Answer: The tulips need to be allowed to grow their foliage until it withers and dries up naturally. This replenishes the bulbs so they have a better chance of reblooming next year.
Note that not all varieties of tulip rebloom well no matter what you do or don't do. But if you do not let them grow, they will definitely not be strong enough to bloom next year. Relocating tulips can also weaken them too no matter what time you do the job.
However, for best results you should let them grow and dry up completely, then dig and store them cool and dry until planting time next fall. Their growth period and curing time can easily take over a month.
Alternatively, if you want to plant something in their place sooner, you can try to transplant them now. Water them the day before, then dig them up taking as many of the roots under the bulb as you can; this means taking a large shovel full of soil with each bulb. Replant at the same depth, water to settle the soil, and hope for the best.
Good luck with your tulips.
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