Stipa tenuissima and Nassella tenuissima are both the same grasses - divided only by the choice of name. Nassella tends to have the common name of Mexican feather Grass. It is a dense tuft of foliage which opens at top, graced my many feathery, pony tail flower spikes. Foliage and flower alike are slender - if massed. As such this grass is absolutely perpetual motion!
In the slightest of breezes - or even if you just walk past it so it seems - the feathery plumes start to sway back and forth like a drunkard. If it were to stay rigidly at attention, the grass would be nowhere as popular. Welcome to Stipa - or Nassella - tenuissima. (Tenuous - the word - meaning; very thin in diameter; flimsy: Aptly descriptive). Whispering Grass is another common name.
Stipa tenuissima forms a narrow fountain from the base, which if no breeze - forms arching mass of foliage and flower stems at a height of around 2ft (60cm). It rarely extends more than 12in across at the base, with the top of the plants flowing out to double that.
It has a reputation for being invasive. this is not because of some hidden root system, but by its eagerness to sow its offspring anywhere ir can find in the garden. Within a year of planting, you will start to find to small tufts in the most inhospitable places - if your garden has such! They can either be potted up and grown on for a while before planting out, grown as container plants, or simply left insitu. In gravel paths or dry gardens they will thrive. You can of course have too much of a good thing, so be careful where you leave the seedlings that you are sure to find.
Nassella tenuissima can be grown from see sown in the spring. It does this quite admirably without help, but if you have just bought the seed - and have no parent plant - then sow the seed in a seed tray in cool glasshouse, coldframe, or any other sheltered place. Pot up the seedlings and plant out in early summer.
It can also be divided. Easy to do, just prise the root system apart with a couple of stout garden forks, and plant out the divisions where you wish them to grow. Do this before the plant breaks into growth in early spring.
No problems with diseases or pests.