What to Do With Cosmos in August for More Flowers

Cosmos are at their best in August, filling flower beds and containers with a froth of daisy-like blooms in shades of white, pink, and crimson. If you grew cosmos from seed this year, August is the month when a little attention can keep plants flowering generously well into early fall.

If you want to grow cosmos, you’ll find they are relatively low-maintenance and easy to care for through the summer. There are also plenty of types of cosmos to choose from, offering different flower colors and forms for borders, containers, and cutting gardens.

Rather than fussing over them, concentrate on regular deadheading, watering when necessary, and giving tall plants a little support. These simple August tasks can help keep your plants healthy and flowering for weeks to come.

1. Keep Deadheading Cosmos

One of the most useful jobs to do with cosmos in August is deadheading – and yes, deadheading is one of those simple garden tasks that can make a noticeable difference. Once a flower has faded, snip it off just above a healthy set of leaves or a developing flower bud. A pair of fine garden snips makes this quick and precise.

For a budget-friendly option, the Felco F322 Pruning Snips are designed for precise trimming and are available from Amazon.

Deadheading prevents the plant from putting too much energy into producing seed and encourages it to keep making new flowers. It also keeps plants looking fresh, particularly in flower beds where faded blooms can quickly become noticeable. If you want cosmos to self-seed, however, you don’t need to remove every spent flower. Leave a few seedheads to mature toward the end of summer, particularly if you like the idea of cosmos returning in unexpected places next year.

Another option is the Corona ErgoCUT Dead Header, available from Lowe’s and specifically designed for removing spent flowers.

2. Water During Dry Spells

Established cosmos are reasonably tolerant of dry conditions, but container-grown plants need more regular attention. If you’re learning how to water plants in a heatwave, check pots frequently during hot or dry August weather and water thoroughly when the potting mix feels dry. A watering wand, such as the Melnor 8-Pattern Soft Touch Watering Wand from The Home Depot, makes it easy to direct water around the roots without disturbing the plant.

Water at the base rather than over the foliage, ideally in the morning or evening. Cosmos are fairly drought-tolerant once established, but still benefit from a thorough drink during prolonged heat. For smaller containers, the Chapin 2-Gallon Tru-Stream Watering Can from The Home Depot is a practical option. Avoid keeping the potting mix soggy, as cosmos prefer well-drained conditions.

There’s usually no need to feed cosmos growing in the ground if they’re flowering well. Too much high-nitrogen fertilizer can encourage leafy growth at the expense of flowers, so focus on consistent watering during extended dry spells.

3. Support Tall Plants

White cosmos flower with a pollinating bee

By August, some cosmos can become impressively tall, particularly if they’re growing in fertile soil. Heavy rain and summer winds can cause them to flop, so check plants regularly and add support before they fall over. The Luster Leaf Link-Ups Grow-Through Round Grid Plant Support from Target is a discreet option that helps keep stems upright, while Bamboo Plant Stakes from Amazon are useful for taller plants that need a little extra support.

Simple stakes, discreet plant supports, or twine can all do the job. Place supports around plants while they’re still upright, rather than trying to lift stems after they’ve collapsed. If you’re growing cosmos in a more informal, cottage-garden-style planting, a little flopping isn’t necessarily a problem. Letting stems lean into neighboring plants can create a relaxed effect, provided they aren’t smothering other flowers.

4. Cut Cosmos for the House

August is an excellent time to cut cosmos for a vase. I like to pick mine in the morning, when the stems are well hydrated and the petals have just opened. I remove the lower leaves and place the stems straight into water, then keep the vase somewhere cool and out of strong sunlight. I also change the water regularly to keep the flowers looking fresh.

For a simple arrangement, a Threshold Tall Glass Vase from Target works well for showing off long cosmos stems, while Floralife Flower Food, available on Amazon, can help extend vase life. I find that regular cutting encourages the plants to produce even more flowering stems, so I don’t hesitate to bring plenty indoors. A generous handful of cosmos makes a pretty late-summer arrangement, either on its own or mixed with other garden flowers.

5. Let Some Seedheads Develop

If you want to grow cosmos again next year, I like to leave a few flowers on the plant to go to seed toward the end of the season. Once the seedheads have dried and turned brown, collect the seeds on a dry day and store them somewhere cool and dry. I find these Glass Mason Jars from Amazon work well for keeping saved seeds dry and labeled until spring.

You can also leave a few seedheads in place and let them shed their seeds naturally. Cosmos can self-seed readily in suitable conditions, although young plants may not appear exactly where you expect them. Keep in mind that some cultivated varieties may not come true from saved seed, so the resulting plants can sometimes differ from the parent.

What Not to Do With Cosmos in August

There’s no need to cut back healthy cosmos simply because they’ve become tall. If your plants are flowering well, leave them be, apart from regular deadheading and a little support for wayward stems. August is also not the time to overfeed them. Cosmos are naturally vigorous, and overly rich soil can encourage lush foliage without giving you more blooms.

Instead, keep your August care simple: deadhead spent flowers, water during prolonged dry spells, support tall stems, and cut blooms regularly for the vase. With these few jobs taken care of, your cosmos can keep flowering well into fall, often until the first frost.