I planted some huge planters with a cottage-gardeny mix of tomatoes,
heartease, pot marigolds, nasturtiums and other stuff.

It's filled the planter, the tomatoes have set and everything's fine, except
that the marigolds and nasturtiums have got a mega-infestation of blackfly.
I have little hope of knocking them out using a contact spray as I couldn't
get to every affected stem: is there a systemic I could use that wouldn't
linger on in the tomatoes?

I reckon the tomatoes are at least 2 - 3 weeks off ripening, so it'll have
time to disperse.

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Sally Holmes
Wakefield, West Yorkshire, England