I don't have a PAR meter but from my experience, I had a 3 gal tank with 2" of mineralized potting soil covered with 1" pool sand. Lighting was a Sunpaq dual daylight 10000k & 6700k 18w located 7-8" above the substrate. No heater, no co2, ph 8.2 (tap). Dry started the HC for 10 weeks, after the tank was flooded the HC propagated quickly along the substrate surface. It took a lot of care for the first 3 months, dealing with the algae, but afterwards it was minimal care. If the light was raised higher the HC leaves would grow smaller. When the light was changed to a Marineland double bright led 6000K the HC dislodged from the substrate in clumps. The roots were very short only 1/4" in length. Even when the Sunpaq light became old the HC never dislodged, the leaves grew smaller and the depth of the growth decreased. Regarding the HC dislodging from the substrate, I could assume that it depended on the temperature of the light and not the intensity.