Also, do not cut back hardy perennials like garden mums (Chrysanthemum spp.), anise hyssop (Agastache foeniculum), red-hot poker (Kniphofia uvaria), and Montauk daisy (Nipponanthemum nipponicum). Leave the foliage. It's important to protect the root crowns over winter.
Some perennials, like hostas, peonies and daylilies, need to be pruned in fall to avoid winter damage. Plants like these should be pruned after the first few frosts in late fall or early winter. Other perennials like mums and coneflowers are better off being pruned in spring just before new growth comes in.
With fall garden cleanup in full swing, maybe it's all the raking and mulching that has people going bananas. But before you start hacking at your trees and bushes, take a tip from a seasoned gardening expert. "The rules of fall pruning are simple: Prune nothing in the fall!
Don't shear the branches of spring flowering shrubs (Forsythia, Lilac, New Mexico Privet, Spirea, Flowering Quince, and others). These shrubs produce flowers on last year's wood, so removing old growth will reduce or prevent flowering. These plants should be pruned immediately after they are done blooming.
Daylilies are best cut back in autumn, once the leaves have had a full growing season on the plant. While there are some evergreen types of daylilies, most are herbaceous, meaning that the leaves die back in the wintertime and sprout anew in the early spring.
DON'T prune during fall.
This can weaken and damage the plant—especially if there's an early frost.
According to gardening expert Mike McGrath, author of Mike McGrath's Book of Compost, and radio host of the show You Bet Your Garden, the rule of pruning is that you should NOT prune in fall. That's right, nothing and no exceptions.
Never shear a shrub in fall (or ever, actually, but that's another article) and leave major pruning or renovation for late winter/early spring, or immediately after bloom for spring-flowering shrubs.
For most trees, the best time for major pruning is late winter to early spring because wounds close faster. Pruning in late summer and early fall may also stimulate new growth, which has little time to harden before cold weather comes.
Cutting back perennials in the fall is not only a great way to keep flowerbeds looking neat and tidy through winter, but it also aids greatly in keeping plants vibrant and blooming the following year. Removing spent foliage and blooms helps a plant focus on recharging it's roots and resources.
Deciduous Trees
Most trees should be pruned during the late dormant season (February through March). February through March is generally regarded as the best time to prune most deciduous trees.
Hostas should be cut back in late fall. Healthy hosta leaves can be left on the plant in early fall to capture much-needed energy, but all leaves should be trimmed off after the first frost to deter slugs and other pests from making your hosta their winter home.
Although it may be cold and snowy outside, winter is actually the best time of year to prune your deciduous plants. Wait for a relatively mild, sunny day, get out your pruners, and take a critical look at your trees and shrubs.
Butterfly bushes do not need to be pruned every year. In fact, you only need to prune them when they get too large for the space allotted. But since butterfly bushes only bloom on new growth many gardeners prune them severely each spring to encourage lots of new growth and lots of flowers.
Winter is usually the best time.
Dormant pruning is usually done in late winter, six to 10 weeks before the average last frost in your area. You can prune shrubs at any time of year if it's necessary—for example, to remove broken branches or dead or diseased wood, or to remove growth that is obstructing a walkway.
A rough rule of thumb is to prune spring-blooming shrubs soon after they finish flowering because most bloom on old wood, while those that bloom in summer and fall usually bloom on new wood and can be pruned in late winter or very early spring.
Good candidates for hard pruning
Due to the severity of this approach, it is recommended only every three to five years. However, woody shrubs such as smooth hydrangea, Japanese spirea, ninebark, and elderberry can be pruned like this more often, even as often as annually if desired.