What’s the halacha regarding kil’ayim with two types of besamim herbs – e.g., mint and lemon verbena? Are they considered edible greens since they’re used for making tea

There’s no problem planting a tree, legume or vegetable in close proximity to the lemon verbena, because lemon verbena is classified as a tree and therefore no separation is required. 

Herbaceous plants such a coriander, parsley, mint and basil are considered vegetables and do require separation between them. In contrast, lemon verbena, wormwood and rosemary are woody plants and so there’s no need to keep a distance between them and other greens.

Vegetables and herbs that are used as spices or for flavor do need separation between them even if they are not eaten as is.