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.
Cutting old and diseased foliage in the fall helps a perennial jump right into new growth come spring. But some plants need their foliage for protection over the winter and instead should be pruned in the spring. Know your plant's particular care requirements for the best results.
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.
The best time to prune zinnias is in the early spring, before they start to grow too much. You can also prune them back in the fall, after they have finished blooming. To prune zinnias, first cut off any dead or damaged leaves. Next, cut back the plant by about a third.
A good rule of thumb is that if the plant blooms on new growth, it can be pruned in winter, before the new season's growth begins and after the final bloom of the year. Be careful with spring-blooming shrubs—those usually can't be pruned in winter without damaging the flower buds and ruining the blooming season.
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.
Remove spent foliage in late fall. Cut back leaves to within a few inches from the ground, also in late fall. If you prefer, you may wait until spring to remove leaves, as soon as you see new growth emerging from the ground. Divided daylilies in late summer after they finish blooming.
The spent flowers quickly turn to a mushy bloom and then dry over undeveloped buds which can prevent them from opening. Removing the dead blooms keeps this from happening. Also, daylilies which have not been deadheaded will form seed pods.
As the first light frosts begin to hit plants in mid to late fall, the foliage of perennial plants will begin to die back. Once this occurs, it is the ideal time to begin cutting plants back.
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.
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.
By repeatedly deadheading zinnias throughout the summer, you can keep plants in the flowering stage, provide long-lasting color. Deadheading also keeps plants looking their best. Most flowers lose their attraction as they fade, leaving an unsightly brown head behind.
Even the planting process is easy. I put down fresh mulch, then sprinkle the Zinnia seeds where I want them. I don't worry about spacing or anything, I just scatter them. When the seeds are down, I sprinkle just a little more mulch over the area for cover.