Thinning perennials that are too jammed

Crowded perennials require some thinning. (Garden Making photo)
Crowded perennials, including peonies and phlox, that require thinning. (Garden Making photo)

My garden is lush with growth from the rainfall we’ve had in southern Ontario this spring and early summer. Fortunately, there’s also been sunshine to bring out lots of flowers, the result being burgeoning clumps of perennials crowding up against specimen evergreens. Truth be known, I’m from the pack-and-cram school of gardening — I don’t leave much space between plants, because that’s the way I can fit more in. This garden bed overload requires careful monitoring because things can get out of hand quickly in a good growing season. All of which is to say, it’s time for me to start thinning perennials that are too jammed – thick clumps of peonies, columbines, early-blooming daylilies, perennial sweet peas and large hostas.

My immediate concern is that removing stems and foliage could reduce the perennials’ energy-making potential this season and mean fewer flowers next year. But if I’m to maintain order and healthy growing conditions, these plants really need to be slimmed down. For example, I’ll remove up to a third of the stems and foliage from peony and daylily clumps. For the columbines, hostas and perennial sweetpeas, I’ll remove the lowest or outer stems (now leaning down to spread on the ground) to reduce the circumference of each plant. My biggest worry with overcrowded plants in a wet season is the proliferation of fungal spores, particularly because I know many fungi produce winter-hardy spores that reside in soil, and then re-infect plants in future seasons. Removing foliage and increasing air circulation around the plants will make less hospitable circumstances for these opportunistic diseases.

After thinning, I’ll feed the plants to compensate for any energy I may have deprived them of making this season and follow up with a nutritious tonic to boost vigour and ward off seasonal diseases. The fertilizer will be absorbed and stored in the plant crowns for making next year’s flowers, and the tonic will be used immediately to keep plants healthy. There are many recipes for plant tonics, all made from organic ingredients and selective plant species containing essential trace nutrients. I’ll confess that I don’t use steeping methods to make plant teas (such as manure, stinging nettle or alfalfa teas), because I can’t stand the odours these fermented tonics produce. I want a tonic that can be prepared and used within the same day, leaving no sticky or smelly residues.

My current favourite tonic is a mix of molasses and Aspirin. (When I have liquid kelp on the shelf, it also goes into the brew.) I dissolve a crushed Aspirin (325 mg) and three tablespoons (45 mL) of unsulphured blackstrap molasses (available from a health food store and some supermarkets) in one cup (235 mL) of boiling water, and then add that mixture to a gallon (4 L) of water. Sulphured molasses can be used, but its preservatives inhibit the amount of nutrients released. If using sulphured molasses, I would double the dose to six tablespoons (90 mL).

I put the tonic in a watering can fitted with a multi-holed rose on the spout and generously apply a foliar drench, allowing any runoff from the leaves to be absorbed into the soil over the root zone. The molasses contains carbohydrate sugars that feed soil organisms, increasing biological activity in the soil. It also packs a nice meal of iron, potassium, magnesium, sulphur and calcium. The Aspirin is a manufactured form of salicylic acid that triggers plant defenses against bacteria, fungi and virus diseases. I’ll apply this tonic now and again next month. Some gardeners use tonics more often, and rely on them more than fertilizers. It’s worth experimenting to see how tonics make your garden grow.


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  1. Elizabeth Fischer says

    Dear Judith

    I love your organic tips for things like the plant tonic that is used when thinning perennials, as well as your organic tips on blackspot remedies and always send them to friends.

    This year with all the rain and moisture I am having a problem with too full beds and like you, am a gardener of the pack and cram school.

    I would like to know if there are any remedies for powdery mildew. My phlox that is over four feet tall and at the back of the bed is full of it. Please let me know if you know what to do about it.

    Love your column.


    • Judith says

      Hi Elizabeth,
      Thanks for your comment, and glad to know there are other pack – and – cram gardeners out there! Mildew on phlox is a nasty business, and usually doesn’t appear until mid to late summer. But in this wet season, it’s already coating phlox leaves. The first line of defense is to purchase (or replace current plants with) mildew-resistant varieties. Reading plant catalogues (or an Internet search) for mildew-resistant phlox should turn up some names. But that’s cold comfort for your phlox plants already afflicted with the disease. Begin by thinning out the phlox clumps, cutting up to half the stems from their base, to increase air circulation around the plants. Then, remove any heavily coated leaves. If the infection is really bad, you might not have any leaves that are clean. It’s quite difficult to stop a fungal infection like mildew when it’s already galloping through the plants. Baking soda and sulphur sprays are most effective when applied before symptoms occur. If you don’t mind working with a somewhat messy organic fungicide, you might try a copper and lime solution, sold as Bordo Mix (or Bordeaux Mixture). Follow directions on the package and hope for drier weather.
      Phlox that were prone to mildew in my own garden seemed to get it every year. Finally I replaced them with mildew-resistant varieties, and solved the problem permanently. That might be something to consider for next year.

