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Regarding construction of a pergola there are instructions for doing
just that in the April copy of The Garden (RHS) page 299.
it says 20 - 25%of the upright should be embedded in the ground.
Hole 2 ft deep for a 9ft upright, 6 x 6.
Hole should be no wider than a spade's width.
Put a half brick or stone under the post, half fill with hardcore and
top up with concrete. It empnasises need to keep checking with a
spirit level etc.
Metal brackets are suggested to avoid lots of joinery.
Happy gardening
Pam in Bristol
pamdotmooreatvirgindotnet