Louise Curley

04 Apr 2024 19:20

How to Grow Cosmos


Every garden needs reliable, stalwart plants that will put on a dazzling summer show without too much effort on the part of the gardener, and for me one of those plants has to be cosmos, also known as the Mexican aster.

Why grow cosmos

Cosmos are my go-to plants for summer-long colour for a whole host of reasons. Firstly, they are so easy to grow from seed, which means you can inexpensively fill gaps in your borders and plant up containers. They flower for months on end – if you keep deadheading them they’ll keep blooming from summer well into autumn, and the open, large, daisy-like flowers are wonderful if you want to attract wildlife such as bees, hoverflies and butterflies. The foliage is lovely too, something that is often overlooked when choosing flowering plants. Cosmos produce lots of luscious, soft, feathery foliage that is not only a lovely backdrop to the plant’s own flowers but also to other blooms in your borders.

Flower types

The flowers range in colour, with pure whites and shades of pink being the most common, however, plant breeding in recent years has created delicate lemony yellows such as ‘Xanthos’ and interesting apricot-pink cultivars such as ‘Apricot Lemonade’ There are also a couple with deep orange blooms. The flower forms offer lots of diversity from those with simple petals arranged in a single ring around a golden centre to those with multiple layers of frilly petals. Some have unusual quilled petals and others have petals that are fused with delicate wavy edges like a cupcake case. There are those with plain coloured petals and ones which are bi-coloured. Heights vary too, from the compact types which grow to around 60cm, mid-height ones and those with statuesque stems several metres tall.

In the garden

The more compact cosmos such as the sonata varieties are ideally suited to containers or planted at the edge of a border and the taller ones such as ‘Purity’ can be used in the middle to back of a border. Plant with perennials and ornamental grasses.

Growing tips

Cosmos are half-hardy annuals, which means they need protection from frost. They can be sown direct into the soil where you want them to grow but you’d need to wait till mid-May to do this, whereas if you sow them in April into seeds trays or pots and keep them under cover in a greenhouse or on a sunny windowsill you can get a head start.

If you’re using a seed tray fill it with compost, then evenly space out the cosmos seeds, gently pushing them into the compost to bury them. Once the young plants have two sets of leaves, gently lift them from the compost and move them into individual 9cm pots where they will have room for the roots to develop.

This year I’m trying out coir pellets. The pellets are dehydrated ‘coins’ of coir which you place in a tray that doesn’t have any holes. Fill the base of the tray with warm water and soak the pellets – they’ll gradually swell as they rehydrate. Using a plant label or dibber make a hole in the centre of the pellet and pop in a seed, then cover up the seed. Place the tray with the coir pellets somewhere warm and sunny. Keep an eye on them to make sure they don’t dry out – to keep them moist put some water in the base of the tray when necessary. When roots start to show through the sides and base of the pellets plant up each pellet into 9cm pots filled with compost.

When it’s warm enough – generally around mid-May – acclimatise the cosmos plants to the outdoors, leaving them outside during the day for a week or so, but bring them back indoors at night, then leave them outside for a week at night as well but with some horticultural fleece over them to provide a bit of protection. After two weeks, and once the danger of a late frost has passed, the cosmos plants can be planted outside.