Q: Various seeds and bulbs require being planted at a certain depth; when you plant at a specific depth, does the addition of mulch add to the depth? Or should the gardener consider the layer of mulch “as part of” the planting depth?
A: Excellent question! Since the soil holds moisture and nutrients around the bulb or seed and protects them from predators and weather I’m sure recommended depths always refer to the depth to which you plant beneath the soil surface. A thin layer of mulch (1 – 2 inches) simply keeps the soil from drying or eroding.
The rule of thumb for bulb-planting depth is two to three times the diameter of large bulbs and three to four times the diameter of small bulbs. Remarkably, some bulbs can move themselves to the correct depth if you make a mistake. Their extensible roots slowly move the bulb into correct position. Even if you plant a bulb upside down, it will grow normal shoots, roots and flowers.