Contents
Cosmos
Cosmos flowers are beloved ornamental annuals known for their vivid colors and classic, daisy-like shape. Blooming atop slender, branching stems with feathery green foliage, cosmos create an airy, informal display in gardens. The flowers typically have a ring of broad petals surrounding a yellow central disk, giving them the open, cheerful look of a wild daisy. Cosmos blooms can range widely in color depending on the species and variety. The common garden cosmos (Cosmos bipinnatus) produces masses of delicate pink flowers and white flowers in pastel shades (along with magentas and bi-colors), whereas the sulfur cosmos (Cosmos sulphureus) lights up planting beds with its vibrant golden yellow flowers and fiery orange flowers. Each blossom is 2 to 4 inches across on average, and many varieties continue flowering continuously through the warm months.
These easy-care flowers thrive in conditions that might challenge fussier plants. Native to hot, sunny regions, cosmos tolerate heat, drought, and poor soil better than many other ornamentals. They typically bloom from mid-summer until the first frost, providing consistent color and nectar for pollinators when many other plants have faded. Butterflies and bees flock to cosmos, making them a favorite choice for pollinator gardens and wildflower meadows. Gardeners also prize cosmos as cut flowers – their long, wiry stems and bright, satiny blooms add a whimsical touch to summer bouquets. Overall, the cosmos is an undemanding, prolific bloomer that brings a natural cottage-garden charm to landscapes with minimal effort.
🌱 Taxonomy and origin
Cosmos is a genus of flowering plants in the family Asteraceae, which is commonly known as the daisy family. This membership makes cosmos relatives of other well-known garden flowers like marigold, sunflower, and zinnias. The genus Cosmos includes approximately 20 to 30 species of herbaceous plants that are native to Mexico and parts of the Americas. These wildflowers are predominantly found from the southern United States through Central America and into South America. Spanish missionaries in Mexico are said to have given these flowers the name “cosmos,” derived from the Greek word kosmos meaning order, harmony, or ornament, inspired by the orderly arrangement of the petals.
The most widely cultivated species are Cosmos bipinnatus and Cosmos sulphureus. Cosmos bipinnatus is often called garden cosmos or Mexican aster – it features finely divided, fern-like foliage and flowers in cool shades of white, pink, and purple. Cosmos sulphureus, known as sulfur cosmos, has slightly coarser, lobed leaves and blooms in warm tones of yellow, gold, and orange. A third species grown in specialty gardens is Cosmos atrosanguineus, the chocolate cosmos, prized for its deep reddish-brown blooms and vanilla-chocolate scent. In their native range, cosmos grow as wildflowers along roadsides and open fields. They were introduced to European and American horticulture in the 18th and 19th centuries and have since naturalized in many warm regions. Today, cosmos is cultivated worldwide as annuals, celebrated for their ease of growth and abundant blooming.
🌸 Bloom time
Cosmos has a long blooming season that stretches through the warmest part of the year. In most U.S. climates, cosmos begin to flower by early to mid-summer (often starting in June or July) and continue producing new blooms vigorously until halted by cold weather in fall. These plants will keep flowering non-stop all summer, especially if conditions are favorable and spent blooms are removed. Regular deadheading (snipping off the old, withered flowers) is often recommended to channel the plant’s energy into forming new buds rather than seeds, thereby extending the bloom period. Even without strict deadheading, cosmos tend to bloom in successive flushes, with fresh flowers opening over many weeks. By late summer, they are usually at peak bloom, covered in colorful blossoms. In mild regions with late or no frosts, cosmos can bloom well into October and even November. Essentially, from the height of summer heat until the first hard frost of autumn, cosmos will provide continuous color. This lengthy bloom time is one of the reasons cosmos is so valued in gardens – they reliably fill the gap when spring flowers are gone and fall flowers have yet to peak, offering a bridge of bright color across the seasons.
