Horticulture Magazine

Cosmos Plant Care & Growing Tips

pink cosmos flowers

Cosmos are excellent flowers for ornamental and wildlife attracting gardens, or even for your vegetable plot.

These flowering plants are extremely popular in a garden – and with good reason. In this article, we will introduce you to this genus and explain why exactly they could be such good choices for your garden. We’ll talk about the types you could choose, and where to grow them. And we’ll give you the information you need to sow, plant and grow them successfully. 

What are Cosmos?

Colourful pink and white cosmos flowers in a garden
Colourful cosmos flowers

Cosmos can be herbaceous perennials or annuals. The species in this genus are native to Mexico, where most of them occur on scrub and meadow land. C. bipinnatus, one species, is naturalised across the eastern United States and Canada, and widespread over the high eastern plains of S. Africa as an introduced species. It is a common garden plant in temperate climes and is therefore often referred to as ‘garden cosmos’. 

Garden cosmos, C. bipinnatus, is a half-hardy annual, though it is common for the plants to self-seed in many situations and so it is common for them to return over a number of years. Cultivated varieties come in a range of hues, from white, through pink, to deep purple. This is a common garden plant for the UK. 

Another Cosmos common in UK gardens is Cosmos sulphureus, ‘yellow cosmos’. This is another half-hardy annual with flowers in many tones of yellow, orange and red. Their height can vary considerably, and there are some dwarf forms. 

The perennial Cosmos atrosanguineus (otherwise known as ‘chocolate cosmos’ is not reliably hardy in UK gardens. But can be overwintered successfully in some areas when the base of the plant is covered with a 15cm thick mulch of straw, dry leaves, or other such material to protect it from the cold. 

Why Grow Cosmos?

Of course Cosmos of the types mentioned above are great ornamental plants – chosen for their attractive blooms that can really brighten up a garden. In fact, the name of the genus comes from the Greek word ‘kosmos’ which means ‘beautiful’. You can not only enjoy the blooms in your garden, but also inside your home, as these can also make excellent cut flowers.

But aesthetics are not the only reason why growing cosmos could be a good idea where you live.

The main reason why Cosmos can be a wonderful garden plant is that the flowers are in bloom for a relatively long period, and are great for attracting a wide range of beneficial insects to your garden. They will attract bees, butterflies and other pollinators. And also attract predatory insects like ladybirds and hover-flies which eat aphids and other pest species and help to keep their numbers in balance. 

The plants themselves are also rarely troubled by pests or disease, and so can be a very useful plant for those who want to enjoy a relatively low-maintenance garden. They can thrive easily even in relatively difficult growing conditions, and as you will find out below, there are plenty of places you could potentially plant them. 

Things are made even easier by the fact that when they are grown in the right conditions, annual cosmos will often readily self-seed. So you can sow them once and expect them again the following year without any extra work on your part. 

You can also very easily collect the seeds from open-pollinated cosmos to sow yourself the following year. (Note, however, that hybrid cultivars will not usually be ‘true to type’, so what the seeds will grow into can be a bit of a lottery.)

One final thing to note is that if you choose organic seeds, and cultivate your flowers without the use of harmful pesticides or fertilizers, the pretty petals of cosmos flowers can also be eaten. They can be added to brighten up a mixed summer salad. 

Choosing Cosmos

white cosmos flower with pink borders
Mexican Aster ‘Picotee’

There are, as mentioned above, a wide range of varietals and cultivars of Cosmos to choose from. C. bipinnatus and C. sulphureus provide many of the ones that are most commonly grown in UK gardens.

Some Cosmos to consider are:

  • C. bipinnatus ‘Apollo White’
  • C. bipinnatus ‘Xanthos’
  • C. bipinnatus ‘Apollo Carmine’
  • C. bipinnatus ‘Apollo Pink’
  • C. bipinnatus ‘ Sensation Pinkie’
  • C. bipinnatus ‘Rubenza’
  • C. bipinnatus ‘Dazzler’
  • C. bipinnatus ‘Picotee’
  • C. bipinnatus Sonata Series
  • C. sulphureus ‘Ladybird’ (Mixed)

Of course, there are plenty more options to choose from, including the perennial chocolate Cosmos.

