How to Grow Cosmos: A Practical Guide from Seed to Flowers

Cosmos are one of my favourite flowers which I use for filling gaps in a summer border. They have lovely feathery foliage, produce masses of flowers.

They're also very easy to grow from seed. Taller varieties are perfect towards the middle or back of borders and make excellent cut flowers, while shorter varieties work well in pots and smaller spaces.

Cosmos like sunshine and don't need particularly rich soil. I generally just plant these up and may be give some water if it gets dry and the plants do there own thing.

Once they start flowering, keep picking or deadheading and they'll carry on for months. Bees and other pollinating insects love the open flowers too.

Give them sunshine, enough room and regular water while they're getting established, and they'll usually provide colour from early summer until the first frosts.

When to Sow

Cosmos grow quickly, so there's no need to start them too early. 

You can sow under cover from March into April. I tend to prefer sowing during March or early April because the improving light means the seedlings grow strongly without spending too long waiting indoors for the weather to warm up.

If you sow very early in a warm house, cosmos can become tall and leggy. A later sowing will often produce sturdier plants and soon catch up.

You can also sow directly outside from late April into May once the soil has warmed, although starting in pots or trays gives you a useful head start and makes protecting the young plants from slugs much easier.

Cosmos are tender, so don't plant them outside permanently while there's still a risk of frost. In my area I plant them out at the end of May to early June but always keep an eye on the weather.

If you've started plants under cover, harden them off gradually before planting out from May onwards.

There's still time for another sowing in May if you've forgotten earlier. Cosmos grow quickly once the weather warms and later plants can flower very well.

How to Sow

Cosmos seeds are a nice size to handle and usually germinate without much trouble.

Fill pots, modules or a seed tray with multipurpose or seed compost. Sow the seeds about 0.5–1cm deep, cover lightly with compost and water gently.

Keep them somewhere reasonably warm until they germinate. You should normally see seedlings appearing within a week or two in good conditions.

Once they're up, give them plenty of light.

This is important with cosmos because the seedlings naturally want to grow quickly. Too much warmth and not enough light will give you long, thin plants that spend most of their time leaning towards the window.

If you've sown several seeds together, move the seedlings into individual pots once they're large enough to handle.

Don't keep potting them into enormous containers unnecessarily. They grow quickly and you don't want half the greenhouse occupied by giant cosmos plants several weeks before it's safe to put them outside.

Keep the compost reasonably moist and give them as much light as possible while they grow.

Growing Seedlings On

Once cosmos seedlings get going, they grow quickly

Keep them somewhere bright and frost-free and water when the compost starts to dry. If the roots fill their pots before planting time, move them into something slightly larger.

When plants have developed several pairs of leaves, I like to pinch out the growing tip. It can feel slightly wrong cutting the top off a perfectly healthy young plant, but it encourages side shoots and gives you a bushier plant with more flowering stems.

You don't have to pinch them out, particularly with naturally compact varieties, but I find it useful with taller cosmos.

In May I start to harden the plants off over a week or two.

Put them outside somewhere sheltered during the day and bring them back under cover at night, gradually increasing the amount of time they spend outside.

Don't skip this because the plants look big and healthy. Soft greenhouse-grown cosmos can sulk when suddenly introduced to cold nights and wind.

By planting time you're looking for sturdy, branching plants rather than tall seedlings desperately trying to flower in their pots.

Planting Out and Spacing

Plant cosmos outside once the danger of frost has passed, usually from May onwards depending on where you garden.

Choose a sunny position. Cosmos will tolerate less-than-perfect conditions, but sunshine is one of the keys to getting plenty of flowers.

They don't need exceptionally rich soil. Ordinary, reasonably free-draining garden soil is fine, and I wouldn't add lots of fertiliser or fresh manure before planting.

Spacing depends on the variety.

Compact types such as the Sonata series can be planted around 25–30cm apart, while taller varieties generally appreciate 30–45cm or more between plants.

Give tall cosmos enough room. They take more space when they mature than you would expect. 

Water thoroughly after planting and keep them watered while their roots establish.

We often plant Cosmos in groups rather than dotted individually around the garden. A few plants growing through one another create that lovely display

Just remember that taller varieties may eventually need a little support, particularly in an exposed garden.