📏 Height and spread
Cosmos plants exhibit an upright, branching habit and can vary in size from compact to towering, depending on the variety and growing conditions. On average, the height ranges between 1 and 6 feet tall. The common garden cosmos (Cosmos bipinnatus) typically grows about 3 to 5 feet in height, with some cultivars reaching up to 6 feet in ideal conditions. Dwarf varieties have been developed as well, which stay much shorter – around 1 to 2 feet tall – making them suitable for containers or the front of borders. Cosmos sulphureus is often a bit shorter than C. bipinnatus, generally around 2 to 4 feet tall, though some selections can approach 5 feet. Cosmos atrosanguineus tends to be compact, usually about 1 to 2.5 feet high.
In terms of spread, individual cosmos plants are somewhat narrow but well-branched. A typical cosmos will spread about 1 to 2 feet wide (its foliage and branches fanning out from the central stem). They are not invasive via roots and stay as discrete clumps, but they have an open, airy form. When planting cosmos, it’s common to space them roughly 12 to 18 inches apart. This spacing allows each plant to develop its bushy form and ensures good air circulation. The foliage of garden cosmos is light and feathery, which means even a large plant doesn’t feel heavy or dense in the landscape. Staking may be needed for the tallest varieties or in windy locations, as the hollow stems can bend or break under the weight of abundant flowers or strong gusts. However, when cosmos is grown in masses or allowed to lean against neighboring plants, they often support each other and create a meadow-like effect. Overall, the height and spread of cosmos can be tailored by choosing appropriate cultivars, and even the tallest types maintain a graceful, transparent quality that doesn’t overwhelm neighboring plants.
☀️ Light
As sun-loving natives of warm climates, cosmos flowers grow best with plenty of sunlight. Full sun exposure (at least 6 to 8 hours of direct sun per day) is ideal to keep cosmos plants strong and blooming profusely. In full sun, cosmos develop sturdy stems and produce the most flowers. They can tolerate partial sun or light shade, but a shadier location will usually result in fewer blooms and taller, leaner plants that stretch for light. For instance, if cosmos is grown in an area that only gets 3-4 hours of sun, they may still survive and flower, but the flowering will be sparser and the plants might become leggy and prone to flopping.
Another reason to provide ample sunlight is that cosmos is adapted to relatively dry, open environments-full sun conditions help keep foliage dry and reduce the risk of fungal issues like mildew. In cooler or coastal climates, cosmos will appreciate all-day sun to reach their full potential. In very hot, arid regions, cosmos can handle the intense sun, though a bit of afternoon shade won’t usually harm them if water is adequate. Overall, to ensure the best performance, plant cosmos in the brightest spot available. Gardens, borders, or meadows with unobstructed sun will yield stocky plants and an abundance of those daisy-like blooms all season long.
💧 Water
Cosmos is quite forgiving when it comes to water and, in fact, prefer a relatively dry regime compared to many other ornamentals. These plants have moderate to low water needs. After planting, young cosmos seedlings should be watered regularly until they establish strong roots – typically keeping the soil evenly moist (but not waterlogged) for the first few weeks. Once established, cosmos display notable drought tolerance. They originate from regions with periodic dry spells and can get by with minimal supplemental watering, especially if grown in the ground with decent soil depth. In most climates, cosmos only require watering about once or twice a week if there’s no rain, and even less often if there is regular rainfall.
It’s important not to overwater cosmos. Soggy or waterlogged soil can lead to root rot and overly lush foliage with fewer flowers. If cosmos is planted in containers or very sandy soil, they will need more frequent watering than those in garden beds, but letting the top inch or two of soil dry out between waterings is recommended. Watch the plants’ leaves for signs of water stress – slight wilting on a hot afternoon can be normal, but prolonged drooping indicates they need a drink. Deep, infrequent watering that soaks the root zone is better than shallow daily sprinkling. Cosmos generally handles dry conditions well, and in fact, too much pampering with water and fertilizer tends to produce lots of leafy growth at the expense of blossoms. A good rule of thumb is to water cosmos when the soil is dry about 2 inches down or when the plants begin to show slight wilt in the morning. Under typical summer conditions, they will bounce back quickly after watering and continue to bloom happily.
