Stop! Don’t Prune That Grass (How to Prune Ornamental Grasses Right)

(Article originally appeared in Fine Gardening Magazine) Most of us know what to do with our big grasses that go dormant each winter: Grab a bungee cord, tie the grass up, and use an electric hedge trimmer to buzz the column of foliage to the ground. But what about those tricky grasses that are evergreen or ones that have a ground-hugging habit? When and how do you prune those garden staples that don’t fit neatly into the “large and goes dormant” category? If you are hesitant to treat your sedge the same as your maiden grass, it’s for good reason. Unconventional grassy plants can’t be trimmed using generalized pruning rules. They require special timing and techniques on your part to look their best. Start by figuring out which category your grasses fall under: evergreen or goes dormant, large or small. This allows you to select the best pruning method, even if you are not sure of the exact varieties you have.

Small and goes dormant

What: Japanese forest grass (Hakonechloa), Japanese blood grass (Imperata cylindrica), switch grass (Panicum), fountain grass (Pennisetum) Japanese forest grass pruning Photo: Before and after pruning Japanese forest grass When: Yearly, late fall to mid-spring How: If you like to prune, these short, spreading grasses are satisfying to tackle. Though you can prune any time after they go brown, hold off on cutting these grasses back as long as possible. Even brown, they provide winter interest and act as sculptural sentinels when covered in snow. If you clean up too quickly, you miss a lot of winter beauty. Birds also love to pack and scratch at the seeds in late winter when food is harder to come by. Depending on your weather though, at a certain point these grasses will start to crumple and look thoroughly messy. When that time comes, use hedging shears to cut these grasses back to a height of 3 inches for the smallest selections – those that are under 3 feet tall, and to 6 inches for taller varieties – those that are over 3 feet tall. If you cut too low, you could be in danger of cutting into the crown of the plant. Moisture then tends to settle into the crowns and rots them out. While some of these grasses have obvious growth points at the base and can be cut a little lower, others form rounded clumps – and it’s not always clear when you are in danger of cutting into the body of the crown. It’s good to leave a couple inches of leeway and not cut directly next to the growth points so that dew or frost settles a couple inches away from the crown. When I cut too close to the crown, I usually lose a few clumps throughout the plant and need to pull out the rotten bits a couple of months into the season. Pruning should be done every year to give the new foliage a clean slate from which to shine.

Large and goes dormant

What: Maiden grass (Miscanthus), feather reed grass (Calamagrostis), Giant pheasant’s tail grass (Stipa gigantea) miscanthus pruning how-to Photo: Before, during, and after pruning Miscanthus While pruning large grasses that go dormant is a similar process to pruning small ones, there’s something about having a huge mass of foliage towering over your head that makes it seem like a more intimidating task. Plus, bigger grasses can have sharp leaf blades, so if you prune without preparing you can get dozens of tiny stinging cuts on your face and arms. When: Yearly, late fall to mid-spring Just like with small dormant grasses, it’s best to hold off on pruning as long as possible to preserve the winter interest and to provide food for birds. You can prune anytime after the plants go fully brown, as long as you do so before they start growing again in spring (you don’t want to nip the fresh new growth tips). The grasses themselves will give you your cue. Maiden grasses start shedding soon after the new year, so as soon as you notice them making a mess, it’s time to prune. How: Even if you choose a sunny day to prune, wear a long-sleeved shirt and gloves so the blades of grass don’t cut your skin. Start by wrapping a piece of rope around the outside of the grass and tie it into a tight column of foliage. This way, the grass will stay bundled as you prune and not explode into pieces everywhere. Once your grass is tied up, use handheld or powered hedging shears to cut the entire grass to about 10 inches tall. If you’re using powered hedging shears, it’s helpful to have a friend hold up the grass so it doesn’t fall on you as you cut. Just be careful not to trim anyone’s ankles! Though small grasses are easy to clean up, big grasses make a big mess. Plan to put down a fresh layer of mulch after you’re done pruning. This covers any tiny bits of grass that won’t rake up. (More on pruning Miscanthus here and here.)

