Depth is one of the other most important factors when planting tulips, especially because they’re generally being planted from bulbs. The general rule for gauging how deep they should be planted when it comes to tulip bulbs is multiplying the size of the bulb you have by at least three – deeper than this, and they might not take root, but shallower than this and they also might not take as they should.
Planting shallower than you should also mean that bulbs could be exposed to harsher cold (or warmer heat) than they should: Obviously, not great for getting plants to grow.
If you’re transplanting existing plants into the ground instead, then you should look at the root system to gauge just how deep down they should go. Leave just enough room for the roots to “take” properly in the ground, and loosely cover around the roots with soil, then water – and make sure you don’t overdo it on watering whether you’re planting from bulbs or transplanting.