Tag: Cosmos Flowers

The flowers are a bright and satisfying choice for beds, borders, and containers, and their drought tolerance make them an excellent choice for arid areas, xeriscapes, and water-wise gardens as well.

You may have heard that these hearty annuals don’t require any fertilizing, which is mostly true – but you’ll find that there’s always an exception to general “rules” like this one.

If you’re ready for a bubbly display of brilliant, bright colors, let’s look at the complete answer to whether cosmos plants need fertilizing!

Here’s what you’ll find ahead:

Cosmos Fundamentals

The Cosmos genus contains a number of different species, and the most commonly grown garden varieties are from two Mexican species – C. sulphereus with orange and yellow flowers, and the mauve, pink, and red-flowered C. bipinnatus.

As low maintenance annuals with brightly colored flowers and tough characters, their hearty nature make them a favorite for gardens as well as for floral arrangements.

These plants need a full sun location in well-draining soil with average to poor fertility, and a slightly acidic to slightly alkaline pH of 6.0 to 8.0.

For an abundance of blooms, it’s helpful to pinch out the growing tips when plants are 18 inches tall – pinching produces bushier plants, and bushier plants provide more flowers!

These are tough and resilient plants that are drought-resistant once established, and only require water during extended dry spells.

However, young plants do require water as they are becoming established. Keep seeded areas lightly moist for germination and provide seedlings with enough water to keep the soil barely moist.

After transplanting to the garden, provide them with an inch of water per week until the first flower buds appear. After that, watering is optional except in very dry conditions.

And when it comes to nutrients, these plants are almost entirely self-sufficient, but not completely.

It helps to sweeten the soil a touch when planting, but rich soils laden with compost are definitely not appreciated.

Plus, you should avoid planting them in beds close to lawns that are regularly fertilized.

Feed formulas for lawns are typically high in nitrogen, and too much nitrogen can produce leggy growth with lots of lacy foliage but few flowers.

If you’d like to learn more about how to grow cosmos, our guide has all the know-how.

To encourage a long flowering season with lots of blooms, deadheading spent flowers is a worthwhile task, because as long as your plants aren’t setting seed, they’ll continue to bloom.

To encourage new growth, remove spent blooms at the base of the flower stalk, just above where it branches from a main stem or other flower stalks. You can learn more about deadheading cosmos in our guide.

Regularly harvesting for cut flowers also helps to promote more bud set.

But if you plan to collect your own seed, allow a few of the earliest blooms to stay on the stem to complete their reproduction cycle. Learn more about how to harvest and save cosmos seeds here.

Fertilizing In-Ground Plants

Cosmos plants thrive in lean soils with meager nutrients. And fertilizing during the growing season can have a negative impact on flowering, causing the plant to produce too much foliage, reducing bud set, or resulting in leggy growth with weak, floppy flower stems.

As such, it’s best to avoid planting cosmos in soil that’s overly enriched with lots of compost, humus, or manure.

Seeds or seedlings can be sown into average garden soil with a little high-phosphorus bone meal mixed in for vibrant root growth plus strong flower and seed production.

If the soil is acidic, depleted of nutrients, or has a gritty, sandy texture, it’s also helpful to mix in a half-shovelful of compost, manure, or worm castings before planting, to sweeten the soil just a bit. 

In the garden, that’s all these cheerful plants need in terms of feeding – fertilizing over the growing season isn’t required and can actually hinder the showiest floral displays.

However, container grown plants are a different story.

Planters and Pots

When cosmos are grown in containers, there is a limited amount of soil and nutrients available to the plants, so they’ll need regular applications of fertilizer to promote vibrant, floriferous growth.

Along with mixing in some bone meal when planting, potted plants benefit from regular light applications of a bloom-boosting fertilizer.

It’s important to dilute the monthly feedings to half strength to keep the plants healthy, upright, and producing ample buds. If applied at full strength, fast acting liquid fertilizers can promote leggy growth and weak flower stems.

Since cosmos plants prefer lean soils, avoid adding large amounts of compost or manure to your potting blend.

The following soil recipe is ideal for potted plants that enjoy arid conditions and scanty nutrients.

• One part compost, well-rotted manure, or worm castings for long-lasting nutrients
• One part garden soil for structure and tilth
• One part coconut coir, peat moss, perlite, or vermiculite to help retain moisture in the root zone
• One part gritty materials such as landscape sand, granite grit, pea gravel, or stone chips to ensure excellent drainage

And it should also be noted that container plants, even established ones, need more frequent watering than those growing in garden beds.

Water your potted comos once or twice weekly, depending on conditions – aim to provide them with an inch of water when the top two inches of soil dry out.

For more information read our guide to growing cosmos in containers.

If you’ve decided you prefer a tidy floral display with only fresh looking blooms, you may have questions about pruning off the faded floral parts of your cosmos plants:

Where do you cut exactly? And how do you tell the difference between a seed head and a fresh, new flower bud?

In this article I’ll present the pros and cons of this decision so you can choose whether or not to bother with deadheading the cosmos flowers in your garden or yard – and how to proceed if the pros win you over!

Here’s what I’ll cover:

Before we get started, if you want to review the basics of cultivating these summer annuals, be sure to read our complete guide to growing cosmos in the garden.

Should You Deadhead Cosmos Flowers?

Rather than looking for guidance about how to prune, are you wondering if you actually need to cut back the faded blooms of Cosmos species in your garden or yard?

Afterall, there are so many other important summer activities that could take the place of this one – relaxing in the gazebo, planning ornamental landscaping around your favorite garden bench, or catching up on reading your stack of gardening books!

Commonly known as deadheading, removing spent flowers is a technique used by gardeners to refresh plants, encouraging more blooms and bushier growth.

I’ll present you with the pros and cons of undertaking this chore with these particular plants so that you can make an informed decision.

