We put in a nice flagstone patio...I had high hopes of planting something between the cracks. But had a neighbor say they tried and gave up because of needing to pressure wash the stone he would kill the plants. That the flagstone would get a green covering on it which gets slippery web wet. I was skeptical...but sure enough my patio now has areas that need pressure washed and it hasn't been a year it has been down.

So my hopes are,that someone here has dealt with this and found a solution...possibly a hardy ground cover to withstand one attempting to individually cleaning the stone with a pressure washer and it handling the stress of water that is forced on the stone beside it to clean the stone.