This week, we experienced the first frost of Autumn 2017 at Wellfield Botanic Gardens. For us, this means fall bed clean-up, and all that task entails, is just around the corner. I will discuss how we handle general bed clean-up in another post, however, one of the most frequently asked questions we receive has to do with what herbaceous perennials to cut back or leave to overwinter, and why.
There are a multitude of answers, not all of them based in horticultural scientific fact, and not all are covered here, of that I am sure. Here are a few reasons to cut or not to cut:
- Control or Promote Self-Seeding – Some plants are very successful self-seeders and depending where they are growing and what you are hoping to accomplish, this could be wonderful or disastrous. For example, Chasmanthium latifolium, or northern sea oats (pictured above), is a wonderful semi-shade ornamental native grass with a tendency in one garden to spread little babies everywhere. This disaster is averted by cutting it the ground in mid-fall. If it were located in a more naturalized setting or causal border planting, it might be allowed to spread joy to all since it has wonderful winter interest. Biennials like Digitalis and Alcea rosea only last two years, and seed pods must be left in place or crushed and spread around near by in order to continue to produce more flowers. This is perfect in a cottage garden design, where self-seeders add a whimsy feel, popping up in unexpected places.
- Disease Prevention – Some plant pests and diseases overwinter in the duff layer, residing upon the ground. Any leaf or woody material known to harbor pests or diseases must be cleared out and destroyed. There is never a reason to leave such material behind.
- Crown Protection – Herbaceous plant material living in temperate zones are designed to survive the harsh winter months by keeping their soft growing points below the soil line. Stalks left in place might capture blowing leaves and snow, creating an insulation layer of loose material, thus protecting the crowns. This might be exceptionally important if you have plant material marginally hardy to this zone. Also, some plants just seem to respond better if left until spring to cut back. Trials have shown, for example, leaving hardy mums alone until spring is the best way to encourage new growth.
- Wildlife Habitat – Some plant material such as certain Echinacea varieties and other native plants are excellent sources of food for birds, etc. We leave these plants alone, choosing to cut them back in the spring, even though I do not find them overly attractive in the winter landscape.
- Aesthetic – When it comes down to it, most often the reason for cutting or not cutting back material is a personal decision. Does that plant look beautiful to you in winter, or does it not? Most people love ornamental grass’ winter look, but it also best to leave them in place, even if they flop, to protect the crown. Some think sedums look great too, but they do turn mushy with the cold sometimes. Leaving the stems in place until spring protects these succulents as well.
Some plants, such as many ferns or Hellebores,retain nice green leaves all winter long, putting up new growth either early in the spring (ferns) or after flowering (hellebores). The foliage looks great and protects crowns from harsh winter temperatures at the same time.
Whatever you do, consider why and then choose your course of action. A excellent resource found in our very own gift shop is The Well-Tended Perennial Garden by author Tracy DiSabato-Aust. Gardening is a great learning opportunity each year. Happy cutting.