  2. Joyce Allen says

    Hi, Judith,

    A propos making tea with comfrey: I’ve been making aerated compost tea for several years, it never smells because, like well made compost, it avoids breeding anaerobic bacteria, which are the ones that make a stink. My sourcebook for this is ” Teaming with Microbes: The Organic Gardener’s Guide to the Soil Food Web” by Lowenfels, Jeff; Lewis, Wayne and Ingham, Elaine.

    Basically, you need an aquarium air pump with air tube and airstone, and a large bucket or barrel. I add a few cups of worm compost (or any good compost), a slug of molasses, a handful of kelp flakes for extra goodness, and fill it with water. Then I let the aerator bubble through for 24 to 36 hours, and the potion is ready to go. I pour it on my plants but if you want to spray it, then you can strain it though an old panty hose. I haven’t yet tried adding comfrey or nettles, but will try that one day soon.

    Joyce A

    • Judith says

      Thanks for this very helpful information. The aeration equipment is simple and easily available. This method is particularly useful for incorporating alfalfa, which I purchase in dry pellet form. I know the pellets must be wet to release their beneficial properties. So far, using the pellets requires them to be covered with soil, to keep them moist and allow decomposition. But if prepared in a liquid brew, they would efficiently release nutrients for immediate use. Thanks!
      – Judith

  3. Kate says


    I have a plot in our local community garden. We suffer terribly from blight with our tomato plants. I spray each week with Serenade and mulch with compost, worm castings and fertilize to keep them healthy. I cut the yellow leaves off constantly and throw them in the trash.

    Any suggestions for other remedies?

    Thank you!

    • Judith says

      Hi Kate,
      Early blight of tomato plants occurs in early to mid-season, during excessively humid weather and after heavy rains — just the kind of June and July weather we’ve been experiencing where I live, in southern Ontario. The disease is a fungus (Alternaria solani) and survives as spores in soil where infected tomatoes have previously been grown. The lower leaves show dark blotches with concentric rings of dead tissue, as well as yellowing and crisping. The leaves will fall off if allowed to entirely succumb to disease lesions. It’s important to remove the affected foliage, wearing disposable plastic gloves when touching diseased leaves. If using bare hands, be sure to wash your hands before touching any still-healthy foliage to avoid spreading the spores.
      The product you’ve been using, Serenade, is a bio-fungicide and works best before disease is firmly established. To deal with the current infection of your tomato plants, try spraying with an organic copper fungicide applied every seven days, sold as Bordo Mix (or Bordeaux Mixture).
      Spores of early blight will live for several years in soil where infected tomato plants have been grown. Your plants have probably been inoculated by spores from previous seasons, and this year’s disease will leave additional spores behind. Moving the location for tomatoes in your plot is essential to avoiding another year with early blight infections. Because the disease seems to be firmly established in your garden plot, it would be wise select blight-resistant tomato cultivars for next year. Avoid overhead watering and crowding plants by too close spacing. Using black plastic or landscape fabric mulch around each plant will help prevent spores from migrating from soil to foliage.
      If you can move the tomato growing location around each year (always avoiding the original location that now is infected with disease spores, and planting resistant cultivars, you can possibly be free from residual spores after five years. If it’s not possible to change the tomato location in the garden plot, consider growing tomatoes in containers (with purchased potting soil) for several years until early blight spores have died away. Judith

  4. Cindy says

    I have a fruitless pear tree in my back yard that looks like it’s dying! It has plenty of water, I see no bugs, what few leaves it has appear to be healthy, but it is not leafing as usual. Suggestions? And thank you for the info on my over zealous Columbines that have taken over my flower beds!!

    • Beckie Fox says

      Hi Cindy: Based on what you’ve described, I’m not sure why your ornamental pear is having a rough year. There’s a balance between enough water and too much water. Do you think it might be getting too much? Has there been any construction or substantial soil disturbance in the area that might have damaged roots? Two suggestions: wait to see how it performs next year (it may bounce back just fine). Or call a certified arborist and ask him or her to take a look at your tree and make a diagnosis. If it’s a significant tree in your landscape, you won’t want to lose it, and it might be best to call in a professional. Good luck!

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