Growing On

Once established, cosmos are so easy to grow

Keep weeds down around young plants, but once the foliage fills out the plants will supress the leaves/

Tall varieties can become top-heavy, particularly when they're covered in flowers. In a sheltered garden they may stand perfectly well on their own, but if your garden catches the wind, a few discreet canes or supports are worth adding before the plants start leaning.

I prefer supporting them early rather than waiting until a summer storm has arranged the whole border horizontally.

If you've pinched the plants out when young, you should see plenty of branching stems developing.

Don't worry if your cosmos seem to be making lots of foliage before flowering. Once summer gets underway, the buds usually follow.

The main job later is keeping the flowers picked or deadheaded.

Cosmos aren't really plants for forcing into perfectly neat little shapes. Half their charm is the way those flowers seem to float above the feathery foliage.

Watering and Feeding

Water cosmos regularly while they're young and establishing.

Once they're growing well in the ground, they're reasonably tolerant of short dry periods. During prolonged hot weather, though, give them a proper soak rather than repeatedly splashing the surface.

Container-grown cosmos need more attention because pots dry out considerably faster.

Feeding is something I'd be cautious about.

Cosmos growing in reasonably good garden soil generally don't need much additional fertiliser. Too much nitrogen encourages lots of lush green foliage and can delay or reduce flowering.

If you're growing them in containers, an occasional feed once flowering begins can be useful because the nutrients in the compost eventually run down.

But don't feed them simply because you think every flowering plant needs feeding every week.

Give them sunshine, reasonable moisture and don't spoil them too much.

Flowering and Picking

Cosmos are brilliant cut flowers, so don't be frightened to pick them.

In fact, regular cutting encourages the plants to keep producing more.

For the vase, cut stems when the flowers are just beginning to open rather than waiting until every bloom is fully mature. You'll get a longer display indoors and the remaining buds can continue opening.

If you're leaving the flowers in the garden, deadhead regularly by removing faded blooms before they start developing seed. You can do this a couple of times each week.

Towards the end of summer, you can leave a few flowers to form seed if you'd like to collect some for next year. I often find self sown Cosmos growing the net year.

Cosmos can keep flowering remarkably late. As long as you continue removing old flowers and the weather stays reasonably mild, they'll often carry on until frost finally finishes them.

One of the pleasures of growing several cosmos plants is they make great cutflowers

Common Problems

Slugs and Snails

Young cosmos seedlings can be very attractive to slugs and snails.

Keep an eye on newly planted plants, particularly during damp weather. Once cosmos are larger and growing strongly, they can usually cope with some leaf damage without much trouble.

Aphids

Aphids sometimes gather around tender shoots and developing buds.

Small colonies can often be rubbed or washed away. Ladybirds, hoverfly larvae and other beneficial insects will usually help once they find them.

Lots of Leaves but Few Flowers

This is probably one of the most common frustrations with cosmos.

Very rich soil, too much nitrogen or insufficient sunshine can all encourage foliage at the expense of flowers. Stop feeding, make sure plants are getting as much light as possible and give them time.

Cosmos really are a low maintenance plant.

Tall, Floppy Plants

Some cosmos varieties naturally become very tall, but plants can also become weak and stretched if they're grown with too little light when young.

Pinching seedlings out encourages branching, while tall mature plants may need support in exposed positions.

If you know your garden gets windy, put supports in early rather than trying to rescue flattened plants later.

Powdery Mildew

A white powdery coating can appear on foliage later in summer, particularly when plants are stressed by dry conditions.

Keep plants properly watered during prolonged dry spells and give them enough space for air to circulate. A little mildew late in the season isn't usually the end of the world.

Plants Damaged by Frost

Cosmos are tender annuals and won't survive a proper frost. Don't rush young plants outside in spring, however tempting a warm spell might be.

At the other end of the season, the first hard autumn frost usually brings the display to a fairly definite end.

Cosmos aren't plants I spend much time worrying about. Sow them when the days are getting lighter, don't make the soil too rich and keep picking the flowers. Most of the time they'll do the rest — and by midsummer it's difficult to remember that those big, airy plants started from such unassuming little seeds.