Grass and rock gardens do not mix.

If it isn't the roots sneaking into the middle of a treasure the seeds surely will and the seedlings are hard to take out.

And some rock plants are very slow to spread. If there is grass there - they can be smothered - which is a waste of money.

It is vital in a rock garden to remove all the big and bossy plants first - grass included. If plants have made seeds in the area you're using then you will have to weed and mulch for several years. In a rock garden you can use rock chips/gravel and this can reduce the number of weeds you have to tackle.

From the pictures you provide - I think that grass seeds will be the problem, more than runners underground. If you can remove them now you've reduced your work for next growing season.

Hope your rock garden is a great success.

thank you vetivert8. I pretty mucn wanted to remove them, yet I need some encouragement and a good reason;-)

If that were growing in my area, it sure looks like what we call watergrass, but is more widely called barnyard grass. It is an annual grass, member of the sorghum family. It develops quite a root system and will bring large clumps of soil if you don't pull it until it is fairly big. It is easy to pull and remove when small. It will spread by seed if left to mature.

Frost will kill it.

This grass doesn't belong in a rock garden. Hen & Chicks doesn't belong in your lawn. Each has it's place, elsewhere it is a weed.