Sunday, November 30, 2014

Grasses - update on facts and fears

Grrrrrasses... I am writing this basically for the 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!

In the wild, this being Brechfa Bog in Radnorshire. Deschampsia cespitosa on the right is clearly, well yes, cespitose. The rest of the green stuff is a mix of the various turf grasses that are far more typical of our north-west European grass flora.

This is how they spread, running through stolons. These turf grasses are at the opposite end of the spectrum to the cespitose grasses. Turf btw, for the Americans = sod.

Many grasses are somewhere in-between, and can be called mat-formers, as they spread out at an appreciable rate, but do not charge out like turf-formers. Turf grasses can be mown, which is why are great for football pitches, sunbathing, picnics, croquet, open air theatre, making love on etc and all the other things we do on them. Couldn't do much of this recreational stuff on cespitose grasses.

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.
In contrast, Miscanthus sinensis is a mat-former, slowly spreading, and in this case, leaving behind some dead patches.

What applies to grasses, applies to sedges too - this is a strongly-spreading Carex glauca (C. flacca).

C. muskingumensis is intermediate, forming tight but distinctly spreading clumps.

But many sedges are truly cespitose. Nice tidy little chaps these, very decorative.

The leading edge of a Calamagrostis and below a Miscanthus, an ill-defined edge, in contrast to the clear end of a cespitose tussock. It is possible to see even at this time of year some new shoots.

Stipa gigantea, a good example of a cespitose grass. Ones in older gardens can be decades old, forming great tussocks. Many cespitose grasses are very long-lived. The ruler is measuring out 30cms.
Nasella tenuissima, formerly Stipa tenuissima, an example of a cespitose species which is very short-lived, and as you might guess, these are seedlings. It can seed around very freely, but not enough in my garden! Species like these are clearly pioneer plants, the longer-lived cespitose stress-tolerant.  Turf grasses are basically competitors. Globally there are more cespitose than turf grasses - but we in northern Europe get a very distorted view of grasses, as our turf grasses are the exception. And of course now they have spread everywhere.

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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Unknown said...

I'm a landscape designer in Virginia. I have always been fascinated by grasses, but I no longer spec Miscanthus grasses in my designs; you can already see them beginning to take over the road edges where they've seeded in from a planting upwind. Our soils and pH combination are quite similar to their native (Japanese) environment. (Aside from digging them out when you notice new seedlings, I can recommend mowing the crowns low. It works.) I'm using natives (Saccharum, Muhlenbergia, Panicum) and grass alternatives like Carex now.

Roger Brook said...

I do find many of these clump forming grasses eventually do get too big and of course they are a devil to dig up, even to get out small pieces as divisions nearly break your wrists.
Readers of my blog will have seen or guessed how I deal with the problem!