The ornamental grass, Calamagrostis x acutiflora ‘Overdam’, is a plant I use more than any other in my own garden as well as being a theme plant in many of the perennial meadows I design.
It forms a dense clump of arching leaves very early in spring. This is a characteristic of those grasses that grow in cool conditions and which tend to become dormant when temperatures rise too high. We call them as a group cold season grasses to distinguish them from the warm season grasses which, not surprisingly, are those that do not start growing until temperatures are high enough.
Needless to say, nature does not only follow the definitions we like to attach to it and there are some grasses such as Sporobolus heterolepis which fall into a middle group that are neither typical of either cold or warm season grasses, but start into growth along with other garden perennials but which tend to take their time to get around to flowering. Most books simply tell you sporobolus is a warm season grass, but actually they are wrong!
The arching foliage of Calamagrostis x acutiflora starts to emerge in late winter (February in the Netherlands) and by the time spring bulbs are in full flower, in April, they make dense clumps that will create a bold background setting for them.
The grass flowers in June, producing amethyst colored clouds that fill the air – held high on tall, slim stems. This is a magical moment that does not last long – two weeks maybe – but there can be a problem. Should it rain, these voluminous flower heads collect water droplets and tend to bend over, sometimes touching the floor. This is not a problem in itself, as once they dry out they regain an upright stance; but should you have planted this grass near to a path, it will be blocked or the flowers trodden underfoot.
The flowers quickly resolve themselves into stiff, buff-colored, upright seed heads that are gathered at the tops of the thin stems to create an uncompromising column that stands out clearly from its neighbors in the flower border. For a garden designer this is an irresistible shape to contrast with anything loose and rounded nearby. The grass can also be planted in blocks and rows to build architectural features. I use it to block access, guide visitors around the garden and to create enclosed areas – all possible, because it is tall enough, and dense enough to block a view.
The cultivar ‘Karl Foerster’ is the most commonly recommended and at a mature height of 1.8 meters or 6 feet it is most impressive. I favor C. x a. ‘Overdam’ as it is slightly lower growing and therefore seems to fit into the domestic gardens I work with much better. Its leaves are actually variegated; on emerging in spring this fine variegation is bright yellow, but within days that gives way to white. The fine leaves with their fine variegation take on a light grey green tone and in no way does the plant look garishly variegated, instead it stands coolly aloof.
The upright form of this grass remains effective throughout summer, autumn and winter. It is hardly damaged by winter’s onslaught, but in early February we must be resolute and cut the plants back to the ground as within days the new shoots will be starting to appear to begin its dynamic cycle once more.
Could C. x a. ‘Overdam’ be worthy of the title of the best perennial grass of all? In my opinion there is no contest.