🌍 Soil and pH
Cosmos flowers are not very fussy about soil, which is one reason they are so easy to grow. They thrive in average garden soil and often perform better in soil that is on the lean side of fertility. A well-draining soil is the most critical requirement. Cosmos do best in loose, well-drained substrates like sandy or loamy soil. Heavy clay that holds water can cause root problems for cosmos, so if your soil is clay-rich, it’s helpful to amend it with compost or sand to improve drainage before planting. Interestingly, cosmos prefer soil that is not overly rich. In very fertile or heavily amended soil, cosmos plants may grow extra-large with lots of lush green foliage, but they will produce fewer flowers. It’s often said that “poor” soil – meaning low to moderate nutrient levels – encourages cosmos to bloom more prolifically, as a bit of nutrient stress spurs flowering.
In terms of pH, cosmos is adaptable. They can tolerate a broad range of soil pH from slightly acidic to slightly alkaline. Generally, a neutral to mildly acidic pH of around 6.0 to 7.5 is ideal. They have been known to do fine in soils up to about pH 8.0 as well, especially Cosmos sulphureus which often grows in alkaline soils. Gardeners need not worry too much about adjusting pH specifically for cosmos, as long as it falls in a normal range. If planting cosmos in a bed with extremely acidic soil (pH below 5.5) or very alkaline soil (above 8.0), adjusting toward neutral will improve overall plant health, but cosmos is more tolerant than many plants of less-than-perfect soil conditions. In summary, provide cosmos with a well-drained, average soil and they will be content. No special fertilization is needed, in fact, avoiding excessive fertilizer is recommended to ensure they channel energy into flowering instead of just foliage.
❄️ USDA hardiness
When discussing USDA hardiness zones for cosmos, it’s important to note that most cosmos are treated as annuals, which means their hardiness is not measured in the same way as perennial plants. Cosmos plants, such as C. bipinnatus and C. sulphureus, are not frost-hardy. The first autumn frost will kill them, so they cannot survive winter in regions that experience freezing temperatures. In practical terms, gardeners across USDA Zones 2 through 11 grow cosmoses as summer annuals – sowing new seeds or planting new starts each spring after the last frost. In the warmest parts of the United States, however, cosmos can behave a bit differently. In USDA Zones 9 and above (essentially areas with very mild winters), cosmos may persist beyond a single season. For example, in frost-free subtropical climates, cosmos might act as short-lived perennials or reseeding annuals that bloom year-round or come back the next season from dropped seeds.
Cosmos atrosanguineus (chocolate cosmos) has tuberous roots and is sometimes categorized as a tender perennial. It can survive winters outdoors in roughly USDA Zone 9 or 10 and warmer, where the ground doesn’t freeze deeply. In Zones 7 and 8, gardeners may be able to overwinter chocolate cosmos by mulching heavily to insulate the tubers, but there is some risk of losing the plant in hard freezes. As a precaution, many people dig up and store chocolate cosmos tubers indoors for winter (similar to how dahlia tubers are handled) and replant them in spring. For the typical cosmos grown from seed annually, there is no overwintering – they complete their life cycle in the growing season. To summarize, cosmos can grow in essentially any USDA zone during the frost-free months (making them very versatile across climates), but they only survive as perennials in the mildest winter regions. Everywhere else, expect to replant each year or rely on self-sown seeds that sprout after winter passes.
🌼 Propagation and longevity
Cosmos is most commonly propagated by seed, and they are among the easiest flowers to grow from seed. Gardeners often simply direct-sow cosmos seeds into the garden after the danger of frost has passed in spring. The seeds are relatively large (narrow and elongated) and easy to handle. They should be sown about 1/4 inch deep in loosened soil and kept lightly moist until germination. Cosmos germinates quickly in warm soil – typically seedlings will emerge within 5 to 10 days when soil temperatures are around 70°F. Starting cosmos indoors is also an option: sow seeds in small pots or seed trays 4 to 6 weeks before the last frost date, and then transplant the seedlings outdoors once the weather warms up. However, cosmos do not like having their roots disturbed too much, so if starting indoors, it’s best to use biodegradable pots or transplant carefully to minimize root shock. Whether sown indoors or out, cosmos grow rapidly. They will usually begin flowering about 8 to 12 weeks after germination, so by mid-summer the first blooms appear. Another method of propagation, if you have existing cosmos plants, is simply allowing them to self-seed – letting some seed heads mature and drop in the garden can result in new volunteer seedlings the next year.
