Errrm...deep enough to get the roots in !!! sorry to sound a bit condescending but that's really the answer.

Thankyou

Best if you dig the area you're planting in generally, if its not been prepared already. Then dig the holes for your plants so that they will be at the same level as in the pots, i.e. , the soil covers the rootball, but doesn't go higher up the main stem than the compost when in the pot.