Technically, very few plants MUST be cut down in the fall. We enjoy leaving as many flower seedheads standing as possible to add both winter interest as well as feed the birds in cold weather.
It's important to cut back foliage in the fall to protect flowering plants from disease and give them a clean start for regrowth as winter starts to turn into spring. However, there are some plants you can keep around through the winter since they benefit wildlife and still offer visual interest for your home.
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.
Deep water plants before ground freeze, and continue to water during winter months when temperatures remain above freezing but without precipitation, Erect physical windbreaks. Wrap problem plants with burlap or other material to protect from wind and subsequent moisture loss to evergreen shrubs and small trees.
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.
But when should hostas be cut back? 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.
Q: When is the best time to cut back in the fall? A: “When they start to look too ratty for you and before the fresh new growth begins,” says Sarah. For plants that are frost-sensitive, wait until after the plants have gone through several hard frosts to ensure they're dormant before cutting back.
When to Cut Back Hostas. As a general rule, hostas should be cut back in the late fall. Start with leaves that have wilted or turned brown. Healthy leaves can stay a bit longer to help the roots store needed energy.
Most houseplants can be pruned and cut, however, there are certain kinds that care should be taken with— these include most palms and tree ferns. Dead fronds and leaves can be removed, but these plants never form branches, so the top areas of growth shouldn't be cut off, otherwise it will effectively kill the plant.
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.
Over-pruning
Any cuts you make to a plant, old or new, are left as open wounds. While plants are able to heal these wounds and come back stronger, they are still wounds that leave the plant vulnerable. Rather than training the plant correctly and aiding growth, over-pruning will only cause further stress.
Cut hostas all the way back down to the ground, but remember to wait until all the leaves are yellow or brown if you're cutting them back in the fall.
Cut The Flowers
Tall scapes of flowers can be costly for hostas to produce and maintain. Cutting them off as they start to emerge will divert that energy back to the foliage, creating a fuller look. Other gardeners just don't like the look! Don't worry; cutting the flowers back won't damage the plant.
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.
Any gardening expert will tell you, (contrary to what you may believe) that pruning encourages new growth just when the plant is trying to go dormant and new growth doesn't have enough time to harden before the first frost and freezing temperatures hit. Pruning at this time of year will severely weaken the plants.
As a general rule, prune spring-blooming trees immediately after they flower. Prune trees that bloom in the summer or fall in late winter or early spring before the end of their dormancy. Routine pruning of dead or dying branches can be done at any time.
Cutting back of dead or dying top growth is the only maintenance that is needed for winter preparation. Cut the stems down to the ground, or to the basal growth if you are gardening in the warmer hardiness zones. Alternatively, the flower stems can be left to cut back in the spring.
Many perennial plants die back as fall moves into winter. Preparing perennials for winter cold often starts with pruning back dead leaves and stems. The foliage of these plants, including peonies, lilies, hostas, and coreopsis, blacken after the freeze.
Some perennials benefit largely from regular pruning during the summer growing season, but the most important cleanups are in the fall and the spring. A host of perennials respond best to a tidy spring cleaning and are seemingly rejuvenated from the practice.