In terms of longevity, individual cosmos plants are short-lived by nature. As annuals, they sprout, bloom, set seed, and then die at the end of the growing season. They do not survive winter freezes (aside from the tuberous-rooted chocolate cosmos, as noted in the hardiness section). However, cosmos make up for their brief individual lifespan with prolific seeding and a long bloom period. Throughout summer and fall, each plant continually produces new flowers, so the overall display can last for months on a single planting. If you want to extend the season or have a succession of fresh plants, you can stagger your plantings by sowing seeds again in early summer, which will yield new blooming cosmos later in the season once the first batch starts to tire. Some gardeners perform multiple sowings (for example, one in late spring and another in mid-summer) to ensure fresh waves of flowers. Collecting seeds from cosmos is straightforward: once flowers fade, the seed heads turn dry and the slender seeds can be shaken out or picked from the heads. Store them in a cool, dry place and they will remain viable for a few years. This seed-saving allows you to grow cosmos year after year without buying new seed packets, and also to share with friends.
It’s worth noting that Cosmos atrosanguineus (chocolate cosmos), which is usually propagated from division or cuttings, has a different longevity profile. If its tubers are dug up and overwintered properly or if it’s grown in a mild climate, the same plant can live for several years, regrowing each spring from the stored root. This makes chocolate cosmos more akin to a perennial in cultivation, despite its inability to produce viable seeds (most chocolate cosmos are clones of a sterile form). For the seed-grown cosmos species, though, the strategy for longevity is to let them renew themselves by seeding or to replant anew each season. In summary, propagation of cosmos is typically through easy seed sowing, and while each plant doesn’t live beyond one growing season, their legacy continues through the abundant seeds and continuous summer blooms.
👃 Fragrance
Most cosmos varieties are not particularly known for their fragrance. The blossoms of Cosmos bipinnatus (garden cosmos) and Cosmos sulphureus (sulfur cosmos) generally have a mild, faint scent, if any at all. When you sniff a typical cosmos flower, you may detect a light, delicate floral smell, but it’s subtle and easily missed. Cosmos were bred and selected more for their color and form than for fragrance, so they don’t perfume the air like some other flowers do. The foliage of cosmos, when crushed or brushed, can emit a slight herbal or resinous odor – some gardeners describe it as a fresh green scent – but it’s not strong unless you handle the plant.
There is one notable exception in the cosmos family: Cosmos atrosanguineus, the chocolate cosmos. True to its name, the chocolate cosmos flower exudes a sweet fragrance reminiscent of chocolate or vanilla. On warm summer days, especially under full sun, a chocolate cosmos bloom can give off a dessert-like aroma that is quite distinct and pleasing. This scent is one of the main attractions of growing that particular species, since the flowers themselves are a rich velvety maroon color but relatively small. Aside from chocolate cosmos, if fragrance is a priority, cosmos might not be the first choice of flower. However, many people grow them alongside fragrant plants, and the lack of strong scent in most cosmos means they won’t clash with scented neighbors. In summary, standard cosmos flowers are effectively scentless to our noses, while the rare chocolate cosmos offers a unique sweet fragrance as an added bonus.
⚠️ Toxicity and pet safety
Gardeners with pets or children often need to consider whether a plant is poisonous. In the case of cosmos, there is good news: cosmos flowers and foliage are generally considered non-toxic. They are not known to be poisonous to humans, dogs, cats, or other common domestic animals. This means you can plant cosmos in your yard or flower beds without significant worry that a curious pet nibbling a leaf will suffer serious ill effects. Cosmos appears on many lists of pet-safe garden plants. If ingested in large quantities (for example, if a dog ate an entire flat of seedlings or a cat chewed multiple stems), a pet might experience some mild gastrointestinal upset simply from the roughage – symptoms like minor vomiting or diarrhea could theoretically occur – but this would be a transient, non-serious reaction. There are no toxic alkaloids or dangerous compounds in cosmos known to cause organ damage or other severe problems.
