Off-season flower induction in on-crop year and improvement of flowering in off-crop year is an economical and scientific objective for longan and other subtropical fruit trees. Recently potaslum chlorate (KClO3) was applied successfully to control flower induction (FI) on some tropical long an cultivars. However, it is still little known in subtropicallongan production, especially in the North of the country. Experiments are described the effects of KClO3 on FI by leaf sprays at 4 concentrations (0.5
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1.5 and 2 percent) in December and soli applications at the rate 40 g/a.l/m canopy diameter on September 23rd and October 4th on Huong chi cultivar grown in Hanoi and Hung yen. The results of KClO3 leaf spray at 1 percent in December on 4 year-old longan trees effectively improved FI leading to considerable yield increase by 71.9 percent with net income 153,000d/tree compared with untreated trees. The soli application in September-October successfully induced off-season crop of Huong chi longan and brought a net income 1.99-2.21 million vnd/tree due to the high price at eariier harvest by 2 months compared with the main crop.