Answer: Kumquats are usually reliable in their cycle of producing fragrant flowers and developing ripe fruit. If the fruit looks normal but is dry on the inside, it indicates inconsistent watering; trees will take the moisture they need to sustain life before they direct moisture to the fruit. If the fruit is drying from the outside in, it may have been left on the plant too long. Unlike oranges, kumquats do not drop from the branches when they are ripe - they must be harvested when they're ripe.
All indoor citrus require bright light (12-14 hours a day, supplemented with artificial light during the winter months). If you're using an incandescent rather than a fluorescent light, it may be too close to the plant. Heat generated from spotlight type grow lights can reduce moisture near the plant, making it too dry in the vicinity of the ripening fruit. Try moving your grow light so it's not too close to the plant (place your hand on the foliage where the light shines to make sure it doesn't get too hot).
Another way to increase the moisture around your plant is to place the pot on a tray filled with pebbles and add water to the tray. (Make sure the bottom of the pot isn't sitting in the water!) As the water evaporates from the try, it will increase moisture around your plant.
Hope these two suggestions help your plant to produce ripe fruit!
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