If you are looking to take advantage of the plants’ ability to attract and benefit pollinators, then single flower type Cosmos are best, though there are also double flowered cultivars to consider for ornamental blooms. 

Two Sulfur yellow Cosmos flowers
Sulfur yellow Cosmos

Where To Grow Cosmos

Cosmos will grow well in most typical garden soils, as long as they are in full sun, and as long as the ground does not get too damp and waterlogged. Though they can cope with relatively dry and low-nutrient soils, they will not like a very dry, very poor site. And they certainly won’t thrive in heavy, wet soils. As long as the soil is moist but free-draining and there is full sun, your plants should do well. Just keep them out of shade. Remember, these plants are native to much warmer climes. 

If you do have a heavy soil or struggle to find a suitable spot, it is also worthwhile considering that these are flowers that you can also grow shorter cultivars in containers. 

In terms of companion planting, cosmos can be a great choice for scattering throughout or around the edges of your vegetable plot. Since they attract pollinators, they can often work well for polyculture planting schemes – perhaps alongside other great flowers for your kitchen garden including marigolds, calendula, borage, nasturtiums etc…

Cosmos look great in many ornamental schemes, especially, arguably, in meadow-style planting – perhaps alongside Verbena bonariensis, Nicotiana, and wispy ornamental grasses, for example. Cosmos can also work well with other showy blooms like dahlias, or zinnias, for instance, in an ornamental border or planter. 

Sowing and Planting Cosmos

Cosmos seeds are sown in early spring. You can direct sow the seeds where they are to grow. Or you can start them in soil blocks, plugs, modules or small pots filled with a free-draining seed compost. 

If you are sowing your own Cosmos from seed, you can improve the quality and vigour of your young plants. As soon as the seedlings have formed 2 or 3 pairs of leaves, you can pinch out the growing tips. This will encourage them to grow into bushier and more flower-laden plants. 

If you do sow in pots, pot on the seedlings as soon as they are large enough to handle. And then plant them out in late April or May after the last frost date in your area. 

If you are growing taller cultivars, these should ideally be grown in a more sheltered position. But even where they are, they may still require staking or some other form of support. In order to avoid damage to root systems, it is a good idea to have supports in place before you plant out your Cosmos. 

In certain areas, it may also be a good idea to provide slug protection for young plants just after they are planted out. (But long term, the best way to control slugs in your garden is to boost natural biodiversity and encourage plenty of the wildlife that eats them.)

Caring for Cosmos

As mentioned above, Cosmos are relatively low-maintenance plants and will require little care. They will only need watering in dry conditions, and should flower over a long period with little work. Often they can bloom from May right through to the first frosts in autumn. (Note: remember that if you are growing in containers rather than in the ground you will usually need to water your plants more frequently.)

In order to ensure that the Cosmos you are growing flower well, it can be a good idea to feed them with an organic liquid plant feed a few times during the summer months. Though this is not an absolute necessity and often Cosmos will flower pretty well without any such intervention. 

As with many other flowering plants, it is also a good idea to deadhead regularly in order to keep the plants flowering over as long a period as possible. Of course, you can also take the blooms to use as cut flowers in your home. And perhaps even, if you are growing organically, harvest a few of the petals for salads. 

At the end of the season, as seeds form, you can leave the plants to self-seed naturally. Or you can collect the seeds from annual Cosmos varieties to plant next year. 

If you are growing the perennial chocolate Cosmos, then you can divide the tubers. As mentioned above, these will need winter protection, and in cooler and more northern areas, it may be best to lift the plants for storage over the winter months, so it can overwinter indoors. When you lift the plants for storage, this would be a good time to propagate the mature specimens by tuber division. 

By now, you should see more clearly why Cosmos can be such great (and pretty easy) flowers to grow. And perhaps you will consider growing some in your garden.

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