Will my Darwin hybrid tulips come back every year?
In his book Ask Ciscoe (Sasquatch Books, 2007), local garden expert Ciscoe Morris suggests planting Darwin and Empress hybrid tulips 12 inches deep (rather than the often recommended 6 inches) so that the bulbs will be less likely to divide and the squirrels less likely to dig them up.
Ann Lovejoy says much the same thing…plant tulips 10 inches deep, in a sunny spot, and in well-drained soil, and some are likely to return for several years. (See Seasonal Bulbs, 1995, p.16)