ha! Anything grows in the cracks of my sidewalks, from artemisias to milk thistle, but they are all dwarfed from not being happy.
Personally, I'd think that killing the grass with vinegar or something would be the best for the sidewalk and the herbs. But if you are determined, roman chamomile seems to do well.

Maybe some moss types would work well. If you didn't want any vegitation there, there is a spray that will kill any vegitation and nothing will grow there for at least a year. The roots of these plants tend to spread and expand the cracks even more. Because ice and water settle in them too, you are looking at the cracks betting bigger every year due to ice expansion.

Have mercy on pram-pushers and the elderly, who find plants on footpaths a curse!

Frankly, I'd be inclined to complain to your local council or whoever is responsible for the condition of pathways in your area, and demand that the path be dug up and re-concreted. If the cracks are large enough for grass to grow through, they constitute a safety hazard. If the cracks have been caused by the spreading roots of trees nearby, they should be removed, too, and replaced with more suitable vegetation.

To kill the grass (but not the seeds), use boiling water or salt - or both. You'll have to keep repeating the treatment, because nothing will stop the birdies or the wind from depositing more seeds!

A strong 20% vinegar is also good for killing weeds. Marshall Grain in Texas sells such a product.

These are good questions you all have raised. Let me explain: the sidewalks I am refering to are those on my property already. We just moved in this house in October, and the sidewalk is full of cracks already. If we replace the sidewalks, it would be at our cost. We won't be ready to replace them for a few years. We've decided instead to grow herbs in the cracks so that when someone walks on them, the herbs will release their scent. We're not worried about expanding the cracks further, as we'll eventually be replacing our walkways. In the meantime, I don't want grass growing in the cracks, nor do I want the cracks to be bare.

Thank you for all your suggestions, and please, if you have more, keep them comming!

Yarrow might work as well. It tolerates mowing and a variety of conditions and it is one of the many herbs that calls my lawn home. It has pretty leaves too - it will be clipped too short for flowers in a walkway.

Dutch white clover, ground ivy, self heal, plantain, chickweed, dandelion, and others all make themselves at home in my lawn. These all tolerate some degree of mowing and foot traffic. Garden escapee feverfew and lemon balm also find themselves places in my pathways and tolerate some degree of foot traffic and mowing as well.

The amount of traffic your walkways get will dictate exactly what plants you can use. High traffic will limit the selection greatly.

FataMorgana

Creeping thyme works very well in sidewalk cracks, mommie_rose. Plus, it'll take a lot of foot traffic, smell great, and stay short.

one word: purslane

If it gets enough sun, Corsican Mint will work. Looks like moss but smells like the best mint ever. Yes, it will be hardy in Z5 because the sidewalk will be a microclimate that will up your zone by at least 1.

How about combos? Some Thyme here, some Mint there, some purslane other places .... And maybe some low growing sedums for foliage contrast?

Martie

Martie, great idea! I do have at least three different areas where the sidewalk is cracked. I should consider different herbs for some nice smell combinations. Thank you to all who have answered my question.