Their wispy, fernlike clouds of foliage and showy blossoms that seem to float ethereally above them will surely satiate the desire for summer blooms buzzing with bees and butterflies. We link to vendors to help you find relevant products. If you buy from one of our links, we may earn a commission. Come and learn all about it, including which varieties are best for the job! Here’s what you’ll find as you read on: For centuries, cosmos have been a favored addition to the garden in many parts of the world. Their charm is obvious – with bright, airy petals in a wide variety of tones that attract pollinators and add the cheer of spring and summer to the landscape. Some species aren’t the best choice for growing in pots because they don’t support themselves well, leaning over as the top heavy blooms weigh them down, but there are a few that make gorgeous candidates. We’ll cover those in detail a bit later on. One common question is often asked by gardeners: are cosmos annuals or perennials? Depending upon which species you choose, there are both annual and perennial types. Cosmos are suited to USDA Hardiness Zones 3 to 11. They greatly prefer the warmth and sunshine of late spring through late summer, and even the perennials among them don’t overwinter well in cold climates without protection. Because they often self-seed, annuals may give the impression that they’re returning each year. Seeds can also be collected for planting the following spring, or the self-seeded containers could be stored over the winter months in a protected location and returned outdoors to sprout on their own. If you’re looking for perennials, there are fewer choices available. Let’s talk about what makes these flowers a perfect choice for container growing next.
Why Choose Container Growing?
Obviously, if you don’t own your yard or you don’t have one at all, it takes a little creativity to have a flower garden. Containers make that possible without the commitment of leaving it all behind or having to tear it out when you move. If space is limited and growing food crops is more important, they can be potted and placed near those crops for improved pollination as they attract many types of beneficial insects. Specimens such as these will receive better protection from the cold when they can be moved to a sheltered area, so those perennials will actually stand a chance of survival until next spring. Bear in mind that a potted root system is more exposed than in-ground plantings, with conditions equivalent to about two zones cooler. Luckily, they can simply be moved into a shed or garage to prevent damage when temperatures dip below 40°F. If it turns out that the location you’ve chosen doesn’t get enough sun, or you want to make your gorgeous blooms more visible on the porch or patio, many planters are mobile, so they can be moved wherever you’d like. A well-cared-for pot can also be reused each year after some sanitation, becoming home to generations of lovely blossoms. It’s easier to keep a small amount of substrate watered in times of drought, and much less likely that ground-dwelling pests such as voles will munch on the roots and foliage. Finally, it’s much easier to amend and refresh potted soil than it is to deal with issues in the garden plot. If maladies such as infestation or disease take hold, it’s simple to dump the soil, sanitize the pot, and start fresh. There’s a vast selection of pots on the market, designed specifically for gardening, but upcycled containers such as barrels or washtubs can work equally well. How can we determine which type is best for this purpose? Let’s take a look.
Which Container Is Best?
Because cosmos are very drought tolerant and heat loving, their roots tend to reach rather deeply. A vessel of appropriate size should allow at least 12 inches of depth and be equally wide, or wider. Plastic or resin pots should be avoided, as they tend to hold moisture for a longer period and the damp environment invites root rot and pathogens that can spread disease. A vessel such as this absolutely gorgeous ceramic Paloma planter is the perfect choice since it has a wide base that will keep it firmly seated. It’s available from Terrain. Ceramic Paloma Planter A container that allows for transpiration, such as clay or concrete, is best to allow airflow. Drainage holes are also a must as soggy soil will lead to certain doom.
