Wait to prune spring flowering plants right after they bloom. Any of your annuals that are no longer attractive can be either cut off at ground level or pulled out of the ground now. Or you can wait until later in the fall or even next spring.
Remove Most Annuals
In general, these plants are easy to spot because after the first hard frost, many of them, including impatiens, begonias, and coleus, have withered and turned brown. If the spent foliage and blossoms on these plants are free of mold and disease, we put them in the compost pile.
Unlike perennials, annuals do not come back from season to season so there is no reason to leave these in the ground. Pull them up, roots and all, and add them to your compost pile. Remove weeds and leaf debris. These are common places for diseases and pests over winter.
Annual cleanup
Remove all of your summer annual flowers, including their seed heads, from your flower beds. (Throw these in your compost bin.) This does more than save you time next spring. Leaving annuals in your beds over the winter will invite pests and disease as the plants decompose.
Another good idea is to remove annual flowers after a killing frost. In addition, perennials that show signs of disease should be cut back in the fall.
Continue to water annuals until freezing temperatures kill them. If your annuals are in containers, move them into a garage or other protected space when temperatures are forecast to dip into the 40s overnight. You can do this until daytime temperatures no longer rise above that threshold.
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.
Shape and reduce the size of overgrown and bulky plants by cutting unwanted stems to the base of the plant or where stems meet. Typically, it is good to remove up to one-third of the stems, especially in overcrowded areas where the foliage is beginning to discolor or die.
Simply put, annual plants die in the winter season so you must replant them every year, while perennials come back every year so you only plant them once.
If you are equipped to compost, your annuals will decompose beautifully for your spring garden. Unless your annuals, both flowers and/or vegetables, died from some sort of disease that could spread to next year's plantings, we recommend using them as “greens” in your compost waste stream.
Tender annuals — Tender annuals thrive in warm soil and warm air temperatures. They should only be planted once temperatures reach and stay above 55°F. They can be severely damaged or die if they are exposed to frost or temperatures below 32°F.
A few start to suffer when temperatures drop below 40. So get them inside in the next couple of weeks. True annuals such as vinca, zinnias and marigolds are eventually going to croak even if you try to milk them through winter.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation.
Generally speaking, the annual plants you want to protect or propagate should be brought indoors as the growing season begins to wane and before cold weather threatens to kill them.
Temperatures below 32 degrees can freeze the leaf tissue of cold-wimpy plants and turn them to mush after just a few hours. Some of the most tender annuals and veggies might not die but suffer cold-induced setbacks even when overnight lows dip below 40.
All cold tolerant annuals can best handle temperatures below zero (to minus two or three) at night when they are well watered in the day. If it is extremely cold, watering off the frost prior to sunrise will eliminate damage to the plant.
Once finished blooming, annual flowers can easily be coaxed to continue blooming throughout the entire growing season by a simple process called deadheading― pinching off spent flowers.
Be Sure to Harden Annuals Before Placing Them Outside
Any annual planted outdoors should be properly hardened to acclimate them to cooler temperatures. Those annuals that are hardened are more likely to survive a frost or light freeze with little to no damage than those that are not hardened.
Spading or tilling the soil in the fall allows for the winter conditions of freezing and thawing to naturally break the soil into its particles. This results in crumbly state desirable for spring planting. Soil is often damp or wet in the spring which makes deep spading more difficult.
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.
A perennial with an insect or disease problem should always be pruned in fall, no matter the type. Trim and discard of any infected plant stems to stop an infestation from carrying over into next year's growing season.