Even if you only have a tiny garden – or just some pots – you have to grow at least one cosmos. They are the lowest maintenance, floweriest plants in the world. For me, cosmos is the classic cut flower and a supremely lovely garden plant. In reward for very little effort, they give you buckets of cut flowers – and they have a very good vase life too. I love to grow lots of different varieties putting together different heights and colours in pots and borders. I hope you enjoy exploring our wide selection ofcosmos seeds and seedlings.
Cosmos are one of the most delightful and easy-to-grow flowering annuals. With their airy colorful blooms and carefree nature they bring beauty to gardens and vases throughout the summer season. Many gardeners enjoy starting cosmos from seed and nurturing the seedlings to maturity. A key step for success is determining the optimal time to transplant young cosmos plants into the garden. Properly timing the cosmos transplanting will support vigorous growth and maximize the profusion of flowers you harvest.
Starting Cosmos Seeds Indoors
While it’s possible to sow cosmos seeds directly into the garden soil, some gardeners prefer the control of starting the seeds indoors. Here are some tips for getting cosmos seedlings off to a great beginning:
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Sow seeds indoors 4-6 weeks before your region’s average last spring frost date
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Use clean seed starting containers and fresh, sterile seed starting mix.
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Sow seeds 1⁄4 inch deep and keep moist until sprouting occurs.
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Place seed trays in a warm location (70-75°F) with plenty of bright light.
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Once first true leaves appear, thin seedlings to leave the 2-3 strongest plants per cell or pot.
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Gradually expose seedlings to outdoor conditions over 7-10 days to harden off before transplanting.
Starting cosmos seeds indoors allows you to provide ideal growing conditions so young plants establish quickly and uniformly. You’ll get a head start on the season with a batch of stocky, green cosmos seedlings ready to move into the garden on schedule.
Determining When to Transplant Cosmos Outdoors
The timing of transplanting cosmos seedlings into garden soil or containers is vital for healthy growth and maximal blooms. Four key factors guide the decision of precisely when to transplant:
Soil Temperature
Cosmos prefer warm soil temperatures. Transplanting into ground that is too cold, below 60°F, can stall seedling growth. Wait to transplant until soils have adequately warmed.
Frost Dates
Since cosmos are frost tender, all danger of spring frosts should pass before setting seedlings outside permanently.
Seedling Maturity
Don’t allow cosmos seedlings to become overgrown or rootbound before moving into the garden. Wait until they are 6-7 weeks old and actively growing.
Day Length
For prolific flowering, cosmos require the longer days of late spring and summer. Transplant too late and plants miss out on full blooming potential.
Taking these elements together, the ideal transplant window is typically 4-6 weeks before your area’s average final spring frost. This coincides with soil temperatures around 60°F and with young, vigorous seedlings developing their first true leaves.
Checking Seedling Readiness for Transplanting
Monitor your developing cosmos seedlings for these signs they are ready to shift into the garden:
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4-6 true leaves have expanded. Older, crowded plants struggle to adapt.
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Roots are filling out the cell but not severely tangled or pot-bound.
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Seedlings have a stocky stature, around 3-4 inches tall, not stretched and leggy.
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Overnight temperatures remain above 50°F.
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Garden soil can be worked and is moist but not waterlogged.
Cosmos seedlings meeting these criteria are in prime condition for successful transplanting into the outdoor garden.
Technique for Transplanting Cosmos
Careful handling of the root ball when transplanting reduces stress on cosmos seedlings:
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Prepare planting holes in beds or rows, spacing appropriately for variety.
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Water seedlings well 1-2 days prior to transplanting.
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Gently remove each plant, handling by leaves rather than stem.
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Tease apart any circled or tangled roots.
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Set seedlings at same soil level as in the pot and backfill with soil.
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Water transplants well to remove air pockets and settle soil.
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Provide partial shade for newly planted seedlings for 2-3 days.
With proper digging, handling, and aftercare, transplanted cosmos recover quickly and start growing rapidly within a week or so.
Growing Conditions After Transplanting
Once settled into the garden, cosmos require minimal care:
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Water regularly until plants establish, about 1 inch weekly.
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Weed carefully to limit competition for young plants.
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Allow enough space for air circulation based on expected size.
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Apply organic mulch to retain moisture and discourage weeds.
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Avoid over-fertilization which can diminish flowering.
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Pinch tall varieties in early summer to induce bushier branching.
Given adequate sunlight and well-draining soil, transplanted cosmos are easy to grow into mature, floriferous plants.
Expected Growth Stages After Transplanting
When transplant timing is right, cosmos seedlings progress through predictable growth phases:
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Week 1: Establish roots and recover from transplant shock.
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Weeks 2-3: Steady leaf and stem expansion as plants take off.
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Weeks 4-6: Flower buds become visible signaling start of blooming.
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Weeks 7-10: Plants reach full size and bloom abundantly for cutting.
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Weeks 10-Frost: Bloom period continues until killed by hard freeze.
Proper transplant timing allows cosmos a long season to display their colorful blooms for cutting or garden enjoyment.