It’s always a relief for gardeners that such a beautiful, easy plant is also safe for the whole family. You can confidently include cosmos in a garden that dogs or cats have access to. In fact, because cosmos is so attractive to butterflies and other beneficial insects, they can contribute to a lively, engaging garden environment for pets to explore. Likewise, for homes with young children, cosmos pose no toxicity risk – touching or even tasting a petal will not harm a child (though it’s of course not recommended to let children eat plants indiscriminately). In summary, cosmos is a pet-friendly, family-friendly choice. As with any non-toxic plant, the main concern would only be potential minor choking if large pieces are swallowed, or minor tummy upset if a lot of plant material is eaten, but overall cosmos are as safe as they come in the ornamental plant world.
🌿 Vase life
Cosmos flowers are popular for cutting, but their vase life – the duration they remain fresh after being cut – is moderate rather than exceptional. On average, freshly cut cosmos blooms will last around 4 to 6 days in a vase of water. With excellent care, some may hold up for about a week, but they generally are not long-lasting compared to sturdier cut flowers like roses or chrysanthemums. Nevertheless, the vivid colors and airy charm of cosmos blooms make them worthwhile additions to casual bouquets, even if each flower’s vase time is brief. The cosmos’ delicate petals and thin stems contribute to their shorter vase life. However, there are strategies to maximize how long they look good in arrangements.
For the longest vase life, it’s best to harvest cosmos in the morning when the plants are well-hydrated, and choose blooms that are just beginning to open (about half-open is ideal). Buds on the verge of opening will often continue to unfurl in the vase, giving you fresh flowers over several days. Immediately place the cut stems in clean, cool water after cutting. Strip off any lower leaves that would be submerged in the water, as this helps prevent bacterial growth that can shorten the blooms’ life. Cosmos benefit from changing the water daily, and giving the stems a fresh cut after a couple of days, to maximize their water uptake. Even with these measures, expect cosmos to look their best for roughly 5 to 7 days at most. One advantage is that each cosmos stem often bears multiple buds: as one bloom fades, another may open to take its place, slightly extending the display. Their airy, casual blossoms pair beautifully with other cottage garden flowers in arrangements. Many gardeners don’t mind replacing cosmos mid-week, as the cheerful pop of color they provide is worth the shorter vase life.
🐛 Pests and diseases
Cosmos is generally low-maintenance and not heavily plagued by pests or diseases, which is one reason they are considered easy to grow. That said, like any garden plant, they can occasionally face a few issues. Among insect pests, the most common troublemakers are usually sap-sucking insects such as aphids. Aphids may gather on tender new growth and on the undersides of flower buds, extracting juices and potentially causing curled leaves or stunted buds. A strong spray of water can knock aphids off, or natural predators like ladybugs often keep them in check. Another insect to watch for is thrips, tiny pests that can infest the flowers and cause streaked or distorted petals. If cosmos blooms have silvery patches or lines, thrips might be the cause, using a gentle insecticidal soap on the flowers can help control them. Spider mites might appear in very hot, dry weather, evidenced by fine webbing and speckled, yellowing leaves, spraying water under the foliage or applying an appropriate miticide can manage spider mites. Young cosmos seedlings, especially when just planted out, can be attractive to slugs and snails. These mollusks may chew holes in the leaves or even clip off seedlings at ground level. Using slug bait or copper barriers around young plants can prevent this damage until the plants are larger and less tender.
Regarding diseases, cosmos is fairly resilient, but high humidity or overcrowded conditions can lead to fungal problems. The most frequently seen issue is powdery mildew, a fungus that causes a white, powdery coating on the leaves, typically late in the season or in very humid weather. While it seldom kills the plant, powdery mildew can weaken cosmos and diminish their appearance. To prevent it, ensure good air circulation (don’t overcrowd plants) and avoid overhead watering of the foliage. If powdery mildew does appear, removing the most affected leaves and improving airflow can help, fungicidal treatments are rarely necessary for cosmos, as they will often continue blooming despite some mildew on the leaves. Another possible disease is Fusarium wilt, a soil-borne fungus that can cause sudden wilting and yellowing of cosmos plants. Affected plants should be removed and destroyed, and rotating where you plant cosmos each year can help reduce the risk of soil diseases building up. Cosmos can also fall victim to aster yellows, a rare disease caused by a microorganism carried by leafhopper insects, it leads to stunted, yellow, deformed growth and flowers. There is no cure for aster yellows – infected plants should be pulled up to protect the rest of the garden.
