Cosmos are one of the most popular annual flowers grown from seed Their bright, cheerful colors and easy care make them a favorite for gardens everywhere But when starting cosmos from seed, many gardeners wonder if they need any special treatment like cold stratification before planting. In this article, we’ll explore if and when cosmos benefit from stratification.
What is Cold Stratification?
Cold stratification is a process used to break seed dormancy in some plant species. It mimics the winter conditions seeds would experience in nature, encouraging germination when planted in spring.
Stratification involves exposing seeds to cold, moist conditions for a period of time. This is typically done by placing seeds between layers of damp paper towels and storing in the refrigerator for 1-3 months depending on the plant. The cold mimics winter, while the moisture allows the seed to imbibe water and begin to break dormancy.
Some plants like apple trees lilacs and peonies have built-in dormancy mechanisms that require this cold period before seeds will sprout. But other plants like most annuals and cosmos, do not require stratification.
Do Cosmos Need Stratification?
The simple answer is no – cosmos seeds do not require cold stratification before planting.
Cosmos are easy to grow warm season annuals that germinate readily when soil temperatures reach 60-70°F. Their seeds have no built-in dormancy mechanisms that need to be broken by cold treatment.
In fact, exposing cosmos seeds to very cold temps can actually damage them and reduce germination rates. The ideal temp for cosmos seed germination is 70-75°F.
So pre-chilling cosmos seeds by refrigerating is not necessary and generally not recommended. You can simply sow cosmos directly in the garden once overnight temps stay above 50°F.
However, some gardeners do like to optionally stratify cosmos seeds, especially when starting indoors or in cold areas. Here’s more on how and when to optionally stratify cosmos.
Optional Stratification for Indoor Starts
While not required, some gardeners like to optionally stratify cosmos seeds for a short 1-2 week period before starting indoors.
This light chilling can help encourage faster, more uniform germination when sowing indoors where temps are lower (60-70°F).
To optionally stratify cosmos seeds:
- Place seeds between damp paper towels & refrigerate for 1-2 weeks
- Check seeds regularly and remoisten towels if needed
- Remove seeds once radicle (taproot) emerges
- Sow seeds immediately in indoor starting trays
Soaking cosmos seeds overnight before refrigerating can also enhance this effect.
Just be careful not to over-chill cosmos seeds for more than 2 weeks or germination rates may suffer. The goal is just a light, brief chilling to mimic nature and sync germination when starting early indoors.
Stratifying for Early Outdoor Planting
In cold climates, some gardeners also like to optionally stratify cosmos seeds for early spring planting outdoors.
This can help get a small head start on the season when soil temps are still cool in the garden. To stratify seeds for early outdoor planting:
- Place seeds between damp towels & refrigerate for 2-4 weeks
- Start seeds indoors 2-4 weeks before last frost
- Harden off & transplant outdoors 1-2 weeks before last frost
Again, be careful not to over-chill seeds. And provide cold protection like cloches or cold frames when planting out early while soil is still cool.
Direct Sowing Without Stratification
No stratification is needed when direct sowing cosmos outdoors once soil has warmed in spring.
Cosmos seeds will germinate readily when soil temps reach 60-70°F, so there is no need to pre-chill seeds.
Wait to direct sow cosmos until after your last spring frost date, when soil temps are warm and overnight lows stay above 50°F. Scatter seeds directly in prepared garden beds.
Water regularly to keep soil moist until seeds germinate. Thin young plants to proper spacing once established. No other pre-treatment is needed for great germination when direct sowing cosmos outdoors in spring or summer.
Fall Planting Without Stratification
Many gardeners also successfully sow cosmos seeds in fall for an earlier spring bloom.
Again, no seed stratification or pre-treatment is needed for fall planting. Simply scatter fresh cosmos seed in the garden in late fall, once temperatures cool down.
Fall-sown cosmos will naturally experience chilling from winter conditions. This helps synchronize spring germination, triggering growth as soil warms the following season.
Key Points on Stratifying Cosmos Seeds
Here are some key takeaways on if and when to stratify cosmos seeds:
- Cosmos seeds do not require cold stratification for germination
- Pre-chilling can optionally be done for 1-2 weeks when starting early indoors
- Take care not to over-chill seeds for more than 2-4 weeks
- No stratification needed when direct sowing outdoors in spring/summer
- Can optionally pre-chill for early spring planting in cold climates
- No pre-treatment needed for fall sowing
Follow soil temperature conditions, not chilling requirements, for the best cosmos germination results. With proper sowing for your climate, you can enjoy a bountiful bloom of beautiful cosmos flowers.
When to Plant Cosmos
Here is a quick guide to when to plant cosmos seeds based on your planting method:
Starting Indoors:
- Optional: Stratify seeds 1-2 weeks before planting
- Start seeds indoors 4-6 weeks before last frost date
- Grow seedlings for 4-6 weeks before transplanting outdoors
Direct Sowing Outdoors:
- Spring: Direct sow after last frost when soil warms to 60-70°F
- Summer: Sow anytime during the extended growing season
- Fall: Sow seeds in late fall for earlier blooms next year
Transplanting:
- Wait to transplant seedlings outdoors until after last frost date
- Harden off plants for 5-7 days before transplanting
- Provide cold protection if transplanting very early
Adjust timing based on your local climate and frost dates. With proper planting techniques, cosmos are easy to grow from seed and will reward you with their vibrant colors and bountiful blooms.
How to Grow Cosmos from Seed
Here is a summary of tips for successfully growing cosmos from seed:
Seed Starting Mix
Use a sterile, well-draining seed starting mix if starting indoors. Avoid heavy potting soils.
