CO-Horts Blog

Thursday, September 22, 2016

The case for not cutting back perennials in the fall

By Irene Shonle, CSU Extension in Gilpin County

Fall is in the air – it is glorious up here in the high country right now with all of the aspen exploding into gold. 
Aspen near the Gilpin Extension office in September

Many gardeners start to think about fall gardening chores.  Pruning back perennials is on many lists, but I would argue that with a couple of exceptions, it’s better to leave them standing until spring. 
Let’s get the exceptions out of the way first:  if you have a plant that produces too many seeds, and it’s starting to take over your garden, cutting back the seed heads before they ripen is a good way to practice plant birth control.  The second reason is if the plant is diseased or infested – pruning back and disposing of this material may prevent further spread in the future.

And now for the case for NOT cutting back perennials and ornamental grasses:

           1. Winter interest.  Colorado has a long winter, and seed heads break up the monotony by catching the snow and the frost in interesting ways.  Many ornamental grasses provide excellent interest all winter (little bluestem is a standout with its rusty-red foliage all winter long).
Rabbit brush has interesting seed heads all winter

Ornamental grasses can provide great winter interest (Little bluestem has the russet foliage)  Photo courtesy Jim Tolstrup, High Plains Environmental Center

2        2. Free bird seed.  Many perennials (particularly coneflowers, Black-eyed Susans and sunflowers, but also some ornamental grasses and even some annuals like cosmos and bachelor’s buttons) attract flocks of birds in the fall and early winter.  Sometimes, they are so voracious that they can limit the number of seeds that fall to the ground (I have to replant Black-eyed Susans nearly every year, because the pine siskins do such a thorough job of eating the seed heads).
Seed heads of Black-eyed Susan provide seed for finches

          3. Future butterflies.  If you planted host plants for butterflies (on purpose or inadvertently), you might accidentally remove overwintering eggs and reduce the butterfly population next summer.

4      4. Extra moisture. Standing plants can help to catch any small skiff of snow, providing extra moisture to the plants.
Standing stalks catch skiffs of snow

5      5. Improved hardiness.  The old foliage can provide some extra insulation (especially with snow on top), helping marginally hardy plants make it through the winter.

There are enough other chores to do in the fall – why not let this one wait until spring?

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