Some Plants Are Especially Cold Hardy
Many cold hardy perennials, shrubs and trees are excellent candidates for fall planting. In general, plants with USDA cold hardiness ratings of 3, 4 and 5, are the best candidates for fall transplanting in cold winter climates.
Fall is a great time to plant another crop of spring greens such as spinach, leaf lettuce, arugula, mustard greens, and mâche because they require cooler soil for seed germination and they mature quickly.
Asters. These beautiful fall-flowering perennials are a lovely companion planting to chrysanthemums, the other stars of the autumn garden. Give asters full sun, and water to get them established (but don't overdo it; they don't like soggy feet).
Late summer and early fall, such as September, is the best time to plant chrysanthemums which are hardy perennials that will grow back each and every year with the proper care. Peonies are also perennials, so after you plant them once in September, they'll come back year after year with the proper care.
For this Blooming Period, we're highlighting some late summer/fall favorites: callas; China asters; lisianthus; perennial sunflowers; and zinnias.
Autumn is a great season for annuals, low-maintenance perennials and evergreen shrubs to shine. In fact, some of the most popular flowers, like colorful mums, roses, dahlias, purple pansies and yes, even bright yellow sunflowers, all bloom around September and October.
Spring-blooming perennials, especially in the bare root form, are best planted early in the fall. Planting in the fall while the soil is still warm will give the roots enough time establish properly. This allows the plants to emerge from well established roots, with a stronger start, the following spring. 4.
Although each plant's needs may vary and some will be more winter-hardy than others, in general about a month before the last fall frost is sufficient in giving the plant enough time to establish itself and take root.
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.
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.
Perennial Flowers: Most perennial plants do not suffer frost damage and do not need to be covered. Frost sensitive perennials include Hostas and Bleeding Hearts. They should be covered to protect their foliage and flowers. Cover any blooming or budded up perennials.
Other tried-and-true, cool weather annuals for you to replace your tired summer flowers in either beds or containers—pansies, violas, dianthus, sweet alyssum, and snapdragons. For green interest, plant dusty miller and ornamental peppers, cabbage, and kale.
Fall is Good for Root Growth
In the fall, the perfect combination of warm soils and milder weather create ideal conditions for newly planted trees, shrubs, and perennials. Warm soil temperatures encourage root growth – more so than in the spring when the soil is still warming up.
Planting in fall allows these plants to establish root systems that they can then bulk up over the winter. Also, in fall, there's usually more rain– which means less maintenance for you.
Answer: Late summer (late August to mid-September) is an excellent time to plant many perennials.
Plant Flowers and Vegetables
If you live in a frost-free region, October is a great time to plant cool-weather flowers and vegetables in your garden. Crops such as kale, cabbage, collards, lettuce, carrots, mustard, onions, Brussels sprouts, cauliflower, beets, and garlic can all be planted in early to late October.
Tip. Mid- to late-August is often an excellent time to plant fall flowers, as long as the weather isn't still so hot that the plants will suffer heat stress.
Asters (Aster Spp.)
One very popular variety is Aster 'Celeste', which forms a clump covered with dense purple daisy-like flowers in late summer and early fall.