Strawberries can be worthwhile - not sure what else but someone may be able to suggest something . Depends what you've got in mind. Depth of guttering, and aspect / location along with time available for watering will make a difference too.
Some people start their rows of carrots in guttering, prior to sliding the lot into their permanent home, but I'd suggest texteditor investigates Hydroponics theories on which to build other crops?
How synchronistic! My (very non gardeny) OH suggested guttering as planters for something we talked about yesterday - I'd never even thought about it before, so thanks for the post!
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Depth of guttering, and aspect / location along with time available for watering will make a difference too.