Why Late Summer is the Best Time to Clear Out Dead Summer Annuals – and What to Replace Them With

If your summer annuals are looking past their best at this time of year, you are not alone, and it is not surprising considering the summer we have had. Having survived the hottest weeks of the year and bloomed for weeks on end, annual flowers are now coming towards the end of their natural life cycle and running out of steam.

Along with looking past their prime, spent annuals can harbor pests and fungi, making now a great time to replace them with plants for late-summer color and early fall interest, as the season is not over yet.

However, if you are already planning your summer borders and containers for next year, why not order some fast-growing flowering annual seed now to get a head start early next spring? With a heat mat, which you can find on Amazon, you can start them off under cover, ready to plant out after the last frost.

Why Clear Dead Summer Annuals

Whether you merely want a change, or the annuals in your borders and containers are looking tired and disheveled, late summer is the ideal time to refresh them before the temperatures begin to fall.

Furthermore, any humid or wet weather at this time of year can quickly encourage stressed summer annuals to rot and develop diseases that may spread to other plants.

From ornamental kale to Japanese anemones, I share my tried-and-tested favorites from my nearly 20-year horticultural career to add a new pop of color and extend the season.

Ornamental Kale

Towards the end of August, many summer annuals such as sweet peas and cosmos are either on the way out or have already gone to seed. Looking past their best, now is the time to harvest seed for next year, pull them up and compost them.

For early fall and winter interest, ornamental kale or cabbages (Brassica oleracea var. capitata), grown for their looks rather than culinary use, make a great choice. Available in a wide array of vivid colors, including white, pink, red, and purple, they are perfect for filling gaps in borders or containers. Furthermore, their striking hues can actually deepen as the temperatures drop.

Often available as plug plants at a local nursery, flowering cabbage seeds are also available to purchase from True Leaf Market.

However, after weeks or months of feeding annuals, container soil can become depleted of nutrients key to supporting plant growth. Before replanting, why not take the time to top dress or refresh your container soil, as this will help give your new plants a boost and see them through the months ahead.

Aster

With simple but stunning blooms from late summer into the fall, asters (Symphyotrichum) come into their stride when most summer bedding is coming to an end and are ideal for replacing dead summer annuals. Available in many varieties, asters' daisy-like flowers can range from white and pink to deep purple and red.

Native to central and eastern North America, the New England aster is prized for its pollinator-friendly flowers and ability to bloom from late summer through the fall. It grows to 3-6 feet tall and thrives in full sun. An easy-to-grow perennial, New England asters return year after year. Considered cold-hardy down to USDA zone 3, New England aster ‘Grape Crush’ is available to purchase from Nature Hills.

As with all replanting, watering is key to helping asters establish, especially during a warm fall. With spent plants removed and space to install, now is a great time to set up a drip irrigation or a soaker hose. Minimizing water loss and delivering water to the base of a plant, irrigation systems, such as Bonviee system kits, available from Amazon, can also save you time.

Japanese Anemone

Flowering from midsummer until the first frost of winter, Japanese anemones (Anemone hupehensis, Anemone × hybrida), also known as windflowers, are often used to extend the season and are ideal for an autumn border.

Available in white, pink and purple, the elegant perennial flowers are held aloft on thin, straight stems and thrive in partial shade. However, even though Japanese anemones are a stalwart in several gardens I manage, they are known to be invasive perennials due to their tendency to spread vigorously via underground rhizomes and can quickly overtake a border if not controlled. Thankfully, though, they also flourish in containers, where they are restricted.

With windflowers being toxic and the sap an irritant, gloves should be worn when handling any plant material. Cowhide leather gardening gloves are available to purchase from Amazon.

Chrysanthemums

Another great flower to replace spent summer annuals with is the Chrysanthemum. Grown for their late summer and fall flowers, hardy perennial chrysanthemums are available in a wide array of colors, shapes and sizes, which makes them perfect for adding interest and impact to your borders and containers.

From shorter, dwarf cultivars to taller, border varieties, there is almost certainly a chrysanthemum to extend the growing season in your garden. Generally thriving in USDA Zones 5 through 9, hardy chrysanthemums can tolerate low temperatures if the soil has good drainage, as they are prone to root rot when sat in a cold, wet soil.

To help protect against frost and insulate the roots over winter, you can apply a layer of mulch 2-3 inches deep, in late fall. Straw, pine needles and hardwood chippings make an excellent insulating layer for hardy chrysanthemums. You can purchase CleanStraw pine straw mulch from Walmart.

Pansy

One of the best fall flowers for pots, pansies (Viola x wittrockiana) are a popular choice for adding some quick color to a container. Furthermore, with fall approaching, pansies are perfect for taking advantage of the cooling weather, as they thrive in temperatures between 45°F - 65°F.

Growing well in full sun or partial shade, pansies are a versatile choice. However, to get them off to a good start, you can plant them in fresh soil, such as Miracle-Gro potting mix available from Walmart.

When choosing containers for fall and winter displays, you will want to choose frost-resistant materials that can handle the worst of the winter weather. Both fade- and weather-resistant, Veradek round outdoor planters are available to purchase from Amazon.

With the fall fast approaching, mulching is on the to-do list and is one task that not only makes your garden look better but also improves the soil.

Gardeners tend to mulch their beds and borders in the spring and in the fall. However, August can also be a great time to put compost down. Helping to reinvigorate spent soil and replace depleted nutrients, adding a layer of compost can also help sandy soils retain more moisture and improve drainage in clay soils.

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