This is the third post in a series I’m doing on perennials and grasses in the larger landscape. I’ve made the claim that perennials and grasses—possibly the most dynamic and interesting plants a designer can use—ought to be used more often in our built landscapes. Imagine our public landscapes, yards, and office parks cloaked in a rich tapestry of sustainable and beautiful perennials and grasses inspired by the patterns and palettes of our native vegetation.
My last post talked about a compositional strategy of massing plants in order to reduce their maintenance and increase their legibility. This post will focus on the second strategy: how to choose the right plants. Plant selection is absolutely critical to the long term success of a planting, and to be honest, it’s not easy. Here are some general strategies that can help.
What Plants Do I Choose?
Choosing the right plant for the right spot is hard enough. With every site, there is a dizzying list of cultural requirements (exposure, slope, soil, climate) that one must consider. In a larger landscape setting, there are additional design factors to consider.
1. Filler Plants to Create Volume
Large perennial and grass beds are most attractive when they create volume against a void, such as a lawn, path, or street. Let me share a secret with you. Great perennial planting in landscape settings is not about perfectly balanced flower colors—though color matters. It’s not really about creating great photogenic combinations—though combinations add style to a composition. Half the battle in creating herbaceous plantings that endure is to simply cover the ground at a relatively uniform height. If you can find perennials and grasses that thickly carpet the ground and range in height from 12-42 inches, you're halfway there.
In his books and interviews, Piet Oudolf talks much about the distinction between structural and filler plants. This distinction is key to his compositions. A structural plant is one whose form is distinctive and architectural, whereas a filler plant has a more amorphous, cloud-like form. Consider the strongly structural flowers of an Echinacea or an Echinops. In a composition, the eye will fall onto these distinct forms. These plants also tend to dry into distinctive seed heads in the fall and winter, creating enduring interest through the year. Filler plants include most ornamental grasses (Switchgrass) and many mounding perennials (Asteromea mongolica). Oudolf recommends using a ratio that heavily emphasizes structural plants to filler plants, around 70/30, mostly to make sure the composition has strong form throughout the year.
My advice is to reverse that ratio. Of course, Oudolf’s work is undeniably beautiful and masterful. But I have two problems with using that many structural plants in landscape settings. First, structural plants tend not to cover the ground as well as filler plants. So when they’re not maintained well, it creates gaps where weeds can fill in. Second, since structural plants are more about a distinctive profile of a flower or leaf, they tend to have shorter bursts of interest. Yes, they look great in winter, but there are moments in early spring, for example, when they’re wiry or just not as full.