Tall Hairgrass is one of the most popular carpeting plants in the aquarium hobby. It's relatively easy to grow, and it's extremely appealing when done correctly. If you're thinking about Hairgrass for your aquascape, let's talk about some of the specifics about the plant, and the optimal conditions for it to grow quickly and healthy.
This plant thrives in a nutrient-rich substrate. While it's not critical to its success, you'll have the best results in your tank with a good substrate. If you're going to have a carpet in your planted tank, especially a Hairgrass carpet, you'll probably want to use some sort of CO2 system. Whether that's DIY or pressurized is up to your budget
Planting Hairgrass is similar to planting any other type of carpeting plant: you'll need to clean the plant after you receive it, separate it into smaller clumps, and plant in a gridlike pattern. This ensures that you aren't getting any detrimental hitchhikers on the plant, as well as prepping it to cover the substrate as quickly as possible.
The key to a great DHG carpet is consistent, even trimming. Left alone, Dwarf Hairgrass grows vertically, not horizontally. That's why it's important to continually trim the plant quite often to stimulate horizontal growth, forcing the growth outwards, not upwards. This is even more true if you're using CO2: Hairgrass has explosive growth in CO2 and high light. However, I wouldn't suggest this for beginners; that's also a very easy way to have algae take over your tank.
It's also a good idea to keep a fish net around when you trim this stuff: it gets VERY messy. Get as much of the trimmings out as possible, but what's left will simply rot away, so don't stress about it.
If you're using CO2, medium lighting, and consistent Fert dosing, you'll want to trim your hairgrass at least once a week. With lower lighting or no CO2, it grow significantly slower, so you can get away with trimming maybe once every two weeks or so