Cosmos Growing Guide

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Cosmos are one of the easiest annual flowers you can grow from seed. With their bright, vibrant colours and long flowering season, it’s no wonder these classic annuals have been garden favourites for decades. Read on to learn everything you need to know about successfully growing cosmos.

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Cosmos Overview

As mentioned above, cosmos are extremely easy to grow from seed and require little maintenance once established. Available in a rainbow of colours including bold reds, pinks, oranges, yellows, and whites, their daisy-like flowers bloom profusely from early summer up until the first frost.

Cosmos grow rapidly, reaching anywhere from 1-4 feet tall depending on the variety. The feathery and fragrant foliage provides great vertical interest interplanted among lower growing annuals and perennials. An excellent choice for cottage gardens, cosmos also make gorgeous cut flowers with their long, sturdy stems.

Choosing the Right Cosmos Variety

With all the different cosmos varieties available, it can be difficult to decide which ones to grow. Here’s an overview of some of the most popular options to help you select the right cosmos for your garden:

  • Sensation Mix – This classic tall variety grows 2-4 feet tall and produces fully double flowers in bright shades of pink, purple, white and red. It’s a great backdrop plant.
  • Cosmos sulfureus (Yellow Cosmos) – As the name suggests, this variety bears vibrant, semi-double yellow flowers on plants that reach 2-3 feet tall. It adds a bright pop of colour.
  • Rubenza – Awarded the RHS Award of Garden Merit, this shorter variety grows 16-20 inches tall and blooms profusely in shades of deep magenta, pink and burgundy. It’s ideal for containers.
  • Cupcakes Mix – Only growing 10-12 inches tall, this dwarf cosmos variety is covered in charming, fully double pom pom-like flowers. An excellent option for borders and pots.

No matter which cosmos variety (or varieties!) you end up choosing, they’re sure to provide plenty of bright, lasting colour in gardens and bouquets.

Planting Cosmos

One of the reasons cosmos are such popular annual flowers is that they self-sow very easily. For best results, cosmos should be directly sown in the garden after all danger of frost has passed.

Cosmos prefer soil that drains well, as they do not tolerate wet or standing water. Before planting, work some compost into garden beds to enrich the soil. Because cosmos seedlings resent transplanting, it’s best to sow seeds directly into sunny garden beds where you want them to grow.

Just follow these simple steps:

  • Choose a site with full sun (at least 6 hours a day) and well-drained soil.
  • Wait 2-4 weeks after your area’s average last spring frost date before sowing.
  • Till compost into the top 6 inches of soil where seeds will be sown.
  • Plant cosmos seeds 1/4-1/2 inch deep, spacing seeds or clumps 12-15 inches apart.
  • Water gently and keep soil moist until seedlings emerge in 5-10 days.

Caring for Cosmos

One of the best parts of growing cosmos is that they require very little maintenance once established. Here are a few simple care guidelines to follow to keep your cosmos looking their best:

Provide cosmos with about 1 inch of water per week. Allow soil to dry slightly between waterings, as cosmos are prone to root rot if overwatered.

Apply a 2-3 inch layer of shredded bark, compost, leaves, straw or other organic mulch around plants to help retain soil moisture and prevent weeds. Replenish as needed.

Deadhead spent cosmos flowers by pinching or cutting them off to encourage continuous blooming throughout the season. Allow the last flush of flowers to go to seed for collecting.

Taller cosmos varieties that grow over 2 feet may need staking to prevent toppling over in wind and rain. Use tall plant cages or drive sturdy stakes around plants and loosely tie stems to supports as needed with soft plant ties or twine.

Cosmos Pests and Diseases

Fortunately, cosmos rarely suffer from serious pest or disease issues, especially when provided with suitable growing conditions. However, here are a few potential problems to watch for:

  • Aphids – These tiny sap-sucking insects can cluster on stems and leaves, stunting plant growth. Blast them off foliage with water and apply insecticidal soap or neem oil as needed.
  • Earwigs – These small crawling insects feed on cosmos leaves and flowers, leaving behind ragged holes. Trap them with overturned flower pots stuffed with straw.
  • Powdery Mildew – This fungal disease coats leaves with white powdery spots. Improve airflow and avoid wetting foliage to prevent its spread. Apply neem oil or a baking soda spray weekly as a preventative.

Here are some tips to help avoid issues with pests and diseases:

  • Provide cosmos with plenty of sun and space for good airflow
  • Avoid overcrowding and overwatering
  • Apply an organic mulch to prevent weeds and retain soil moisture
  • Rotate cosmos planting locations each year

Following good growing practices goes a long way towards keeping cosmos vital and thriving for seasons of vibrant colour!

Harvesting and Saving Cosmos Seeds

One of the best things about growing cosmos is that collecting seeds for future seasons is quick and easy.

As flowers fade, they give way to a ring of plump brown or black seeds. Wait until most of the seed heads have fully dried out and turned brown on the plant. Next, cut entire mature seed heads off the plants and place them in a paper bag. Leave in a dry, well-ventilated area for about a week to fully dry.

Then, simply give the bag a shake to separate the seeds from the flower chaff. To clean, pour seeds from bag onto a plate and blow on them or use your fingers to separate out any remaining flower bits. Finally, store cleaned cosmos seeds in an airtight container in a cool, dark place until ready to plant next year.

When stored properly, cosmos seeds remain viable to sprout for 2-3 years. And since cosmos readily self-sow, chances are you’ll have a whole new batch of seedlings popping up this spring!

Frequently Asked Questions

Q. How long does it take for cosmos seeds to germinate?

Cosmos seeds usually germinate quite quickly in 5-10 days when sown directly outdoors after soils have warmed. Make sure to keep seeds continuously moist.

Q. Do cosmos need a lot of water?

Cosmos don’t require frequent watering, but soils should not be allowed to completely dry out. Provide cosmos with about 1 inch of water per week, allowing the soil to slightly dry between waterings.

Q. How tall do cosmos grow?

Some cosmos varieties grow quite tall, up to 4 feet! Popular mid-sized varieties reach 2-3 feet, while dwarf cosmos and Cupcakes cosmos grow just 10-12 inches tall – perfect for containers.

Q. Can cosmos be grown in containers?

Yes! The Cupcakes and Rubenza dwarf cosmos varieties thrive in containers. Provide containers at least 10 inches deep with drainage holes, using a quality potting soil. Site containers in full sun.

Q. How do I prevent cosmos from toppling over?

Tall cosmos top-heavy with dense flowers may need some support. Drive sturdy stakes around plants and use stretchy plant ties to loosely secure taller stems. Allowing adequate garden spacing also improves airflow and stability.

So there you have it – everything you need to know to successfully grow cosmos! Give these easy, vibrant annuals a try this coming season and enjoy their brightly coloured, carefree flowers.

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