Small and stays evergreen

What: Sedge (Carex), sweet flag (Acorus), blue oat grass (Helictotrichon), Mexican feather grass (Nassella tenuissima), lily turf (Liriope), mondo grass (Ophiopogon) cleaning blue oat grass Pruning Mexican feather grass Top: Getting the brown blades out of blue oat grass Bottom: Mexican feather grass before, a month after, and three months after pruning These little charmers are some of the easiest plants to tuck into your garden, because they fit almost anywhere, have year-round good looks, and need little care. Yet even the most easy-going of grasses need periodic attention to perform their best. When: any time for cleanup, early to mid-spring for rejuvenation How: By the end of the growing season, brown foliage can pile up inside these plants and give them an unkempt appearance. Luckily, there’s an easy fix to clean them up: just put on some rubber gloves (cheap dishwashing gloves work great) and run your fingers through the grass as though you were combing its hair. The spent foliage clings to the rubber and comes out in easy clumps. You may not be able to clean out all the spent blades, but removing some will trigger the grass to refresh itself. Sometimes, of course, a stronger solution is needed. If painters have trampled on your evergreen grasses or if wind or winter cold have damaged even the freshest leaves, it may be time to go in for the big chop. In early to mid-spring, use your hand pruners or hedging shears to reduce the height of your grasses by two-thirds. While this leaves your grasses looking like awkward hedgehogs, these grasses bounce back fairly quickly and usually look good again in 2 to 3 months. Cutting these grasses back too much will allow moisture to gather on their crowns, which can cause rot. When I’ve experimented with cutting back more than two thirds, portions of the grass died a soggy death. If you’re overly zealous with the pruners, you could also cut into the growth points on the crowns without knowing it – especially on sedges, which can form a mounded crown. Rejuvenation pruning shouldn’t be done more than every 2 to 3 years because small evergreen grasses have slightly less vigor than grasses that go dormant. When you cut off all that foliage, the plant is losing energy stored in its leaves, so it ends up with less energy to put into producing new growth. I like to give the grasses time to recover before subjecting them again to a stern pruning. The exception is Mexican feather grass, which can be pruned back hard any time its foliage clumps into unsightly dreadlocks:

Large and stays evergreen

What: Flax (Phormium), Cordyline (Cordyline), Yucca (Yucca) how to prune phormium or flax Photo: Before, during and after pruning a Phormium/ flax Although technically not “grasses”, these large, spiky plants stand as focal points in the landscape, drawing attention with their bold colors in dramatic shapes. This makes it all the more important to prune right, because a poor pruning job will be noticed by everyone. Unlike with large deciduous grasses which are whacked back almost to the ground, subtlety is key when pruning large evergreen “grasses”. There are many reasons to prune these plants, ranging from the removal of dead flowers and ratty leaves, to keeping plants in scale with their surroundings. With brightly colored flax, there’s another reason to prune: The new growth is more brilliantly-colored. When: Anytime for cleanup and resizing; mid-spring for rejuvenation How: When pruning to freshen up foliage, I simply select the oldest or most damaged leaves and cut them out at the base. This might seem like a time-consuming task, but once you get into a rhythm, it goes pretty quickly. Use the same technique to prune for size. Grasp the tallest leaves, and one by one, cut them out as far down towards the base of the plant as possible. When pruning for size, move around the plant as you go, removing up to two thirds of the leaves, which is the point at which the pruning becomes obvious. Sometimes, however, selective pruning just doesn’t cut it. If your plant is overgrown, has significant winter damage, or must be cut to make room for construction, you can prune severely in mid-spring. Use hedging shears to cut off all the foliage at the base. You’ll end up with a mound about 1 foot tall. While cutting off all the foliage is not an ideal approach, these varieties grow back quickly and look good again in about four months. They do, however, have an awkward phase during their regrowth: When the blades start to regrow, some will look damaged and have clipped tips, so you’ll need to selectively prune again to remove those. This allows the fresh new growth to shine. Over time, some varieties of Yucca and Cordyline grow quite tall and develop a long trunk. If you don’t want yours to look like a tree from a Dr. Seuss story book, cut the plant midway down the stem; it should pre-sprout from just under the cut point. In areas where these plants are marginally hardy, however, cut the trunk back by only one third. Sometimes that stem will re-sprout, but occasionally, the plant will sprout up from the base, instead. One last caveat: Be sure to wear eye protection any time you are pruning spiky grass-like plants. When you are focusing on removing leaves at the base, it’s easy to lean down and get stabbed in the eyeball with a sharp leaf tip. That’s a definite pruning “don’t”.

Comments

  1. Ken Green says

    Read your review on the Fiskar 6201 and found your web site. Thank you for both the review and Grass plant care. It was very helpful and I am going to acquire the 6201 reel mower. We live in Canada so our season is short, but the lawn and grasses still need attention.

  2. barbara says

    This is great info. I have a question you might be able to answer. I live on long island, ny and transplanted an ornamental grass (not sure of the exact type). I dug it up, split in 2 and then replanted in a dif spot. I used potting mix with fertilizer. The problem is that the leaves are now wilted. I am not sure if I killed the plant or should leave it. I am wondering if it will regrow or not. Please let me know what you think. Thanks!!!!

    • Craig says

      Hi Barbara,

      I had the same problem last year living in CT, used potting mix also.
      Potting mix fertilizer contains nitrogen which is good, but causes the plant to go limp thru the growing season.
      The grass will come back next year straight and tall avoid over fertilizing.
      Cheers, Craig

  3. liz bell says

    I have a HUGE NZ Flax in my very small UK garden – it was planted 3 years ago, and has now virtually taken over the raised bed – if I cut it right down in the autumn, will it grow again in the spring?

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