There are several reason you may want to consider deadheading cosmos aka Mexican aster flowers:

If you want to encourage bushier growth in these flowering plants, deadheading as well as pruning can help.

Do you want your plants to focus all their energy on producing more flowers rather than making seeds? Then deadheading is the best choice for you.

Are the Cosmos species you’re growing perennials rather than annuals?

Annual species like C. bipinnatus and C. sulphureus tend to produce loads of flowers even without deadheading.

Perennials such as chocolate cosmos (C. atrosanguineus) don’t tend to have quite the long-lasting and overeager bloom load as the annuals.

So to keep C. atrosanguineus blooming, deadhead the spent flowers to encourage a longer blooming season.

Another practical reason that you may want to deadhead your cosmos is to prevent volunteers popping up in your garden.

C. bipinnatus self-seeds readily – so much so in fact, that it has become invasive in some areas. If you don’t want to see unplanned seedlings growing in your garden next season, removing seed heads before they mature is a good plan!

Of course, there’s also the possibility you absolutely cannot bear the sight of messy, faded flowers, in which case you’ll want to do some trimming.

Finally, if you are worried that your homeowners association will slap you with a steep fine for growing imperfect plants in an otherwise perfect neighborhood, well, yes, you better grab those pruners and start removing spent blooms.

Now that we’ve covered reasons in support of deadheading cosmos flowers, here are a few arguments in favor of allowing the faded blooms to complete their life cycle in your yard or garden:

Often the motivation for deadheading is to encourage “more blooms.” How many do you need exactly?

As a convert to the “let them be” camp, I have never once looked out at my garden and thought, “wow, my favorite ornamental annuals just aren’t blooming enough!”

If you feel like you’re not seeing enough blooms on the Mexican aster plants you’re growing, maybe you should sow more seeds!

These plants can also be grown in pots if you’re running out of room in your garden beds.

You can learn more about growing cosmos in containers in our guide.

To some of us – myself included! – the spent blooms are actually quite attractive. The dried up petals only remain in place for a few days before falling off and leaving behind a ripening seed head.

And that ripening seed head can be quite difficult to distinguish from a new flower bud to the untrained eye.

Personally, I love looking out over the large swaths of these blooms in my garden and seeing them at all different stages of their life cycle, from bud, to bright bloom, to maturing seed head, and I find this mix quite captivating.

For that matter, what is your garden aesthetic? If you enjoy a wild looking, cottage garden style for your ornamentals, there’s certainly no reason to deadhead cosmos.

And as a more practical motivation, if you skip deadheading, the seed heads will continue to ripen, eventually producing fully mature seeds that you can save to grow in future seasons or share with friends and family.

On the other hand, if you prune the seed heads before they mature, the seeds won’t be viable. They need to ripen on the plant.

Want to learn more about harvesting and saving cosmos seeds? Check out our article!

Another argument in favor of letting faded cosmos flowers remain rather than pruning them off is that the mature seeds actually provide good food for wild birds.

So if you’re interested in gardening for wildlife, you can easily leave this homegrown “bird food” for visiting wild birds to add to their menu.

Finally, if you have a large number of these plants growing in your garden or yard, deadheading cosmos can be a time-consuming chore – unless you whack the plants back to a certain height, removing seed heads, new buds, and every floral stage in between.

In my very large polyculture garden where borage is rubbing elbows with cucumbers, and nasturtiums grow as a ground cover below my pole beans, I have more pressing work than deadheading cosmos flowers. There’s lettuce to be harvested and squash bugs to control!

I bet you may have other, more compelling gardening tasks to dig into as well.

Perhaps you’d like to spend some time learning how to create a gorgeous color scheme in your garden, extend your garden vertically, or xeriscape your yard to save water?

But if you were swayed by the pros of deadheading, I harbor no hard feelings. Read on to learn how to proceed!

How to Recognize Spent Flowers

Before you start snipping, you’ll want to be sure you know how to recognize spent flowers once the petals fall.

When the petals are still attached, it’s quite obvious that you’re dealing with a spent flower.

However, once the petals fall, there are some similarities between a yet to open flower bud and an unripe seed head – and the two can be difficult to tell apart if you don’t know what you’re looking for.

You certainly won’t want to trim off any flower buds if you can help it!

An unopened bud on the far right and an unripe seed head in the bottom center of the photo.

Unopened buds are rounded on the top, while unripe seed heads typically have a tufted appearance.

As seed heads start to mature, the seeds will turn brown and the seed heads will take on a spiky shape.

In contrast, the unopened buds have a smooth, rounded shape as you can see in this picture below.

Once you know what you’re looking for, it’s not so hard to tell the difference between buds and seed heads!

Where to Cut Faded Cosmos Flowers

Before you make your cuts I do recommend sterilizing your garden pruners by rubbing them down with hydrogen peroxide.

This will help you to avoid spreading disease throughout the garden.

Each flower has a stalk attached to a larger stem of the plant and you’ll need to make your cut at the base of the flower stalk, close to this larger stem.

Make your cut next to the stem, so that you don’t leave a beheaded flower stalk behind, which would frankly be much more unattractive than the spent blooms you are removing!

And that’s it!

This method will work for gardens and yards with small numbers of plants but will be very time consuming in larger spaces. If you have large numbers of spent blooms to trim back, you may need to use a faster method.

For larger sized plantings, wait until most of the blooms on the plants have faded, then use garden shears or pruners to trim the plants back. You can cut them back to 12- to 18-inches tall if you’d like.

Of course, you’ll be sacrificing fresh blooms if you use this method, but the plants will become bushier and soon return to creating their floral show.

Would you like to dig even more deeply into the fine art of pruning off faded blooms?

Read our article to learn more about deadheading blooming plants in the garden.