How to Grow Cosmos From Seed
Cosmos is a genus that includes a range of flowering herbaceous plants. Some are annuals while some are perennial. All of these plants are native to Mexico, where they grow in areas of meadow land and scrub.
In this guide, we will talk about Cosmos bipinnatus, also known as cosmea, a half-hardy annual that it is very popular to grow in UK gardens. It is also commonly referred to as garden cosmos, to distinguish it from other species.
In the daisy family, these daisy like flowers come with petals in various shades, usually of red, pink or white, with central disc florets, typically with a vivid yellow hue. They flower through late summer and into autumn, often blooming until frost occurs.
They are popular for their ornamental appeal. But they can also be very useful to gardeners because they can attract a range of beneficial insects for pollination and pest control, so they can be a good choice as companion plants in an annual vegetable garden.
Varieties of Cosmos to Grow
There are many beautiful named varieties of Cosmos to consider. So of course, before you can grow Cosmos from seed for your garden, you will need to decide which variety or varieties to grow. There are many named cultivars to consider.
Some good options to consider, for example, when choosing Cosmos bipinnatus are:
- Apollo Series Cosmos (AGM – award of garden merit from the RHS) various hues, c. 65cm.
- Dazzler – deep pink flowers, c. 1m tall.
- Picotee – picotee blooms
- Rubenza – red to deep pink petals, c. 75cm tall
- Sensation Pinkie – (AGM) – mid pinkish purple flowers, 0.5-1m tall.
- Sonata Series Cosmos – various hues, compact plants to 30cm tall.
- Xanthos – unusual lemon yellow to white blooms, 60cm tall.
Of course, these are just some of the highly regarded options from the many more options on offer.
When choosing which Cosmos to grow, one of the key considerations is the colour of the blooms. But you should also think about other things – including the height to which the different options grow.
Ideal Conditions For Cosmos Seeds
Cosmos seeds can be sown in any moist but free draining soil in full sun. They are ideal for a location with a south or west facing aspect, and can cope whether the conditions are sheltered or more exposed.
As long as cosmos get enough sun, and don’t sit in waterlogged conditions or dry out too much, these accommodating flowers can be happy in a wide range of situations.
You might place them in a bed or border of a cottage garden, or prairie planting scheme. You might incorporate them in an annual meadow patch for wildlife, use them as companion plants in your vegetable garden, outdoors or in a polytunnel, or grow them in pots.
When To Sow Cosmos Seeds
Cosmos seeds are sown in spring. You can either sow them in early to mid spring indoors, or you can sow them where they are to grow after the last frost date in your area, typically in April or May in the UK.
For best germination, the temperature should be around 16 degrees. Germination rates are generally high and you should not find it challenging to grow your own garden Cosmos from seed.
Sow the seeds by broadcasting or sowing thinly in rows where they are to grow, or place them into seed trays, modules or small pots filled with a suitable seed starting potting mix, covering them lightly with the growing medium.
If you have sown indoors, prick out and pot on the seedlings once they are large enough to handle, placing each one into its own individual pot. Then harden them off and plant them out into their final growing positions in April or May once all risk of frost has passed where you live.
How To Plant Cosmos
If you have not direct sown your Cosmos seeds, you should make sure that you prepare the growing area before planting. Make sure that the soil is suitably free-draining, and add organic matter to improve drainage if the soil is at all heavy or prone to waterlogging.
Another thing that you might like to think about is providing some support for taller Cosmea, especially if these are in a slightly less sheltered position.
Plant indoors sown Cosmos out in their final growing positions once you are sure that there is not danger of frost. (If an unexpected late frost threatens after planting, use cloches or row covers or mini polytunnels to protect your young plants.)
Make sure that you plant the Cosmea so that they sit at the same level in the soil that they did in their previous pots.
In suitable soil and full sun, direct sown seeds should establish and grow well. However, these, and newly planted Cosmos sown from seed indoors, can be vulnerable to pest problems – especially slugs that can decimate your young plants.
So make sure that you keep slug numbers down organically by introducing plants and features to attract their predators to your garden. And create physical barriers to protect young plant from slugs where necessary.
Adding mulch around your plants can be beneficial to add nutrients, keep soil moist but promote draining around the plants, and suppress weed growth that could put plants under strain due to competition.
How To Care For Cosmos
Cosmos are easy and hassle-free plants to grow. They are, other than slugs and snails, typically not bothered by many pests or diseases.
To get the most from your plants however, you should:
- Water when the weather is dry for longer periods, or when growing in pots or undercover, making sure that the soil or growing medium does not dry out entirely but also taking care not to overwater and making sure excess water can drain away freely. Remember that when you are growing Cosmos in containers, they will typically need to be watered more frequently than those that grow in the soil in your garden.
- One seedlings form their first two or three pairs of true leaves, pinch off the growing tips of each plant. This leads to bushier plants that will typically produce more flowers.
- Once the plants are in bloom, feed with a potassium-rich organic liquid plant feed such as comfrey tea to promote healthy blooms and a longer period of blooming.
- Deadhead spent flowers to prolong the blooming period. Cut back the faded flower and stem, cutting back to the first leaf below the flower.
- If you want, you can collect seed from your Cosmos to sow next year. However, where conditions are right, these flowers can also self-seed fairly reliably. So you could have Cosmos popping up in your garden next year without any work from you. Just remember, if you would like the plants to produce seed, that you will have to stop deadheading near the end of the blooming period so seeds are produced.
Companion Plants For Cosmos
As well as thinking about Cosmos as an isolated plant, and learning how to grow it yourself from seed, it is also a wonderful idea to think about Cosmos has part of your overall garden plans.
In any organic garden, we want to introduce as much biodiversity in plants and wildlife as possible. This helps keep things in balance and makes pest control, fertility management etc. a whole lot easier.
When growing Cosmos, it is useful to think about companion planting, and, beyond this concept, about creating polycultures with beneficial combinations of multiple plants.
Whether you grow Cosmos as a companion plant to aid crops in your vegetable garden, or want to find the right combination of plants for an ornamental bed or border, the goal is to find plants that like similar growing conditions, and which can either aid cosmos or be aided by it. You want to avoid placing plants together that will be overly competitive for space or resources.
Cosmos can be a good companion plant for many other crops and for any other insect- pollinated plants because it can help attract bees and other pollinators to your garden. In a vegetable garden, this characteristic makes it a particular good companion for insect pollinated crops such as tomatoes, peppers and squash, for example.
Cosmos also benefit a wide range of other plants by attracting predatory insects like ladybirds, or hoverflies, or lacewings, that help keep down populations of aphids in a garden.
Of course, we can also think about aesthetics when choosing what to plant with Cosmos. These versatile plants pair well with a huge range of annual and perennial flowering companions.
References:
https://www.hortmag.com/plants/growing-cosmos
