Perennials can be cut back in spring or autumn

  • Article by: NANCY ROSE , Contributing Writer
  • Updated: November 8, 2000 - 10:00 PM

QIs now the time to cut perennials back? Or is spring a better time?

AThere are advantages and disadvantages to cutting back perennials in either season. It's a matter of preference and convenience rather than any hard-and-fast scientific rule.

There are several reasons to remove all dead stems and foliage in the fall. The main reason is to remove dead plant material that might harbor disease organisms or damaging insects, such as stem borers, overwinter. If you have had a disease problem, such as a fungal leaf blight on a specific perennial, it's worth cutting that plant back before all the diseased leaves fall to the ground. However, if you're not seeing many disease and insect problems in your perennials, there's not a compelling reason to cut them down in the fall.

Another advantage of cutting back perennials in the fall is that there often seems to be more time for doing it then. While there's no shortage of garden chores in the fall, I feel a bit more leisurely in the garden at that time. There are a million things to be done in the spring, it seems, and all in the month of May!

However, a downside of fall cleanup is that you don't have any interesting stems or seed heads in your perennial beds overwinter. In my garden, the dried seed heads of purple coneflowers, 'Autumn Joy' sedum, Siberian iris, and balloon flower (Platycodon) provide a range of textures and some color (mostly shades of brown) to my winter landscape. I also enjoy watching the small seed-eating birds peck at the seed heads when they visit my garden in winter.

QI purchased lily bulbs to plant this fall but I'm worried that the squirrels will dig them up and eat them, as they've done with other bulbs. Can I pot the bulbs, let them grow in my house, then plant them in the spring?

ALilies such as Asiatic hybrids, Orientals and martagon lilies all need cold, dormant rest periods during winter in order to grow and bloom properly. This means that they can't be grown indoors as a houseplant. The only lilies that might be grown this way are nonhardy species, such as calla lilies, which wouldn't survive Minnesota winters in the ground.

What you can do, however, is to plant the bulbs in a pot with well-draining potting medium and store the pot in a cold but not freezing spot. A spare refrigerator, set between 32 degrees and 40 degrees, is ideal. Or you could put the pot of bulbs into a large bag of dry leaves and put the bag in an unheated attached garage, preferably pushed up to the house foundation where it will be a little warmer. The trick is to keep the bulbs cool, but not let them freeze.

You also can plant the lilies outdoors and try to foil the squirrels. I've read suggestions about making little hardware cloth cages to place around each bulb before planting, but this sounds like a lot of work and might negatively affect the bulb's growth. An easier trick would be to place sections of chicken wire fencing flat on the ground over the spot where you planted the bulbs. Anchor the fencing firmly into the ground with sod staples or other long pieces of wire bent like a hairpin. This should keep the squirrels from digging into soil in that area. Then remove the fencing in the spring when the lily foliage emerges.

QI would like to build up about 6 inches of soil around a tree to make a flower bed. Will this hurt the tree?

AAdding even a few inches of soil over a tree's root system can hurt the growth of the tree. Most of a tree's roots grow in the top 6 to 12 inches of the soil, where there is enough oxygen available for the roots. By piling soil over the root system, you can suffocate the roots. Different tree species have different tolerances to reduced soil oxygen, but I definitely would not recommend adding 6 inches of soil over the root system of any tree.

-- Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. She spends her spare time gardening, inside or outside, depending on the weather. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.

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