On the whole, cosmos is not a preferred food for larger critters. Deer and rabbits typically leave cosmos alone, likely thanks to the plants’ slightly aromatic foliage and rough texture. While no plant is absolutely deer-proof if food is scarce, cosmos is considered marigold – sorry, considered deer-resistant and usually will not be browsed when other options are available. This makes them a good choice for gardens where deer or rabbits are a concern. In summary, with minimal attention to spacing and basic garden hygiene, cosmos tend to stay healthy. A quick check for aphids or mildew now and then will cover most issues, and many plantings of cosmos sail through the season with nothing more than a few insect visitors and perhaps a touch of mildew very late in fall.
FAQs
Are cosmos annuals or perennials?
Most cosmos is grown as annuals, meaning they complete their life cycle in a single growing season. In climates with cold winters, cosmos will die once frost arrives and must be replanted the following year. However, in warm regions that don’t experience frost (roughly USDA Zone 9 and above), cosmos can sometimes behave like short-lived perennials or return yearly by self-seeding. Cosmos atrosanguineus (chocolate cosmos) grows from tubers and can survive as a perennial in frost-free areas or if its tubers are lifted and stored over winter. Generally, though, you should treat cosmos as annuals in the garden and plan to either collect their seeds or let them self-sow for the next year’s display.
Do cosmos reseed themselves?
Yes, cosmos readily reseed themselves under the right conditions. If you leave some spent blooms on the plant toward the end of the season, they will form seed heads filled with narrow black seeds. These seeds often fall near the base of the plant. In spring, you may find new cosmos seedlings sprouting in the same area without any effort on your part. Many gardeners deliberately allow cosmos to self-seed, creating a naturalized patch that returns annually. To encourage self-sowing, avoid heavy mulching or soil tilling in the area where cosmos grew, so that the dropped seeds have a chance to settle into the soil. Keep in mind that self-seeded cosmos (especially if hybrids were grown) might exhibit slightly different colors or heights than the parent plants, but they will still produce the familiar daisy-like flowers. Overall, cosmos are enthusiastic self-propagators, often popping up year after year once established.
How can I get my cosmos to bloom more profusely?
To encourage cosmos to produce more blooms, make sure they have optimal growing conditions and a bit of routine care. First, plant them in full sun – a lack of sunlight can cause leggy growth and fewer flowers. Second, be cautious not to over-fertilize. Cosmos do best in average or even poor soil, too much fertilizer (especially high-nitrogen types) will lead to abundant foliage but scant blooms. Often, no fertilization at all is needed for cosmos. Regular watering is fine, but avoid overwatering – let the soil dry out slightly between waterings, as slight stress can actually prompt more flowering. Most importantly, practice deadheading. By snipping off the spent flowers before they set seed, you redirect the plant’s energy into producing new buds. Deadheading cosmos consistently can significantly prolong the flowering period and increase the number of blooms. Additionally, when plants are young (about 12 to 18 inches tall), pinching back the growing tips will encourage them to branch out, yielding more flowering stems. With plenty of sun, moderate water, lean soil, and regular deadheading, cosmos will reward you with a bounty of blossoms all season long.
Do cosmos flowers attract bees and butterflies?
Cosmos is known to be excellent pollinator-friendly flowers that attract a variety of beneficial insects. Their simple, open flower form (with a central disk and surrounding petals) makes it easy for insects to access nectar and pollen. You’ll often see honeybees and bumblebees visiting cosmos blooms throughout the day, collecting pollen or nectar. Butterflies are also drawn to cosmos – species like monarchs, swallowtails, and painted ladies commonly flutter around cosmos patches, sipping nectar from the colorful blooms. In addition, cosmos can attract hoverflies and other beneficial insects that prey on garden pests, which is a bonus for ecological pest control. Some gardeners even report the occasional hummingbird investigating cosmos, although hummingbirds typically prefer tubular flowers with more nectar. Planting cosmos in your garden can help support local pollinator populations, especially because cosmos bloom over a long period (summer into fall) when some other sources of nectar might be scarce. Whether in a dedicated pollinator garden or mixed into your flower beds, cosmos will contribute to a buzzing, fluttering display of wildlife as they bloom.
