so that it was covered beyond the graft, it might form enough roots on the scion to be able to combat the incompatibility problem. I did try this, and some roots did come, but I grew impatient and pulled the tree out. Roots had formed and I feel that if the mounding was continued and the tree left in the ground, it very well may have overcome its problems. Ah well, that's another one that I have lost.
Mr. C.A. McConchie, during his address at the Lychee Seminar held recently in Cairns, said that the pollen of both Lychee and Longan flowers was compatible. If 'cross' pollination occurs, then this may help explain why incompatibility does occur between the seedlings and the scion material.
If longans are marcotted or air layered, this problem does not arise, however, the resulting tree does not have a tap root and therefore is not as stable in high winds. Stability could be achieved by either mounding up with soil as previously described or approach grafting one or even several seedlings of the same variety.
DATE: January 1990
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