Almost any herbs which have a strong smell will help to reduce pests to some extent. The lemon-scented ones are usually favoured (eg lemon balm, lemon-scented geranium, citronella, which is related to lemongrass), but basil, rosemary, lavender etc also play their part. Pyrethrum is an obvious choice, too.

These are most effective when placed in such a position that passers-by can rub against them, or reach out to crush a leaf or two. This helps to release those essential oils.

Your problem is that these don't do so well in a cool/cold climate. The heat and sun help the plant to produce those essential oils - the less of both they get, the less EO they produce, and the less effective the plants are at repelling bugs. And of course, these strong-smelling plants won't always survive a freezing winter.

I think you might just be between a rock and a hard place!

Mountain Valley Growers claim that 2 of their lemon-scented thyme groundcovers are hardy to z5, plants only offered: Goldstream and Lemon Frost. Often ads show this type of creeping thyme planted between stepping stones on garden paths where it could rub off on shoes. Daisy, do you think it might keep mosquitoes from biting ankles?

I started sweet basil inside from seed to plant at the perimeter of my patio to repel mosquitoes & hope it works! I see Rchters carries lemon basil seeds. In a recent post here, someone brings basil inside for the winter, grows new plants from cuttings inside in the early spring & has decent size plants to put out & to propagate after the last frost.

I've been researching herbs that repel mosquitoes, but don't have practical knowledge to offer & I'm killing all my locally bought lemon thyme. Internet info says to sow seeds of thyme in the fall. Has anyone tried this?

In my person experience, the bug-repelling herbs work best when you rub a few leaves directly on the skin.

Will they repel mosquitoes if crushed on ground?

Wormwood (artemisia) is a good choice. It grows in a few years to a perennial bush. The leaves have a strong camphor smell when rubbed that seems to repel both four- and six-legged pests.

Wormwood, (and other artemesias), mint, rue, pennyroyal are some of the classics. All dead easy to grow.

Grasshoppers love mint! When I was a kid we'd catch grasshoppers all the time by going to the mint patch. It would be covered.

Tansy.