How to Prepare Your Container
Planters that are being reused should be thoroughly sanitized prior to planting. Pest eggs and pathogens are known to persist in crevices and under the rim between uses. Scrub out any old soil with a brush and some soapy water. A new pot may be less likely to harbor pests or pathogens, but it isn’t impossible, so give those a scrub and a rinse as well. Prepare a solution of nine parts water to one part household bleach and submerge the entire pot for 15 to 30 minutes. A deep storage tote full of this solution provides ample space to allow for full submergence. Remove the container and rinse it thoroughly to remove the bleach; even trace amounts can cause damage to the roots. Now that the vessel is prepared, choosing your substrate is the next step. Many varieties of cosmos originated in Central America, and they prefer sandy soil that is low in nutrients. A mix of one part potting soil or coconut coir to one part sand is perfectly adequate, as anything richer or more dense in organic material can lead to weakened stems and low blooming rates. With the vessel lovingly prepared, it’s time to add the specimen that will call it home.
How to Grow
Unlike some flowering plants, established cosmos will happily bloom and grow with a great deal of neglect. Soil that’s lower in nutrients is preferred, and they won’t wither and die without daily watering. Seedlings can sometimes be purchased from a garden center in early spring, but more often than not, seeds will be the starting point. Bare root tubers, such as those for the chocolate species (C. atrosanguineus), are also a possibility when planting. They grow very quickly – from germination to blooming, it usually only takes a few weeks.
Planting
Bare roots should be soaked in warm water for an hour or two to rehydrate them prior to potting. Purchased or saved seeds can be started indoors, or you can simply press them about half an inch deep into the soil in a prepared pot to skip the transplanting later on. Cover them lightly with soil and water them in well. Seeds started indoors four to six weeks before the last frost date will typically bloom earlier in the season, but those started outdoors won’t need hardening off. Space dwarf varieties about two to three inches apart, while taller species should be spaced eight to 10 inches apart, or planted individually in containers. Place the pot in a warm, sunny spot away from harsh winds where the seedlings will receive at least six hours of sunlight per day, or harden off seedlings and transplant them to a container in the same type of location. Until the roots are well established, which can take four to six weeks, seedlings should be bottom watered or misted daily to keep the soil moist. Be sure that excess moisture is draining off, as standing water or soggy substrate will surely kill the seedlings. After about six weeks, gradually taper off watering until the soil begins to dry out in between. If extreme heat or a total absence of rain continues for more than a few days, begin watering more often.
Staking
While they are not climbers, taller species may need some support to remain upright. The long stems don’t always fully support the top-heavy flowers.
Watering
Feel the surface of the potting medium daily, especially during periods of high heat. If the top two inches or so feel completely dry to the touch, water deeply and allow the excess moisture to drain off. I prefer to use a few bamboo stakes interwoven with green twine as brown or white twine can look obvious and detract from the beauty of the flowers. Green Jute Twine Press the bamboo stakes in deeply prior to planting or while seedlings are still small to avoid damaging their roots. Wrap the twine in a square or star-shaped pattern between the stakes to give the stems something to lean against as they grow. In the absence of rain, most container-grown cosmos will need water about twice per week – but again, weather conditions and temperatures can change that schedule. Providing consistent moisture will encourage more prolific blooming.
Pruning
Even though the main stems are rather woody, cosmos are considered herbaceous and they will not need pruning. Bear in mind that some species can reach an excess of five feet in height, so if size is a concern, choose a dwarf or more compact variety instead. They do, however, benefit from taking regular cuttings. Snip stems at eight to twelve inches in length and use them as cut flowers in bouquets and arrangements. Trimming regularly encourages the production of new buds. Deadheading should also be done regularly if you don’t plan to collect seed for planting the next season.
Winterizing
As I mentioned, perennial types will need some winter protection in regions where winter lows regularly dip below about 40°F. Moving the container to a sheltered location is the safest bet since the root system is more exposed to the elements in a pot than it is in the ground. You can also choose to wrap the pots with burlap or bubble wrap for extra protection. Perennial species will generally continue to bloom until mid- to late fall in regions that experience cold winters. After the last blooms have withered, trim the dead stems back to the rhizome and wrap the pot, then move it to shelter. In the case of annuals, if you decide to allow them to self-seed in their containers, move these to a basement or garage for the winter months, and take them out when spring rolls around.