Enjoy a Prolific Cosmos Harvest
A bit of planning and care when transplanting cosmos seedlings is the key to success with these easy, rewarding flowering annuals. Give them suitably warm soil, frost protection, adequate moisture, and full sun after transplanting. Time it right and you’ll be rewarded with drifts of delicate cosmos blossoms to cut or admire all season long.
planting our jumbo seedlings or 9cm potted plants
We’ve done a bit more of the legwork here for you, so if conditions are right, you can plant these straight into your containers or the garden by following the instructions below. If weather conditions aren’t right (i.e., they are too cold or too wet) then you can pot them on into a 1 or 2 litre pot before planting.1. Prepare the planting area by forking over the soil and incorporating plenty of organic matter (well-rotted manure, home-made compost or multi-purpose peat-free compost). Pick out any large stones and visible weeds as you go. 2. Rake to level the soil, breaking up any large clumps. Try to create a fine, crumbly soil texture. The young plants roots will like this. 3. Mark out planting positions with plant labels or empty pots, making sure you allow enough space between each one. Learn more about spacings for specific plants on the specific plant page. 4. Dig planting holes deep and wide enough to cover the roots. Adding rootgrow into the bottom of the planting hole will help plants establish roots 5. Position the plant in the hole, leaving the crown of the plant (where the roots meet the top growth) just level with the soil surface. Fill the space around the plant with the displaced soil, and gently firm the soil with your hands. You should be able to tug gently on the leaves without the planting lifting out of the ground. 6. Water well, and once it looks like all the water has been absorbed, water again with a really good amount of water (not a sprinkle). Do this every 5-7 days (if there has been no rain) to encourage roots to be drawn down deep into the ground, resulting in stronger plants. 7. Keep well-watered during dry spells, until established. For more top tips on watering visit our guide here
The shorter varieties of cosmos are perfect for pots and seedlings can be planted out from May.1. Select an appropriate size pot for the type of plant or plants you have chosen, and the look you want to achieve. For seasonal summer displays you can pack in your plants to create more impact, we suggest about 9-10 plants per 40cm diameter pot. 2. Make sure you have plenty of drainage holes in the containers and broken pottery at the bottom to prevent the drainage holes from blocking. 3. Fill your chosen container with fresh, peat-free multi-purpose compost.4. Dig planting holes deep and wide enough to cover roots and position the plant in the hole. Fill the space around the plant with soil, pressing firmly so the plant can’t be easily lifted.5. Water well and add more compost if the surface has sunk more than an inch below the rim of the pot. 6. Keep well-watered and feed regularly during the growing season for the best results.
can cosmos get mildew?
Yes, cosmos can get powdery mildew, which is a fungal disease that shows up as a powdery coating on the foliage. You can remove any affected leaves and try an organic solution such as homemade comfrey tonic to keep mildew at bay. The best way to avoid powdery mildew in the first place is to ensure the plants are well spaced and not too crowded, and that you water your cosmos plants regularly (but do not allow the soil to get soggy).
How to Fix Leggy Cosmos Seedlings | Tips to Get More Flowers | Perennial Garden
FAQ
How big should cosmos seedlings be before planting out?
Sow cosmos in early spring so the plants can become established before hot weather comes. Cosmos also can be started indoors in containers and set out when the seedlings have at least five leaves, or purchased as bedding plants. Plant seeds in prepared soil about 2 inches (5 cm) apart and one-half inch (1 cm) deep.
How well do cosmos transplant?
Dividing & Transplanting: Cosmos are easily started from seed and transplanted into the garden. Their root system is rarely large enough to support dividing, so it’s best to start new plants. Perennial forms of Cosmos, such as Chocolate ( C. Altrosanguineus) are propagated by tuberous roots since their seed is sterile.
When should I transplant Cosmos seedlings?
Timing is crucial when transplanting cosmos seedlings. It is best to wait until the soil has warmed up and all danger of frost has passed. For most regions, this typically occurs in late spring or early summer. Transplanting cosmos seedlings too early can lead to stunted growth or even death.
How do you transplant Cosmos seedlings?
To transplant your cosmos seedlings, start by digging a hole in the prepared soil that is slightly larger than the root ball of the seedling. Gently remove the seedling from its container and place it in the hole, making sure the top of the root ball is level with the soil surface. Backfill the hole with soil, firming it gently around the seedling.
Can a cosmos plant be transplanted in the summer?
It is not recommended to transplant cosmos plants in the summer because the high temperatures and dry soil can cause stress on the plant and make it difficult for it to establish itself in its new location. When should I fertilize my transplanted cosmos plant? Wait two to three weeks after transplanting before fertilizing your cosmos plant.
Why do Cosmos seedlings need to be transplanted?
Transplanting cosmos seedlings allows them to have more space for root development, better access to sunlight, and increased air circulation. Transplanting also helps them to establish a stronger root system, enabling them to better withstand disease and adverse weather conditions.
What happens if you wait too long to transplant Cosmos seedlings?
On the other hand, if you wait too long, the seedlings can become root-bound and experience stunted growth. A good guideline is to transplant cosmos seedlings when they have reached a height of about 4-6 inches and have at least 3-4 true leaves.
How long after transplant can you fertilize a cosmos plant?
Wait two to three weeks after transplanting before fertilizing your cosmos plant. This will give it time to settle into its new location and reduce stress on the plant. Transplanting cosmos plants is a simple task that can be done by anyone. By following these steps and tips, you can ensure that your cosmos plant thrives in its new location.