The roots of your groundcover plants can range about under the flagstone to find what they need. One of the problems with putting anything other than sand between the stones is that soil in that anything else makes a hospitable home for weed seeds. Not that sand is an ultimate answer, not even mortar is that, because organic matter does settle even on flat stone, and hosts weeds. When you weed, even in sand, you pull up dirt through the sand layer and create even more weed territory. I would use sand and, until the groundcovers grow in, plan on careful maintenance.

KarinL

Thanks KarinL for getting back to me on this. I was afraid no one was going to answer my question. The only thing I'm concerned with is that I want to make sure the wooly thyme is going to grow and fill in properly. I have read that if you're planning to put ground cover in between the stones, you need some kind of loam. A friend of mine put in the same kind of patio last year, and all her ground cover died. She was told that you need some dirt mixed into the sand to get the ground cover to get established. So, now she is taking out most of the sand from inbetween the cracks and replacing it with a mixture. I just don't want that to happen to me. Maybe I should mix like 70% sand and 30% soil and even add some fertilizer to help it along. I will be watering it. What do you think?

You could use sand until your stones are secure then fill in the top half inch or inch with soil for your groundcover.

Brat

If you are using flats of groundcover or even purchasing them in individual plugs or pots, there is sufficient soil already attached to the roots to get them established. One characteristic many groundcovers share is a very shallow and spreading root system - they don't need much soil to keep them happy. I have quite the assortment of various groundcovers growing in several inches of crushed rock under my GC tables at the nursery - little chunks broken or dislodged from the larger containers - and they thrive in virtually no soil at all!

I'd fill between the pavers with the sand and plant the plugs of groundcovers as is, with the container soil attached to the roots. As long as drainage between the pavers is good (and no reason it shouldn't be), they get enough sun, you don't walk on it too frequently for a couple of weeks and water as necessary, you should be good to go. No need to fertilize - most GC's require none.

This is the method I've used both professionally and at my home. I've got a variety of different groundcovers growing between recycled concrete pavers set in a crushed rock base - no added soil except what was attached as they came from the pots.

I just wanted to say thanks for the info. I had the same question...though it might be a year or two before I get around to building some paths. The couple books that I looked at completely glossed over this topic.

- Brent

Thanks for explaining that to me. Maybe the reason my friend's patio GC didn't live was because she didn't water it enough or something like that. I know how fast little pieces of GC can die in our summer drought conditions without adequate water. So, I will use the sand to fill in the cracks and just plant with the dirt from the GC plugs and just keep an eye on it and make sure it is watered well during the summer until it gets established. Thanks so much for all your help!

Planting in mid-summer is often a bad idea. I'd put them in in early spring or in fall. I suspect that, aided by the original soil plug from the pot, the roots are good to go once they get through the sand base under the flagstones and into the soil below.

KarinL

Even though this is a very old post, it is exactly what I was searching for as I just put in a flagstone path with a crushed rock base and sand layer. I was doubtful about planting between the stones until now. Thanks for the confidence gardengal48!

By the way, the original poster asked about loam; it is a class of soil. It packs and holds together well and is probably why some have recommended it for this application.