Cosmos are one of my favourite flowers in the garden, especially the deep pink flowers which just look spectacular. Cosmos has ferny foliage, grows to about 1 meter tall, they are easy to grow and can be grown in various styles from pots, densely planted or planted as a singular plant. We have grown several varieties in our garden which are sensation, dwarf vega and bright lights.
Bright lights which is an orange flowering cosmos can become a bit weedy as it self-seeds easily. We grow bright lights in the vegetable garden to help bring in beneficial insects and find it flowers all winter long when not much else is flowering.
When to Plant Cosmos
Cosmos is a heat loving plant, so they grow best during Spring and Summer, however in Perth where we live, they can be grown all year round. We like to sow are seeds in late Winter, so our cosmos is ready for flowering in Spring.
Where to Plant Cosmos
Cosmos prefers full sun which is between 6-8 hours of sunlight a day. We have found they will also grow well in part shade which is about 4 hours of sunlight a day.
How to Propagate Cosmos
Cosmos is easy to propagate and there are various methods to propagate them. Cosmos seeds germinate best when soil temperatures are between 20-26 degree Celsius and take 15-20 days to come up.
Our preference for propagating cosmos is to start them in seedling trays. By starting them in seedlings trays, you can still start your cosmos off when it’s still cold outside, getting a head start on Spring and it also protects them from pests such as slugs and snails.
Cosmos seeds can also be direct sown and one of the easiest methods to direct sow them is a called scatter sowing. You simply scatter the seeds around in the garden, ruffle up the soil to cover the seeds. Just be on the watch out for pest trying to eat your seedlings.
How to Transplant Cosmos Seedlings
Cosmos seedlings are ready to be transplanted out when they there first true leaves and are between 5-10cm tall. Before transplanting your cosmos seedlings, we recommend in improving the soil by adding some compost, manure, organic slow release fertilser and mulch.
When do Cosmos Flower?
Cosmos flower 85-95 days from the time they were transplanted. If you transplanted your cosmos on the first of September, they would begin flowering in December. If you want your cosmos to flower in Spring, we recommend in sowing your seed in Winter, pot them up in a green house, then plant them out as semi established plants in September.
How to Care for Cosmos
Cosmos is an easy plant to grow and doesn’t require any major maintenance, but there are a couple of things to make your cosmos into a bushier plant and extend its flowering season.
How to create a bushier cosmos plant
When the plant is young, pinch out the growing tips, this help to create a thicker, dense bushier plant.
How to extend your cosmos flowering season
To extend your flowering season, cut off old flowers just above a set of leaves. This will encourage the plant to produce more flowers.
Can You Put Cosmos in Pots?
Cosmos do well planted in pots and look great when densely planted in a pot.
Can I Save The Seeds from My Cosmos?
Saving the seed from your cosmos is quite easy, just allow the flowers to dieback on the plant, remove once dry and place in a cool dry place until your ready to plant them.
Common Problems with Cosmos
Other than when they are seedlings cosmos have been a trouble-free plant for us. When they are seedlings, slugs and snails seem to be attracted to them. Here are some common problems you may come across when growing cosmos.
Slugs and Snails
Slugs and Snails seem to be attracted to seedlings, to protect your seedlings we recommend for the first few nights go out with a torch light and just check your seedlings and remove slugs and snails. Here is more information on controlling slugs and snails.
Aphids are a small sap sucking insect that you will find under leaves or on new shoots. To control aphids you can use a high pressure jet of water to wash them off, squish them with you finger or spray them with a horticultural oil or potassium soap spray.