Answer: You didn't say what type of mandarin you're growing. Some mandarins need a second tree nearby to act as a pollinator and some mandarins (such as the Satsuma) set fruit without pollination. If your tree has set fruit in the past but did not develop fruit this year, the most likely cause is cold, wet weather while it was in flower. Pollinating insects avoid cold, wet weather and as a result do not transfer pollen. So, even if you have what is considered a self-fruitful mandarin, inclement weather can stop the flowers from producing fruit. Hope you have better luck next year!
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