What sort of compost is in the pot? Some composts eg multi-purpose are too insubstantial to retain enough moisture for long-term planting like a shrub.
I would use a very large pot with John Innes No 3 loam-based compost with a few handfuls of MPC added just to lighten it a bit.
A shrub like buddleja, with a lot of upright stems, needs quite a bit of moisture for its roots to pump up to the tips of those stems.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
As above really but yy buddleia in the ground have began to show signs of drought as well as we haven't had any meaningful rain for a very long time. It's a combination of the summer heat and drying winds around here. Mine are in shady spots which reduces their need for water quite a bit but if they begin to droop then you should give them a good soak.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Mine are in shady spots which reduces their need for water quite a bit but if they begin to droop then you should give them a good soak.