With effortless elegance, cosmos plants add an abundance of brightly colored flowers to the summer garden.
These sparkling annuals are the epitome of low-maintenance, putting on long-lasting floral displays with little effort from the gardener, hardly any water, and minimal fertilizing!
Pollinators love the flat or cupped flowers, and the plants self-seed readily, creating attractive stands of cheerful maroon, orange, pink, scarlet, white, and yellow blooms that return the following year.
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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:
When and How to Fertilize Cosmos
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.
Bloom booster feeds typically have a higher phosphorus ratio than all-purpose fertilizers, which is ideal for promoting flowers.
I like to use this organic Rose and Bloom plant food from Burpee. It’s a granular product with an NPK ratio of 4-6-4.
Apply slow release pellets in early spring and again in early summer, or use a liquid or water soluble solution diluted to half strength every month.
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.
Tougher Than They Look!
Tougher than they look, cosmos are an excellent choice for providing brightly colored flowers in garden beds, borders, and containers – as well as in water deprived areas.
For the best floral display, avoid planting in soil that’s been heavily enriched with organic materials like compost.
A little mixed into lean soil at planting time is fine but use a light touch to prevent excessive foliage and leggy growth with few flowers.
Also, hold off fertilizing garden plants – only container grown cosmos need additional feeding, which should be done at half strength to keep plants strong and vibrant.
Any questions about how and when to fertilize cosmos plants? Drop us a note in the comments section below.
And for more information about growing cosmos in your garden, read these guides next:
