Fall Perennial Maintenance Primer

October 9, 2022

Cutting Back Perennials

Perennials (as opposed to woody shrubs) can be cut back either in fall or early spring. Both options have benefits and drawbacks:

Cutting Back in Fall
PROS:

Reduces places where insect pests and diseases can overwinter.
Saves you from doing the task in spring.

CONS:

Reduces places where beneficial insects can overwinter.
Dead growth left over winter helps hold in mulch and snowcover.
Many perennials provide winter interest if the dead growth is left over winter.

Perennials can be cut back as they yellow off or start to look bad over the course of the fall (some perennials will need a hard frost to kill off the foliage). We strongly recommend cutting back peonies in fall to prevent botrytis (a fungal disease).

Some perennials should NOT be cut back in fall. This includes evergreen perennials, and the small number of perennials that are actually woody, but too small to be considered a shrub. Here are some examples:

Bearded Iris (cut back to 6” only)
Bergenia (remove dead leaves in spring)
Bishop’s Hat (cut back to ground level in early spring)
Bitterroot
Coralbells, Foamy Bells and Foamflower
Hardy Cacti
Hellebore
Hens & Chicks
Lavender (remove any dieback in spring)
Liverleaf (remove dead leaves in spring)
Pinks
Russian Sage (cut back to 12” in fall and remove any dieback in spring)
Most groundcovers (many are woody or evergreen)

With the exception of the above, any perennials you don’t cut back in the fall should be cut back in early spring, before new growth starts.

Cutting Back Perennial Vines

Most perennial vines should not be cut back in fall. Virginia creeper, honeysuckle, grapes, bittersweet, Wisteria, and kiwi are all examples. Prune out any dead or weak growth in spring, as the vines start to leaf out.

Hops are an exception – these die back to the ground and therefore need to be cut back to ground level in fall or spring.

Clematis vary in their pruning requirements, but all except herbaceous “bush” types are best pruned in early spring. Spring-blooming types flower from old wood and require little pruning. Summer-blooming types generally flower from new wood and are pruned back to 1 foot above ground level in spring. “Bush” types are cut back to ground level in fall or spring.

Watering and Fertilizing

Fertilizing in the garden should have stopped by August 15 – we don’t want to be encouraging growth at a time of year when plants should be shutting down for winter. You can still add compost and bone meal to your soil – these will release nutrients slowly and the latter will aid in root development; they won’t delay or interrupt dormancy. Fall is a great time to replenish your soil by adding compost – it will save you from having to do it in the spring (choose spring OR fall to add compost to your beds annually).

Watering should continue in the fall, although as the weather cools, you want to ease off on watering to help encourage dormancy. This fall has been very warm and dry, so water has still been important, especially for new plants which need to be kept evenly moist. When fall is turning to winter – the ground is about to freeze, and you are putting your hose away – give all your perennials, trees, and shrubs a deep final watering to ensure their roots are moist going into winter. Dry roots going into winter lead to losses. This is what we call “watering in”.

Moving & Dividing Perennials

Fall is a great time to move and divide many types of perennials. You can get away with moving almost any perennial at this of year, as long as you do it on a cool day and keep it well watered, but choosing the right time of year to divide perennials based on their growth cycle is important. As a general rule of thumb, plants that flower up until July 15 (i.e. spring to midsummer) are best divided in fall, while those that bloom later in the season (many of which are still blooming now) are best divided in early spring.

Why would you divide perennials?

To rejuvenate plants that have lost vigour, stopped blooming, or died out in the centre.
To propagate (multiply) your plants.
To reduce the size of clumps that have outgrown their space.

To divide perennials, dig the entire clump out of the ground, being sure to keep as much of the root system intact as possible (do the same when moving perennials). Shake or rub off most of the excess soil, then use a sharp knife to cut the clump into pieces. For some plants, it might be easier or safer to pull clumps apart with your hands. Always make sure that each division has several growth points and plenty of healthy roots on it. If part of the clump has died out, cut that portion out and discard.

Here are some examples of perennials that are best divided in fall:

Cranesbill
Daylily
Hosta (seldom need division, can also be done in spring)
Iris
Lily
Peony (seldom need division)
Pinks

Some perennials should never be divided, usually because they have a taproot or because they are woody. Here are some examples:

Clematis and other vines
False Indigo
Lupine
Russian Sage

Mulch & Winter Protection

Most perennials don’t need special winter protection. If you have some tender plants or are growing in an area that doesn’t get good snow cover, then adding a protective layer of mulch over winter will help prevent winter kill. You can apply as deep a layer of mulch as you want – 2-4” is usually sufficient, but for really tender plants you can go up to 12” deep if needed. For deep mulching, wait until the ground has frozen before applying. Remove mulch in layers as it thaws in spring.

Here are some examples of good materials to use for mulch:

Bark mulch
Straw
Peat
Wood shavings/sawdust
Clean (pest/disease free) leaves

Harvesting & Sowing Perennial Seed

Growing perennials from seed can be a rewarding challenge. Not all varieties come true from seed (or produce viable seed), and different types of perennials vary in their difficulty. It’s a good idea to research the specific plant you’re trying to seed and learn its particular requirements for germination. A good starting point for first timers is to harvest seed when the pods are fully ripe (brown, dry, and opening), and direct sow into the garden in fall (seeds will normally germinate the following spring). Usually, Mother Nature will take care of the germination requirements, although you won’t get as many seedlings as you would starting them indoors in late winter/early spring.