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Cutting Back Perennials- Do It In the Spring

arbo_retum
17 years ago

Thanks to the U of Minn Yard and Garden email newsletter, I have just read their reasons for cutting back perennials in the spring, rather than doing fall cleanup. In our gardens,we started doing this a few years ago for really one simple reason: the amount of plant material that is left to dispose of- after the winter- is 1/10 of what it was in the fall(because of all the leaf dessication and loss).We just don't have the room to compost all that material in the fall. But as you will see below, they give many reasons for spring cleanup. My exceptions to the spring-cutting back rule are : Peonies because of diseased foliage(we always have botrytus problems on some plants) and siberian iris (spring iris foliage is very difficult to cut back because it's still somewhat turgid). Hope youall find the article useful.

best,

mindy

Trimming Back Herbaceous Perennials- Should we do it in fall or spring?

David C. Zlesak, Regional Extension Educator, Horticulture

As we look out over the fall garden many of us see flowered-out perennials which were once glorious, but now are in decline. Some are succumbing to foliar diseases like powdery mildew or just going through the natural aging process and are losing their leaves and slowly turning brown. What can be done to both preserve the health of such plants so they will come back strong next season and clean up some of the unsightly material in our gardens? Each gardener needs to choose their own balance between these two objectives, weighing factors such as each particular plant species, their use of particular garden space for the winter season, disease pressure, and time availability.

As long as there has not been a freeze and there is green foliage to undergo photosynthesis, chances are there are still carbohydrates being generated and transported to the crown of the plant. Interfering with this process by removing stems and foliage will limit the amount of energy reserves the plant has going into winter and from which to fuel new growth the following spring. For the sake of the plant, it is typically best to wait to remove top growth until it naturally dies back or at least after a hard freeze in order to allow as much energy reserves as possible to be generated and stored. Some perennials when cut prematurely in the growing season may try to regrow from the base, especially if we have a long, warm fall. Such plants deplete stored energy reserves and may not survive the winter.

For especially marginally hardy plants such as mums and lavender, it is beneficial to leave old growth through the winter and remove it come spring. Stems can act as small windbreaks to accumulate snow, blowing leaves, and other materials which act to insulate the crown of the plant. In the University of Minnesota mum breeding program led by Dr. Neil Anderson, winter survival of mums were compared with those having top growth cut back in the fall after a few hard freezes versus leaving it through the winter. There was a clear trend that those with top growth present throughout the winter in these open research fields were much more likely to survive.

Another reason to leave top growth in tact through the winter is the winter interest it provides. Many perennials have relatively strong canes that do not readily lodge and provide interesting architecture, fruiting structures, or seeds desirable as food to wildlife. Some perennials which provide good winter interest include many of the daisy or composite flowers such as Echinacea (purple cone flower) and heliopsis, most ornamental grasses, and other sturdy plants with attractive fruit capsules such as Siberian iris.

For very hardy plants such as bee-balm, daylilies, hostas, and peonies, they are so durable that removing their foliage in the fall is generally no problem at all. In fact, removing foliage in the fall and carefully discarding it can help reduce disease pressure, if a problem, the following season. Even for the more tender or marginal perennials where diseases have been an issue, removal of diseased tissue in the fall is a wise choice. Mulch can be brought in as an alternative to help insulate the more tender perennials.

Barring disease pressure, leaving top growth on herbaceous perennials is the easiest way to manage herbaceous perennials and is what is recommended. Top growth can provide continued ornamental interest throughout the winter in itself and the wildlife it may attract. In addition, in spring top growth of many herbaceous perennials is more brittle and much easier to remove compared to the fall. Consider and weigh your particular perennials, winter garden preferences, and disease pressure and make the best choices for your situation.

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