This is exactly when you should cut back perennials, according to a gardening expert

Many perennials are past their prime in late summer. After the first few frosts occur, you may wonder if it’s okay to cut back those blackened stems and dried blooms in your autumn garden to save you the trouble next spring. But before you start tidying everything up, you may want to reconsider your timing.

It’s important to understand that you don’t absolutely have to cut back every perennial in the autumn. It’s often just a way to make the garden look neater and to get ahead for next spring when you’ll have all kinds of other garden chores you want to accomplish.

But many beneficial insects, such as leaf-cutting and mason bees, hibernate in hollow stems of plants during winter. Other insects, such as bumble bees, snuggle up underground or in brush piles of fallen leaves. Many butterflies overwinter in various stages on and under plant debris, too.

For this reason, you may want to hold off cutting back plants, and leave fallen leaves, sticks, and other debris in place until next spring. That doesn’t mean your whole garden has to go wild or look messy; you can rake up key areas and leave a few areas untouched to provide habitat and protect native species.

Here’s how, when, and if you should cut back certain perennials in your garden this autumn.

It’s really up to you. Some gardeners tend to decide on a plant-by-plant basis. Leaving some types of perennials in place can feed and shelter birds and insects. But removing those that are diseased or slimy after a frost (we’re looking at you, hostas!) is also an option if you like a neater look.

Remember to dispose of, not compost, diseased plants or those that struggled with an insect infestation. It’s best to get rid of these plant materials rather than to allow diseases and insects a head start next spring.

Also, don’t get ahead of yourself. Wait until several frosts have occurred so that you aren’t cutting into green stems. And don’t just remove every single plant. Even in cold climates, many perennials, such as ornamental grasses, look good in the winter and offer something to look at, other than your bare flower beds or brown lawn.

Once the first few frosts have passed, use secateurs to cut back the plants to a good few centimetres above ground level. You don’t want to cut plants level with the ground because some perennials survive winter better when you leave the crown (where the stems meet the roots) intact for insulation.

Leaving a few centimetres of stems in place also helps you remember where perennials are planted, so that you don’t accidentally dig them up next spring before they pop back up again! This is especially helpful for perennials that are slow to emerge in spring, such as Russian sage, balloon flower, and butterfly weed.

Cut these plants back after the first few frosts to remove unattractive foliage and pests that hang out on these common garden plants over the winter:

• Hosta foliage should be cut to a few centimetres above ground level because it may hide slug eggs and provides cover for burrowing rodents, such as voles, which devour plants year-round.
• Catmint foliage turns gray and unattractive after a frost so it can be cut back to within about 6 inches of the ground.
• Phlox harbors powdery mildew, so remove it now and dispose of the stems.
• Daylily ugly brown leaves and stems can be cut off and composted now.
• Yarrow foliage turns brown and unsightly, so cut back the plant to the new basal leaves at the base of the plant.
• Monarda is also susceptible to powdery mildew, so cut it back to about a foot after a few frosts.

Some plants provide food and cover to birds and beneficial insects, so you shouldn’t mess with them until spring. Leaving them intact also provides insulation for their crowns and offers structure and winter interest to your bare garden. This includes perennials such as:

• Echinacea, also known as coneflower
• Rudbeckia, also known as black-eyed Susan
• Hardy geranium
• Chrysanthemums
• Russian sage
• Ornamental grasses
• Lavender