Are cosmos flowers safe for pets (cats and dogs)?
Yes, cosmos is generally considered safe and non-toxic to pets. According to pet safety resources (such as the ASPCA), cosmos do not contain harmful compounds that would poison cats or dogs. If a dog or cat nibbles on cosmos leaves or flowers, you may not need to worry beyond mild digestive upset. At most, ingesting a large amount of any plant material can cause a pet to have an upset stomach or possibly vomit, but cosmos itself won’t cause serious symptoms. This makes cosmos a pet-friendly choice for gardens frequented by animals. Of course, it’s always best to discourage pets from chewing on garden plants in general – even non-toxic plants can cause some gagging or discomfort if eaten in quantity – but you can be reassured that cosmos pose no special risk. Many people include cosmos in their yards specifically because they are both beautiful and safe for households with curious pets and children.
When is the best time to plant cosmos?
The best time to plant cosmos is in spring, once all danger of frost has passed. Cosmos is sensitive to cold, so wait until after your region’s last frost date to sow seeds or transplant seedlings outdoors. In practice, this means late spring for most temperate areas. You can direct-sow cosmos seeds into the garden at that time, as the soil warms (above roughly 60°F). Alternatively, start seeds indoors 4-6 weeks before the last frost and move them outside once the weather is reliably mild. Be sure to harden off indoor-grown seedlings before planting them out. By late spring, when night’s stay warm, you can plant cosmos in the ground or containers and they will quickly establish. From sowing to flowering usually takes about 8 to 12 weeks, so cosmos planted in spring will begin blooming by mid-summer and continue until fall.
Interesting tips
- Pinch for fullness: To create bushier cosmos plants, pinch off the top inch or two of the main stems when the plant is about a foot tall. This simple technique causes the cosmos to branch out more, resulting in a fuller plant with extra flowering stems. Pinching early won’t reduce the overall number of flowers – instead, it encourages significantly more buds to form over the season due to the increased branching.
- Provide support for tall varieties: If you’re growing a tall cosmos cultivar (some reach 5 feet or more), consider staking the plants or growing them along a fence for support. Tall cosmos have hollow stems that can bend or snap in strong winds or heavy rain. By giving them a support structure – whether individual stakes for each plant or a horizontal trellis line for a row of plants – you’ll help keep the flower-laden stems upright. Another approach is to plant cosmos in groups, closely planted cosmos can lean on each other and are less likely to topple compared to isolated plants.
- Deadhead and succession sow: Regularly deadheading cosmos (removing spent blooms) will keep the plants blooming continuously by preventing seed development. A cosmos plant that isn’t allowed to go to seed will put its energy into producing more flowers. Also, for an extended season of color, try succession sowing. Plant a second batch of cosmos seeds about 3 to 4 weeks after the first planting in late spring or early summer. This later sowing will bloom a little after the first, ensuring you have fresh flowers coming along well into late summer and fall, even as the earliest plants begin to slow down.
- Save the seeds: Cosmos are one of the easiest flowers for saving seeds. At season’s end, leave a few flowers on the plants to mature and dry out. The centers will turn into dry seed heads filled with elongated black seeds. Collect these seeds and store them in a labeled envelope or jar in a cool, dry place until next spring. By saving seeds, you can grow cosmos year after year without buying new packets – and you may find the plants gradually adapt to your local garden conditions. Seed saving also lets you share this flower with friends or move it to new spots in your yard at no cost.
- Leave some for the birds: If you resist deadheading toward the end of the season, cosmos will produce lots of seeds that not only fall to the ground but also provide food for wildlife. Birds such as goldfinches and sparrows love cosmos seeds and will perch on dried flower heads to pluck out the seeds. Leaving a few seed heads on your cosmos into fall effectively turns your flower patch into a natural bird feeder. This can help sustain local bird populations and also adds another element of life to your garden as the seasons wind down. Plus, any seeds the birds don’t eat might drop and sprout into volunteer cosmos plants the next year.