A few months ago I purchased dahlias as an offer in Gardeners World, from Thompson and Morgan.
I'm not a young lad but by no means am I an expert gardener and tend to follow information provided.
T & M, in their planting instructions, suggest a planting depth of 6" and I followed this app. 5 weeks ago when the frosts had finished ( I had started them in shallow pots in the greenhouse).
At the time I thought that 6" was a little deep and last Friday Monty stated "Plant them just below the surface"