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.
Spent blooms should be cut back as soon as they begin to fade to keep reseeding issues to a minimum. But as you progress deeper into the autumn season, the foliage of daisies will begin to fade as well. At this point, it is time to cut the entire plant back near the ground for winter.
Always cut back to a leaf or part of the stem where you can see a new bud forming. Later in the season, when the plant begins to produce fewer blooms, you can just let them be. Let the plants stand during the winter to provide food for birds. In late winter, prune them back to the ground.
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.
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.
After zinnias flower, cut off the old flowers (a process called “deadheading”) to encourage more flowers to form. Zinnias are annuals and will die with the first hard frost of fall. If you want them to reseed, let the last flowers of the season mature fully and scatter their seeds.
To extend the blooming season, cut some black-eyed Susan stems back by a third in early summer. The ones you cut back will bloom after the ones you didn't cut back, so you'll get to enjoy the flowers for a longer period of time.
Black-Eyed Susan Care
Black-eyed Susans are easy to establish, they naturalize well, and require little maintenance other than deadheading. Regular deadheading of the faded flowers keeps the plants in bloom longer.
If you want to do any cutting of your roses in autumn, wait until after the first hard frost (temperature below 25 degrees Fahrenheit overnight). If you cut back before the first hard frost, it may send a signal to the roses to grow when they should be going dormant.
WHEN TO PRUNE ROSES. The best time to prune roses is in late winter or early spring, around the time new growth begins. This could be as early as January or as late as May, depending on your climate.
It's easy to neglect roses, because they don't need to be pruned regularly in order to grow and bloom year after year. But, if you want to keep them healthy and full of flowers, trimming helps a ton! If you never cut them back, over time they will bloom less, and look more scraggly.
Deadheading Daisies
So yes, deadheading Shasta daisies (and other varieties) is a good idea. Deadheading daisies not only improves their overall appearance but will also inhibit seed production and stimulate new growth, which encourages additional blooms. By deadheading regularly, you can extend the flowering season.