How thick is the mulch? Even sedums need to root in soil, and thick gravel may prevent it from spreading effectively.

Is removing the gravel mulch an option? This time of the year, an ad in your local paper or craigslist.com for free gravel mulch may get some takers who are willing to remove it for you. If you don't remove that mulch, you are going to be fighting it for years. Trust me on this. The former owner of my property was fond of gravel mulch, and it took me many years to get rid of it. Another option may be to use the wretched gravel yourself and fill in garden paths with it. Of course, this assumes that you actually need paths.

Here is a link that might be useful: groundcover forum

Wooly thyme just seems to love the gravel I have between the stones on my terrace. Although I agree with vicki that the gravel needs to be pretty shallow for anything to take root. Although eventually weeds will definitely find a way.

I like the idea of sedums though, give it a try. I'm always moving plants around because they clash more than I thought they would but sometimes something works out better than I though too - it could be smashing.

Now this is from someone who has been removing gravel steadily from her backyard....

You need to think about this carefully or you will have mistakes added over mistakes. Think about whether is is best overall to maintain a gravel "look" in which case you would do things that make gravel stay neater longer, but you want to add some living presence. Do you have a photo? I can't tell whether you are dealing with an area that could be treated kind of like a patio, like a garden floor, or whether it is totally not suitable for that. You could scope out southwestern and desert-type gardens and find examples such as gravel area with one or more large stone planters planted with sedums, even desert-y shrubs--often a low bowl shape looks really good, or sometimes even a driftwood look. Or you could go sort of Japanese, with JM's in containers and other related plants. It can take some doing to make it look restrained and architectural and not just tacky. In these example you are not going for a lot of planting, but some kind of living focal points in the gravel area.

There is a book, female author, on planting in gravel, also book on growing on hard surfaces! This works when you are in fact planting somewhat into an underlayer of soil, or, as I have done, in really "dirty" gravel which functions like sharply draining soil. There is more maintenance with this approach as it is planting stuff through a gravel mulch deal and can get messy or look spotty. If you have six inches deep of pure fresh gravel, I don't think you culd dot around small plants and expect them to establish well and spread fast--but they might--but you certainly could plant a large specimen shrub or small tree.

You also might be able to edge the area with plantings as you would be accessing the soil there, and keep the center clear.

If it is an unmitigated boo-boo, only you will know whether y'all can agree to undo it.

In his book "Gardening Success with Difficult Soils," Scott Ogden lists some plants that will thrive on raised beds of granite. Perhaps they would also grow in gravel. They included sedum, pink iceplant, low-growing junipers, lamb's ears, santolina or artemisia, Ceanothus, butterfly weed, south african gazania daisies for annual summer bedding. He also lists many bulbs that will thrive and become permanent in crushed granite beds. Just a thought. I'm not an expert gardener.

Ugh, I'm sorry Diggin. I have been removing the lava rock mulch that a previous owner spread all around our house and I know the challenges you are facing. : (

I'm just a landscape designer and have no real advice. If I were you I would cover it up with a large hot tub surrounded by nice tropicals in pots. Your hubby only gets to enjoy the hot tub AFTER he removes the pink rock mulch. : )