Some people say that planting very small groups like a few individual stems is the best way to get it to grow.
I originally did it like you and had very little luck with it. That is my experience. You have to make sure that you remove as much of the potting material as possible and have over 2 and closer to 3+ wpg IME. I think that part of my original problem is that My fish kept messing with it and uprooting it. Also I have a really high flow in the area that it was planted so that also made it come up. IMO the best way to start is to get a bit that has been grown submersed in a forum members tank and start with very small groupings, 2 to 3 individual stems.
BTW are you using CO2 and ferts, I think it helps immensely with this plant.
I just tried again with it as above and hopefully it will work better this time. Others on the forum have had better experience with it than me so I hope your's goes like that.
You can do that or you can spend an entire evening seperating them into small pieces and using tweezers to plant individual stems. I've done both and both will work well. Now I only do as you did it. I saw no need or benefit from seperating stems. The most important factor is having the right tank conditions. Good ferts, good light (3+ wpg) and plenty of CO2 or Excel is what's really going to get the HC growing.
I did kinda both. I got one pot and planted small clumps, and then I planted stems in other places. The HC has nearly covered the foreground and it grows really, really fast. IMO good light, ferts and co2 is what makes the difference between a nice carpet of HC and a sparse foreground.