You did the perfect job of planting the hairgrass, after a week trim it down to about a half inch and it will spread like wildfire through the open area. If you want to let it spread through the tank do nothing, if you want to restrict it to that area use strips of clear plastic inserted into the substrate as far as you can push them, and sticking out above the substrate about 1/2 to 3/4 of an inch. This corrals the hairgrass and stops it from sending runners everywhere. You will still have to watch for the occasional stray that gets around the end. It isn't easy to see the plastic, when the plant club came to my house I had to point out the plastic I used to keep an open area in the front of my ten gallon tank. This was a group of people looking very hard into my tanks.