MyGardenSchool students, for whom i wanted to clarify a point which I clearly had failed to do in the material, given how many ask about this - but this is an important issue so I putting it up as a blog post. The point I want to make is about how grasses grow and what this means for us as designers of plantings and as gardeners who have to manage them long into the future.
The Achnatherum brachytrichum (Calamagrostis brachytricha) is a good example of a cespitose grass, one that forms a distinct tight tussock. Actually there are many which are a lot tighter, and of course like everything else in nature this is not a hard and fast category. Diversion - it is much easier to think of gradients rather than categories very often when trying to classify natural phenomena: the infinite shades of grey between black and white (joke for workshops - there many more than fifty shades of grey).
We love cespitose grasses, we like the visual appeal of that neat bunchy habit, and they stay where they are put, unlike many of the older generation of ornamental grasses, that ran all over the place and gave them all a bad name. However, they may seed all over the place - and I am going to come on to that.
The one on the left above is an Eragrostis sp. (old pic, lost name - sorreee!). The other is Calamagrostis 'Karl Foerster' raised by the great man himself and pretty much one of the best ornamental grasses we have. Notice how different in habit they are - the Eragrostis stems can all be followed in the mind's eye back to a tight base, so it is clearly cespitose, while the Calamagrostis stems go straight down - that tells you that this is going to be more of a mat former, and it is going to continually spread, in theory, ad infinitum. Cespitose grasses on the other hand, get to a certain size and more or less stay that way, forming a dense tussock. They do grow outwards after this point, but so slowly it can be virtually disregarded.
A cespitose grass, drawing thanks to Ye Hang. The bunch - get the point!
Molinia caerulea - an old clump at Hummelo. See how tight it is, and that very clear edge. Go up to a really old cespitose grass and if you give it a kick, you will appreciate how hard and solid that base is.
What about seeding more generally?
Calamagrostis 'Karl Foerster' is sterile, so that is ideal. Nor does it spread strongly. I grew one of its parents, once, C. epigejos, a mistake, very aggressive runner, especially on our moist and fertile soil. Had to get the Roundup out - fast.
Miscanthus have gotten a reputation as invasive in some US states. I worry about others, like the Achnatherum I started with here. This Asian species has begun to take over the High Line, so the question will be, how will it compete with the other plants there? Will it establish a balance? Or will it lower diversity? In the garden at home Molinia and Deschampsia are in their element , as they are locally native plants, and do they seed? A lot! To the point where I am beginning to wonder whether I really want them. Or what should we say to clients about long-term management? Or could we end up with relatively stable forb-cespitose grass combinations?
Warm-season American or Asian species which do not seed in our summers, even if they are very slow to get going, have huge advantages in this respect. Unlike many forbs, when grasses seed, they often do so in massive quantities.
There are lots of questions. I feel we are only just beginning to learn some answers.
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