Perennials to Interplant in Grasses

Of all my plant obsessions, herbaceous plants are among my favorites. Several years back, I remember walking through the woods with a colleague, a tree expert. After several hundred yards, we laughed at each other. He was always looking up at the canopies, and I was always looking down at the ground—scanning the forest floor for herbaceous plants. It is my perpetual posture: head down, scanning right to left.

Amorpha canescens
As a perpetual ground-scanner, I’ve recently had a revelation about the way many perennials and grasses are meant to grow together. This revelation has influenced the way I design.

It’s actually quite simple: many perennials from meadow/prairie ecosystems have evolved to grow within a matrix of grasses. While that is not a particularly ground-breaking concept, it does challenge the way many perennial gardeners arrange their plants. Most perennial gardens focus heavily on forbs (blooming perennials) that are scattered one by one in planting beds. Grasses, if used at all, tend to be added as specimens or accents. But if you consider the way most meadow perennials grow, this ratio should be reversed. The grasses are meant to be the dominant plants with forbs emerging through this matrix.

Consider the morphology of an Echinacea (Cone Flower). Echinaceas typically have low basal foliage and tall spindly stems which support the flowers. This very structure is designed to help the plant grow out of a lot of grasses. The low foliage first emerges in late spring before the warm season grasses emerge, grabbing sunlight to ready the plant for its flowering. Once the grasses put on their height, the Cone Flower sends up its flowers on delicate stalks. The grasses support the flower (like a stake). If you’ve ever had perennials flop over, it may be because it is missing its support system.

As a gardener or designer, this does not mean that your perennial gardens need to be mostly grasses. It does, however, provide a real opportunity for people interested in designing with ornamental grasses. I love the look of large masses of ornamental grasses in a landscape. They are easy, low maintenance, have a long season of interest (particularly in winter), and add a wonderful looseness and spontaneity to a landscape.

Dalea purpurea growing in grasses
Any time you plant a mass of ornamental grasses, you can interplant them with perennials. It just so happens to be an excellent strategy for low maintenance gardening. The grasses cover the ground and provide a long-season of reliable interest; and the perennials emerge through the grasses as a colorful accent. I typically use a ratio of 90:10 or 80:20 grasses to perennials. That allows the grasses to still read as a continuous mass, providing legibility and tranquility to the composition.

This strategy is particularly good for people who are worried that grasses look too wild or weedy in a landscape setting. The presence of blooming perennials makes the grasses helps the composition look more gardenesque, less wild.

Aesclepias tuberosa and Veronicastrum virginicum

So how do you interplant perennials in grasses? Be sure to carefully select the right perennials to plant with the right grasses. Any time you interplant two different species, the species compete with each other. Sometimes the grasses (who tend to have superior root systems) will “eat” the perennial accents after a few years. The key is to pair perennials and grasses of the same basic height and competitiveness. If the perennial towers over the grass, the composition can look too chaotic. Use perennials that are equal or just slightly taller than the grass it is planted in. It is also important to pay attention to the origin of the perennial. Use perennials that have evolved from a meadow setting as opposed to a woodland floor setting. They have very different competitive strategies.

Echinacea, Liatris, and Aesclepias interplanted in a mass of Sporobolus heterolepsis
Through lots of trial and error, I have found that perennials with vertical habits (as opposed to spreading or mounding habits) look the best in a sea of grasses. Mounding perennials can create larger “holes” within a mass of grasses, weakening the visual legibility of the massing.

Below, I have created a chart of perennials that are especially good to interplant within a mass of ornamental grasses. How you arrange your perennial accents depends on the scale of the grasses their planted in. In really large grass area, it’s possible to create drifts and hints of patterns with the perennials. In smaller massings, you may want to dot perennials individually or in small clusters of 3 or 5.

Interplanting is one of the more challenging aspects of gardening. To be honest, the majority of my experiments in mixing perennials have resulted in messy, chaotic compositions. But the handful of successes is worth the effort. Have you successfully interplanted perennials in grasses? What worked for you?

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