What kind of fish are bumping into them? and what kind of plants.
I'm using Turface (light colour of the same stuff you have) and the plants that grow roots can hold up to a 14.5" Sailfin pleco (if he doesn't snap the stem).. Now if I could get the loaches to stop helping him...
Even Glosso can get an excellent hold in it. I would imagine plants like Cabomba and anacharis might never get a hold in it since they don't have roots. In those cases, try lead type anchor (not many things are lead anymore) and bury it with the plant in the gravel.
120 Gal SW, 150lbs Prem. Fiji, 23Gal Sump, 2x250w 15K MH, 4x65w Actinic
3 x Halichoeres Chrysus (1 surfed), 3 x Nemateleotris magnifica, 1 x Centropyge bicolor, 11 x Scarlet Hermits, 6 x Zebra Hermits, 40 x Astrea Snails, 6 x Nassarius Snails, 3 x Cerith Snails