By Chris Erickson
The mountain mints, of the genus Pycnanthemum, deserve more of a place on intensive green roofs. There are roughly 20 species of Pycnanthemum and several are regularly encountered in the native plant nursery trade. Here I will focus on two: Pycnanthemum muticum (commonly called short-toothed mountain mint) and Pycnanthemum tenuifolium (commonly called slender mountain mint), that can fill different needs in the planting landscape.
Pycnanthemum muticum (Short-toothed mountain mint)
P. muticum is a great plant for creating large masses. It grows to about three feet tall and creates a dense, pillowy leaf canopy. During the spring and early summer the leaves are a vibrant light green but by mid summer white flowers develop. While individual blooms are not showy they are accompanied by small off-white leaf-like bracts that, when seen from afar, create a delightful effect. It looks as though the top leaves of the plants have been dusted by a silvery powder.
The most notable quality of P. muticum may be its excellence as a pollinator plant. The flowers are beneficial to a wide range of insects: butterflies and bees in particular. In a study of various plants’ value to pollinators by the University of Penn State P. muticum ranked first in both total numbers and diversity of insects attracted. The blooms last for roughly ten weeks and while the flowers are open I rarely see a patch of P. muticum that doesn’t have a hive of diverse insect activity around them. In my own gardens this plant has certainly been the most consistent, dependable pollinator attractor I’ve worked with.
My favorite quality of P.muticum to share with others is its smell. When the leaves are crushed they release a wonderful spearmint aroma. In my experience it is the most aromatic of all the mountain mints, with almost a creaminess to it. Even brushing against the plant can be enough to trigger the odor.
P. muticum is tough, beneficial to wildlife, and smells wonderful. The downside is that it can be an aggressive spreader and sometimes flops over later in the year. This plant spreads via underground runners and I have seen patches grow by a solid three feet a year. If you need to cover a large space this can be an asset but in a small garden it can be difficult to manage. To keep a patch of P. muticum from spreading, cut and remove roots that have reached beyond the edge of the desired boundary. The best time to do this is in the fall as roots will send up strong vegetative growth in the spring. The best strategy, however, is to use existing infrastructure to your advantage and box them into a corner where they have less room to spread.
Highview Creations has featured P. muticum on two of our roofs. At Gracie Square Hospital we planted them along a fence which keeps them hemmed in on one side. During the fall we pruned spreading roots to maintain the planting boundary and cut back some of the longer stems that had flopped over at the edge. At the Lenox Hill Neighborhood House we mixed them into a mixed-species prairie-like planting. In this planting they a
re paired with other robust, taller species such as big bluestem, swamp milkweed, and joe-pye weed. These plants are able to compete with the P. muticum and keep its propensity to spread in check. Both of the above sites utilize irrigated green roof soil.
Pycnanthemum tenuifolium (Slender mountain mint)
Among the mountain mints, P. tenuifolium occupies a different niche in the planted landscape than P. muticum. P. tenuifolium is a bushier, more compact plant whose delicate narrow leaves (where the common name of the plant comes from) add great texture to the garden. When viewed from afar the shape of P. tenuifolium’s foliage is clumpier than the dense, smooth canopy of P. muticum. Overall P. tenuifolium are more structural and work well as individual plants in the landscape.
Like P. muticum, P. tenuifolium flowers in mid to late summer, developing flat-topped dense clusters of white flowers that are showier than the flowers on short-toothed mountain mint. However, P. tenuifolium does not develop the same whitish bracts that give P. muticum it’s silvery coloring. Where the flowers of P. tenuifolium really outperform P. muticum is in the winter. The stiff stems of P. tenuifolium hold their form very well through the winter and the dense flower clusters of summer become dense seed clusters that provide great winter texture. P. tenuifolium is also valuable as a pollinator plant, though in my experience not to the same degree as P. muticum.
For the garden designer the biggest difference from P. muticum is that slender mountain mint is not a vegetative spreader. It is a strong seeder though, and if you let the flowers go to seed you can expect to find slender mountain mints popping up around the garden the next year. I personally prefer to remove seedlings as opposed to vegetation that is spreading from roots. Vegetative rooting has the benefit of being tapped into the parent plant’s resources, and growth from seeds is often much more delicate. In many cases new seedlings are just out-competed by the plants around them. In a loose, wild garden P. Tenuifolium can actually be helpful for naturally filling in bare spots because of its ability to scatter its seeds around. However, on a roof that receives minimal maintenance P. tenuifolium’s ability to seed itself could become an issue.
Both P. muticum and P. tenuifolium are tough plants. P. tenuifolium is more drought tolerant, preferring sandy, poor soils. This makes it an excellent candidate for more lightly or even non-irrigated green roofs. Both plants, and mountain mints in general, are hardy from zones four to eight (approximately southern Mississippi to northern New York), and prefer full sun though they will tolerate some shade. These are delightful, workhorse plants that are beautiful, a benefit to local wildlife, and an excellent resource for the growing footprint of green roofs in our cities.