Thursday, January 12, 2012

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?


  1. Great insight. You ought to have used it and the lists to sell a book. I've always been unable to accept the mathematical\geometrical formulae for planting meadows; this makes much more sense to me. No offense to Piet.

  2. Spot on! I've actually been considering this approach for my front parking strip this summer...we'll see if I'm able to pull it off :-)

  3. I agree with Chris about the book, but I am going to go ahead and print off those awesome lists for free while I still can! :o)

  4. Great work, Thomas! I know how much effort and time you put in to figure out this chart.
    I know because "yes, I have" is my answer to your question: I'm inter-planting often, on small scale borders - appr. 2.5x8 m or so. Having Piet's private garden close by (very different from his public "architectural" creations!), having many more examples of other Dutch designers and enthusiastic gardeners who are into grasses+perennials combinations, I'm learning and experimenting in my own garden. My method is very similar to yours, also based on scale and combination of forms for better all-year round look. I have to say that already first cycle, when completed, shows where the gaps are, and it's an ongoing joy to carry on such an "experiment". Sometimes grasses act unexpectedly, even similar species in the same border - one suddenly grows enormously tall, while the other stays normal, etc- so there is never a dull moment. Which is exactly what i like. Now pondering a new planting plan for such a border in a shady area, checking in Piet's and Ruy's books, but also your table is great to look into. Thank you, like your blog.

  5. You are singing my song. Great topic and excellent information! If you do not mind I will be including a link to this post in an upcoming lecture I am doing on ornamental grasses.

  6. Hi Thomas, very nice and interesting post! I enjoyed especially the last picture with echinaceas, liatris and sporobolus. I never tried to create a matrix yet but this would be eventually my next step. Interplanting perennials in my grass garden has produced some good combinations as well as messy combinations, anyway grasses soften fatal error that occur in a garden... I mean: a messy border is better than a horrible one, isn't it?


  7. Hey Chris,

    Not sure I have it "together" enough to do a book, so I must inflict you poor souls with my various rants. I understand why forbs are such an attraction, but it sure is a lot easier to design with a higher percentage of grasses.

  8. Scott,

    I'll be eager to see the results. If it is anything like the rest of your garden, I'm sure it will be smashing . . .

  9. Mary,

    I'm a voracious stealer of free information! I'd say my advice is about as reliable as anything else on the internet ;)

  10. Olga,

    You make some great points about experimentation that I wish I emphasized more. Interplanting is so unpredictable. Even combinations that worked for me on other gardens fall apart in a new setting. But this kind of experimentation is at the heart of gardening. I find it endlessly fascinating, in part because it is so challenging.

  11. Danilo,

    Thanks for the comment and please link away. I think large masses of ornamental grasses interplanted with perennials would look great in your part of the world. Adds to PA's rustic charm. Best on all your great projecs. Thomas

  12. Alberto,

    Thanks for the comment. I totally agree about grasses "softening the errors". They are a great filler material with a long season of interest.

  13. Thomas, I guess I've been doing something similar, though not exactly what you describe. My mixture is probably more 60/40 grasses to forbs, but my plantings use large plants, and don't resemble a meadow, unless it's a meadow on steroids. But I thoroughly agree with your basic tenet that mixing grasses and forbs is a great idea, and with your recognition that many forbs evolved to grow in grasses.

    You're doing some really important work. I do wish you had time to turn your thoughts and experience into a book.

  14. This is a revelation--funny how looking to nature can do that. I have struggled to create appropriately scaled beds of massed perennials in a small yard, and I also desire grasses for their multi-season interest. This could be the finishing touch my garden needs!

  15. I like lists and associations. Thats the best way to think about design, is summing up parts...and you've proven I must get back to my own lists of companion plants.

    Perennials (and bulbs) in grasses...MMMM! Soft & softer, not just soft & sharp...I like it!

  16. Living in the Flint Hills of Kansas provides an plethora of mixed plantings. Mainly Amorpha,Panicum,Schiza.,Dalea p.,Lezpedeza, Baptisa,Echinacea,Aesclepias spp. and many more in a natural state. It's incredible and "true". I was fortunate to tour a private 9000 acre tall grass prairie that was pristine and not overgrazed. Many of your plants on your list are present. We just need to follow natures example for successful communities of plants in our designs and aspirations.

  17. Thanks for this wonderful "master class".
    I have one concern about ornamental grasses planted among perennials. Experts recommend that we dig up and divide grasses when they begin to grow like donuts with empty spaces in their centres. I can't imagime digging up a prairie style installation. It must be an overwhelming project. Thomas, have I missed something?

  18. Very interesting post and very helpful set of lists. I am attempting this proportion between forbs and grasses in my garden but am finding the perennials and grasses still flop. It could be that they are not yet sufficiently mature, but I suspect it is more a problem of adequate sun exposure. I think a scheme like this is more likely to succeed with maximum sun exposure. I'm still dedicated to achieving this effect, so i'll need a few more seasons to experiment with the right grasses and perennials with a little more shade tolerance. Afterall, not all meadows, edges of meadows, or large grassy forest clearings have sun from dawn to dusk, but the grasses and forbs grow densely and don't become a floppy mess by September. I'm glad to know that others are sensitive to this more natural approach.

  19. This is very useful information. It is such a sensible approach! And the lists are very helpful.
    I would add that the perennials, besides adding esthetic value, provide food for pollinators.

  20. Hola! Donde estas mas posts? All on board with grasses now eagerly awaiting your next post...two weeks...long time...need inspiration and information:0

  21. Hi, I think you mean Amorpha canescens.

  22. Hiya, is this exact blog is your one and only site or you personally have others?

  23. Howdy! What a nice looking personal blog you own! Did you apply all the settings to this website by yourself?

  24. wow very useful blog thanks. I am currently working on a concept design, i'm thinking of doing a grasses garden. I was just wondering how practical that is. What do you think? After reading your blog i might consider inter-planting as well.


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