Answer: The most common causes for non-flowering are 1) the bulbs are overcrowded, the bulbs are planted too deeply, and 3) the foliage is removed while it is still green. Tulip bulbs store energy which is transported from the leaves through a process called photosynthesis. If the foliage is not allowed to mature and turn brown on its own, the carbohydrates will not be transported down into the bulbs for storage. Without adequate energy, the bulbs cannot flower. Simply allowing the foliage to die back on its own will assure the bulbs have enough stored energy for the following spring's floral display. If the bulbs are overcrowded, the best course of action is to allow the foliage to die down, then dig the bulbs and replant, placing them 3-4" deep and giving them 8-10" of space all around.
Best wishes with your tulip bulbs!
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