how and when do I feed herbs?

Any nitrogenous feed, like growmore, or a liquid version, will be fine for you to eat. It's the pesticides you might spray on you have to be careful with if its a food plant. Bear in mind, though, that most herbs, provided they're in sun and not suffering drought, grow like weeds anyway!

I personally would never feed herbs - the conditions they grow in out in the wild are extremely lean and feeding could result in lanky, overgrown, out of character plants.

Just as Moongrower says, Im very much into herbs and wildflowers and am a firm believer in letting it do what it would naturally. definatly taste much better,

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