Repotting
Repotting is not generally necessary with annuals, unless you started with a vessel that is too small for the mature size of the root system. Perennials may need to be repotted periodically to maintain adequate root space and freshen the substrate. Start by preparing the new vessel as previously described. Add some fresh potting medium to the bottom to elevate the root system so it sits just below the brim. Before moving the plant to its new pot, be sure to give the roots a quick visual check to make sure there aren’t any that are dead or rotting. Trim away any that you see, then place the root ball into its new home and backfill around it with fresh medium. Water it in well to settle and move it back to its sunny location.
Growing Tips
Choose a container that is at least 12 inches deep with a wide base to avoid tipping over, especially for taller species.Provide at least 6 hours of sunlight per day and avoid overwatering – allow the soil to dry out in between.Sandy soil that is relatively low in nutrients is preferred to avoid weak stems and poor blooming.
Cultivars and Species to Select
If you’ve read our guide to 25 of the best cosmos cultivars, you already know that there are many gorgeous options that can be grown in containers. There are some cosmos that are best suited to this container planting because of their more compact growth habit. Cosmos are also heat and drought tolerant, and in regions with mild winter weather, they may continue to bloom into fall. Here are just a few of the best types for potting:
Capriola
Known as a semi-dwarf type, C. bipinnatus ‘Capriola’ tops out at less than 30 inches in height. ‘Capriola’ The three-inch blooms are white to pink with deep pink to red tips and appear between June and September, floating above feathery green foliage. Packets of 50 seeds are available from Burpee.
Rose
Another dwarf cultivar, C. bipinnatus ‘Rose’ adds soft pink-purple color to the potted garden. The two- to four-inch blossoms have petals that are slightly darker in the centers, with stems reaching a height of up to 24 inches, although they’re often shorter. ‘Rose’ This early-season cultivar begins setting buds in spring, so their blooming season can be several months longer than what you’ll see with some other varieties. ‘Rose’ seeds are available by the packet, by the ounce, or by the quarter-pound from Eden Brothers.
Sulfur
If the pastels don’t do it for you and you want something bold and bright, C. sulphureus aka sulfur cosmos is the one for you! The bright yellow-orange, marigold-like blooms float atop dwarf stems that reach only 24 inches in height at the most. This species fits well in planters and containers, it rarely needs staking, and has slightly smaller blooms than C. bipinnatus. Sulfur cosmos is tolerant of temperatures over 100°F, so it’s a great choice for southern gardeners! Sulfur Carpet Mix features bright orange, red, and yellow blooms on compact plants that reach just 12 inches tall. Sulfur Carpet Mix You can find seeds available in a variety of package sizes from Eden Brothers.
Managing Pests and Disease
Cosmos grown in containers are rarely affected by pests or disease, although aphids, spider mites, and thrips can sometimes make an (unwelcome) appearance. Fortunately, they’re also considered deer resistant. Diseases are a relatively rare occurrence, but afflictions can include aster yellows and powdery mildew. Smut can occasionally become a concern as well. Learn more about the pests and diseases that you should keep an eye out for in our complete guide to growing and caring for cosmos.
Blossoms Floating Like Stars in the Sky
I’ve never been able to decide which type of cosmos is my favorite. Each has its own appeal, from soft, muted pinks and purples to the deepest crimson, to bright, pure white – but no matter which I’ve planted, they all make stunning additions to my garden. Even if you already have some growing elsewhere in your landscape or garden, a few potted beauties never hurt! Are you growing cosmos in containers? We’d love to see what you’ve got growing – please share your photos in the comments below! Questions are always welcome as well. For more information on growing flowers in containers, check out these titles next:
Tips for Growing Coneflowers in ContainersA Pot of Gold: How to Grow Sunflowers In Containers5 Tips for Growing Asters in Containers
