Minnesota,
United States 
I planted tulip bulbs last fall but no signs of any growth this spring. Other mature tulips eslewhere in the yard pushed through the dirt weeks ago. Did I not plant them deep enough (planted 6"-8" deep) in our MN climate?
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tulips page
4 Apr, 2010
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That sounds like a good depth. Was there any sign of critters digging? Pocket gophers and voles can decimate a tulip bed, newly planted ones first. What kind of tulip were they? Some of the Mediterranean species tulips can't stand a Minnesota winter,
Another thought: have you planted tulips before? If they were planted upside down, that will delay them severely--you might see leaves this year, but 2-4 years before they flower again. No insult intended--I had to learn the hard way, too!
And you never know, they may just be slow to start. Sometimes new bulbs are a little slower than established clumps.
5 Apr, 2010