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Favorite Early Summer Gardening Task: Trimming Back Perennials

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 One of my favorite early summer gardening tasks is trimming back perennials. Above: trimming Solidago.

Trimming back perennials is easy and fun

Early summer (May-June) is the perfect time to trim back your perennials. This is the time of year when you turn around and find that many plants seem to have tripled in size overnight while you weren't looking. After a heavy rain, they may start to flop.

What to do? If you wish to tidy the plant up, or prevent future flopping, a quick trim is in order.

Other reasons for trimming include:

Shaping the plant into a more attractive clump or moundDelaying or staging delayed bloom times for a longer overall bloom period, or to coordinate blooming with nearby plantsPreventing late-season legginessMaking room for neighboring plantsImproving air circulation to prevent problems such as powdery mildewControlling the ultimate height of the plantGenerate new flowering branches to increase the number of blooms

Trimming back perennials is not the same as shearing or deadheading. Shearing refers to cutting off a significant portion of a plant's vegetation (typically after it has bloomed) in wide swaths, without regard to where on the stems it is being cut. It's usually done to shape a plant or rejuvenate it following a period of growth. Deadheading refers to removing spent blossoms.

Pinching is a form of trimming back, but only the growing tips of each stem are removed. Trimming back removes a greater portion of the stem.

General guidelines

Trimming back perennials is somewhat more of an art than a science. Each gardener has his or her own methods and preferences, but there are a few general guidelines to keep in mind.

Here is a patch of wild bergamot (Monarda fistulosa) that was partially trimmed a few weeks ago, but needs more attention:

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Monarda fistulosa pre-trim.

As you can see, it is not a very well-defined clump. There are straggling stems leaning out to the sides, and the clump is fairly thick. This plant is susceptible to powdery mildew later in the summer. If left untended, it will get quite tall (3-3.5 feet); the lower leaves will senesce to a wrinkled and ugly brown mess; and most of the blooms will come and go within a few weeks.

My goals now are to tidy up the plant, thin it out a bit, and attempt to trim it back in a way that promotes blooms over a prolonged period of time.

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Trim stem by one-third to one-half, back to a leaf node.

First I trimmed a number of stems back to slightly different heights, always back to a point just above a leaf node. I aimed to cut off about one-third to one-half of the total height of the stem.  If you cut too far down, you'll wind up with a weak and unattractive plant that may not be able to rebound. If you don't cut enough, nothing bad will happen, but you might not effectively control the ultimate height of the plant by the end of the season.

Generally, the earlier in the season that trimming is done, the more leeway you have. If you are late in getting to your trimming, then err on the side of caution and trim conservatively. If the plant is already in bud, you will certainly delay flowering, possibly by a month or more. It's best to trim before the plant starts forming buds. 

Some gardeners use hedge shears and just cut across a clump. That works if you have a ton of plants to get through, or people won't be looking at the plant up close. If, like me, you tend to be a little more persnickety, then pruners or scissors offer more precise control.

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Trim leaning stems.

After I trimmed back the stems for height, I cleaned up leaning stems, stragglers, and other criss-crossing or weak stems. This thinned out the clump and created a more coherent shape.

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Monarda fistulosa post-trim.

After I was finished, the clump looked a little airier and neater. I attempted to shape it with slightly shorter stems near the front and taller stems in the back. Some stems near the middle and back were left untouched. They will bloom first. The shorter stems will be stimulated to grow side shoots at each trimmed node, which will flower a bit later. If done judiciously, it's hard to tell when a plant has been trimmed back; it just looks slightly better than it did before. 

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Monarda fistulosa regrowth after trimming.

I did some initial trimming a few weeks ago. Already the plant had begun to regrow, as shown above. Where previously the stem had a single leader, now a pair of stems are growing, each of which will produce blossoms.

Which plants can be trimmed back

Not all perennials can be trimmed back before blooming, or you will lose this year's flowers. It's always best to check first if you are unsure whether a certain plant will allow trimming back.

Summer- and fall-blooming plants are better candidates for trimming back. Spring ephemerals and early summer bloomers typically should not be trimmed back. Also, plants that send up single flower spikes tend not to be good candidates. 

Plants with opposite leaves often take trimming well, and will produce more flowering shoots:

MintsSalviasAgastachesMonardas

Trimming back can also benefit the following plants:

Asters (Aster or Symphyotrichum)Goldenrods (Solidago)Perennial sunflowers (Helianthus)Joe pye weed (Eutrochium or EupatoriumPhlox RudbeckiaEchinaceaBalloon flower (Platycodon)

Plants that will not flower if their leading stem or flower stalk is trimmed include:

LupinesDaylilies (Hemerocallis)HostasIrisOriental poppies (Papaver)Bear's breeches (Acanthus)Wild indigo (Baptisia)Red-hot poker (Kniphofia)Geum hybrids

The Well-Tended Perennial Garden, by Tracy DiSabato-Aust, is a fantastic resource which provides a number of handy lists, such as which plants can and cannot be trimmed back. It also gives an overview of perennial plant care, garden design, and maintenance schedules. Detailed information on many common perennial plants can be found, but the reference lists offer the most useful information overall.

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Impacts of trimming back

Depending upon the timing of a trim, flowering can be delayed. If done early in the season, a judicious trim will not affect bloom time at all. If you are going away for vacation in August, it is possible to time your trimming so that you delay flowering until after you get home! How is that for a useful gardening trick? It may take some practice and close observation, but if you want to try it, simply trim back a few stems and make notes or take photos to see what happens. That way you'll get a chance to see how and when impacts occur.

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 Solidago rugosa regrowth after trimming.

This photo shows a goldenrod stem that was trimmed back in May. It put forth three new shoots, and my bet is that it will flower at the same time as the other uncut stems.

Trials and tribulations of trimming back perennials

Do not be afraid of pruning your perennials. When in doubt, do a little. Then stand back to see how it looks. Try a little more. Most plants are surprisingly forgiving. Even if you mess up and hack a plant, in all likelihood it will come back. You may lose this year's flowers, or it may look sad for a while, but you probably won't kill it. Part of the fun of gardening is in learning. You won't learn unless you make mistakes.

Here's a mistake I recently made, if it helps you to feel better.

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Over-trimmed Phlox subulata.

Technically, this was a case of shearing rather than trimming back, but the effect was deleterious nonetheless. My moss phlox had bloomed and was looking kind of scraggly, so I cut it way back. Too far, as it happens. I think part of it will be okay, but I may have to replace this clump. A slightly different cultivar in the background was also sheared back hard, and I worried that it was toast, but fortunately it seems to be recovering. Mostly. Live and learn!

Dealing with the trimmings

The ideal way to handle the stems you have trimmed back is to lay them on top of the soil in your garden. They will quickly decompose and add organic matter back to growing plants. If aesthetics are an issue, tuck them behind or under spreading vegetation, or use them in another bed that gets less visual attention. Otherwise, compost your trimmings in your compost bin or pile if you have one. 

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