Is it too late to lose the rock and start over with an organic mulch?

The only thing that fabric does is help keep the rocks from working into the soil. You're going to get more weeds in your rocks from weed seeds blowing in and germinating on TOP of the landscape fabric than you will from them coming up from underneath. And you'll tear up your hands pulling them, compared to hoeing them up or lifting them with a trowel as you can when it's a bark or leaf mulch.

I've cursed the previous owners of our house (or, more accurately, the team that the builders hired) as I've shredded my hands trying to get dandelions out of sharp-cornered flint pebble mulch. All the beds I've put in have been topped with compost and bark mulch, and I'm gradually removing the rock and fabric from the existing beds.

Sorry; I spent Sunday dealing with those damned rocks and Monday bandaging my hands.

To answer your question, I would think that adding Preen in the crown area of your plants would burn the foliage badly. Plus, it would take out any new perennial seedlings, which you might want to keep in your garden. And Preen won't take care of the weeds popping up on top of the river rock.

ok thanks and now I am scared---yep too late to get rid of the 6 tons.....but we shall see

I have a rock bed that I has only thyme growing in it and plan to put in another. Before I put the river rocks in I laid down newspaper and pages from old telephone books to help control the weeds. I also use needle nose pliers to pull out whatever weeds that pop up.

I too have torn out that nasty fabric and hauled out rock....
If you don't have a problem yet....be aware you will!
I sometimes use preen, but I don't put it on the plants....but sprinkle it like pepper on the soil where I don't want seeds to grow. And I have stunted and killed plants by sprinkling Preen before I put in the plants....and gotten the Preen on the roots.
Linda c

I have stuff double the strength of regular Preen. I killed half the foliage of my boxwood too. I wasn't too careful the first time i applied, but next time i will take care, and not spread on a windy day.