February 2019 issue of the Cornell Flower Bulb Research Program Research Newsletter:
The days of digging 6” or 8” deep holes are over! A lengthy series of landscape trials conducted since 2009 reveals the desirability of relatively shallow planting, followed by a mulch layer, for multi-year perennializing of tulips. For example, even in Ithaca (warm end of Zone 5 to cold end of Zone 6), excellent results come from digging 2-3” deep holes, placing the bulbs, then mulching with 2-4” of mulch. Deep planting dramatically reduces the ability of tulips to perennialize, possibly by exposing the bulbs to higher soil moisture levels.