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.
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. Here is a mix of annuals and perennials in early November.
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.
Take Special Precautions for Potted Plants
To protect them, you can wrap the pots in an insulating material (think burlap, old blankets, or even bubble wrap), place them close to the foundation of your house, and arrange them close together. You can also put a layer of mulch over them for added protection.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation.
Many pests and diseases will persist in the soil and plant debris over winter, if they're given the opportunity. Removing spent annuals and vegetable plants from the garden, improves our chance of eliminating numerous future pest and diseases that would otherwise survive on that dead plant material.
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.
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.
Remove Most Annuals
If the spent foliage and blossoms on these plants are free of mold and disease, we put them in the compost pile. If we see traces of powdery mildew (zinnias are often afflicted), downy mildew or other diseases, the plants are put into trash that's hauled off the property.
Since annuals only have one season to create seeds for the “next generation”, they tend to have a long bloom period. They seek as many opportunities to create seeds. And you can make the most of it by removing the flowers as they fade. This will encourage them to bloom again so they can make more seeds.
Annuals are structured to freeze and die with cold weather. Their goal in life is to save their offspring (seeds). Annual plants pull water from their seeds before winter comes; therefore, when the low temperatures come, there is very little water to freeze, swell and rupture the cells within the seeds.
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.
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.
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.
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.
Focus on frost hardy annuals. Some of these annuals that can withstand 20 degrees or so, including pansies, snapdragons, dianthus, alyssum, dusty miller, viola, and osteospermum. Keep in mind that flowers may be a bit ragged after such cold but the plants should make it through ok.
It's time to be brave and grab the trimmers, though, because cutting back your annuals will rejuvenate them for the end of the growing season, boasting bigger, better, and healthier growth into the fall.
I usually go through and lightly rake my flower beds at least once in the fall and that cuts down on the amount of clean up I have to do in the spring. If you have a problem with rotting perennials or rotting bulbs, it's especially important to get as much leaf debris off your flower beds.
Some cool-season annuals can tolerate soil temperatures as low as 45 degrees Fahrenheit; however, most cool-season annuals will grow best in soil temperatures at 65 degrees Fahrenheit or warmer.
Bring Plants Indoors
Bringing plants indoors is the easiest way to protect them from frost or freeze damage. Potted tropical and annual containers can be pulled into a heated room, porch, or into a protected and warm structure, such as a garage, to protect them overnight.
Frost damage is detrimental to annuals and bringing them into our homes can prolong our plant's life beyond a single season. While overwintering annuals feels a bit like bypassing the system, it is a lot easier than it seems and, because you are growing them yourself, can even save you money!