Planting Depth
Sow cosmos seeds 1⁄4” deep indoors or outdoors. Don’t cover too deeply.
Spacing
Space seeds or thin seedlings 12-15” apart. Crowding causes weak, leggy plants.
Watering
Keep soil moist, but not soaked, for good germination. Reduce watering once plants establish.
Light & Temperature
Cosmos prefer full sun and warm 70-75°F soil and air temperatures.
Fertilizer
Avoid over-fertilizing, which can cause leggy growth. Use compost tea or dilute solutions.
Transplanting
Transplant seedlings carefully to avoid disturbing roots. Harden off for 5-7 days first.
Timing
Follow recommended planting times for your climate. Cosmos are very frost sensitive.
With the proper growing conditions, cosmos are easy to grow from seed and will provide non-stop color in the garden from midsummer until fall frost. Give them full sun, warm temperatures and well-draining soil for success.
While optional in some cases, cold stratification is not required for cosmos seeds. Their seeds have no built-in dormancy and germinate readily in warm conditions.
Simply follow soil temperature conditions and recommended planting times for your climate. A light 1-2 week chilling can be used when starting very early indoors or outdoors. But take care not to overdo it.
With the proper care when sowing, cosmos are among the easiest flowering annuals to grow from seed. Their bright blossoms in shades of pink, orange, red, yellow and white will give you a steady flush of color from summer through fall frost.
So try growing cosmos from seed this year for a low maintenance, high impact display! With a couple simple tips, these beautiful annuals are easy to cultivate and will thrive with minimal care. Just sit back and enjoy their vibrant colors lighting up your garden.
How to Grow Cosmos Throughout the Season
Growth Habit: Cosmos are multi-branching plants, with hollow tubular stems. Keep the flowers cut back after first bloom, to prompt new and continuous growth. After a while, don’t just cut off the flowers on your Cosmos; trim it a third of the way down. Water and wait for a new crop.
Staking: When planted as a group, Cosmos can hold each other upright. Protect the taller varieties from the wind, and be ready to stake them so that the tall, thin stems don’t break in heavy rain or wind.
Watering: New plants need water to grow, but older plants can handle dry conditions well and would rather have less water. When watering, focus on watering the roots, and avoid splashing the foliage.
Fertilizing: Too much fertilizer in the soil will make plants grow quickly, but it will also weaken the stems and make flowers scarce. Prepare the beds properly, yet hold back on too much compost and fertilizer. Add a side dressing of compost or fertilizer mid season, as needed.
Mulching: Putting mulch around the base of the cosmos plant in early spring will help keep weeds away and keep the soil moist.
Trimming & Pruning: Keep Cosmos trimmed of spent blossoms, to encourage new growth and continuous buds. When you deadhead in the middle of summer, don’t just remove the flowers; cut the branches back to about a third of the way down the plant. Water the plant and wait for a new crop for a late-season show.
When and Where to Plant Cosmos
Light: Cosmos prefer full sun conditions, except in extreme heat where they can tolerate part shade.
Soil: Prepare the garden with loose, weed-free soil. Cosmos prefer dry, arid soil over wet conditions. Soil that is too moist may lead to disease. While adding compost is good, don’t use too many nitrogen-based fertilizers because they can speed up the growth of leaves instead of flowers.
Spacing: Plant the shorter types (C. sulphureus) 10-12 inches apart, while the taller varieties (C. bipinatus) prefer 1- 2 feet in between. Plan to stake your Cosmos if plants get too tall, and gangly.
How to Plant: Cosmos is one of the easiest flowers to grow from seeds planted right in the garden bed.
Planting in the spring: Because sprouts grow so quickly, make sure to spread the seeds out after the risk of frost has passed in the spring. You can also start seeds indoors in plug trays 4 to 5 weeks before your last frost date if you have a very short growing season. This will give your plants a start on growth. Plant in the garden after the frost danger has passed, but keep them out of the wind and cold until they are established. Cosmos are tender annuals, and will quickly turn brown when temperatures drop below freezing.
Fall Planting: Cosmos may also be direct sown in the fall. The trick is to do the exact opposite of what you need to do to plant in the spring: scatter your seeds after a few hard frosts have passed. This ensures that your seed wont germinate until its ready next spring.
How to Grow Cosmos Flowers From Seed – How to Prune For More Flowers and General Care
FAQ
Do cosmos need to be cold stratified?
Can I just scatter cosmos seeds?
Are cosmos difficult to grow from seed?
Do I need to striate Cosmos seeds?
You’ll want to go back and thin out or relocate surplus seedlings if you surface seed them too densely. Seed Treatment: None required or even recommended, but for kicks (or practice) you can subject your cosmos seeds to cold stratification or striation if you’re starting them indoors.
Can Cosmos seeds grow in cold weather?
If the temperature is too hot or too cold, your cosmos seeds may not germinate. Ideally, the temperature should be between 65-75 degrees Fahrenheit. Finally, it’s important to keep the soil moist, but not overly wet. If the soil is too wet, your cosmos seeds may develop mold or rot.
Can Cosmos grow in cold weather?
Cosmos can tolerate warm, dry weather very well. They are even drought-tolerant. Direct-sow seeds outdoors once the danger of frost has passed. Alternatively, sow seeds indoors 4 to 6 weeks before your last spring frost date in trays or pots with a good seed-starting mixture. Move them into 5-inch pots as soon as they’re 3 or 4 inches tall.
What factors affect germination rate of Cosmos seeds?
Light and moisture levels also have an effect on the germination rate of cosmos seeds. The seeds should be kept consistently moist but not soggy. When it comes to light, cosmos seeds should receive at least 6 hours of direct sunlight a day. Seed Quality The quality of the seed is another important factor.