I've kept hair grass but never actively restrained it, could you elaborate on how to "train" it like this? are you just cutting the runners down to the substrate level?
You can create "barriers" of other plants by planting around the hair grass. While it's viral, it has a great deal of difficulty overcoming other plants in the way of it.
I discovered this in my first iwagumi by mistake. Where I had already mostly grown an HC carpet, and when I wanted to add hair grass, it never seemed to grow. The reason was because the HC essentially had blocked out it's growth in all but a small area. When faced with this challenge Hair grass does one of a few predictable things:
1. Stops growing
2. Grows underneath the substrate
3. Shoots runners over the plant hedges (easy to cut)
I reinforced this mistake by doing it in my Mini S and Mini L in Seattle - where hair grass absolutely would not grow past the hedge of HC, if you look at the double tank shot on page one, notice the Mini S (Front left tank), where there is absolutely hair grass in the tank, but it's almost imperceptable because the HC blocked out the growth of the hair grass.
You can manipulate the growing points of other plants this way as well. In this particular layout I have a few artificial barriers:
1. Wall of moss stones. Hair grass will grow underneath the stones and prop itself up in spots not covered by moss if allowed.
2. E. Tenellus. This will through out underground runners and does not grow as thick and dense as Hair grass initially, so it will cover more ground and is a larger plant, so the roots will block growth inevitably.
3. HC and stones on the right. This will create the hedge on the right side that blocks hair grass from penetrating to the other side of the layout.
Techniques to stymie growth in the beginning:
1. Not trimming it as early as usual. I'm letting the emmersed growth stay for at least another week. This will drain the plant of 'energy,' while it focuses on growing out and trying to keep the old plant matter alive. This also will ensure that the fresh new green growth will grow vertical.
Trimming hair grass will encourage it to send out runners in a massive dash for survival. So by not trimming, we signal the plant that it's in a good spot and can just grow up, not out.
So by stacking the odds in favor of the other plants, and with practice I can manipulate exactly how much hair grass will be in the layout.
Essentially a ratio of 5:1 other plants to hair grass will help keep the odds balanced.
This is why learning the growth patterns of each plant is essential to a mixed carpet.