Sunflowers are one of the most iconic and beautiful summer flowers. Their bright, cheerful faces seem to radiate sunshine and warmth. Growing these summer classics from seed is easy and rewarding. Starting sunflowers indoors allows you to get a head start on the growing season so you can enjoy their blooms sooner. This complete guide will walk you through when, how, and why to start sunflowers indoors for a gorgeous garden.
When to Start Sunflowers Indoors
Sunflowers are a warm weather, frost-intolerant plant. They will thrive outdoors when temperatures are consistently in the 60s and 70s Fahrenheit. This means sunflowers should be started indoors about 4-6 weeks before your last expected spring frost.
Most gardeners start sowing sunflower seeds indoors in early to mid April. This timing allows the seedlings to grow large enough to transplant outside approximately 1 month later in mid to late May, right after the danger of frost has passed.
Starting too early runs the risk of stunting the plants. The seedlings may get leggy and weak trying to survive in low light indoor conditions for too long. Wait until about a month before transplanting outside to start your seeds.
If you live in an area with a very long growing season and late first fall frosts, you can sow seeds indoors in early March. Just be sure to provide ample light and space for the seedlings as they grow into young plants over the 2+ months before outdoor planting.
Stagger planting sunflower seeds every 2 weeks indoors for continued blooms. The last indoor planting can be done in late May or early June to achieve late summer and fall harvests.
How to Start Sunflower Seeds Indoors
Starting sunflowers inside is simple with a few basic supplies and these easy steps
Choose Your Seeds
Sunflowers come in a stunning range of sizes, colors, and bloom forms. Popular varieties for home gardens include:
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Mammoth – Gigantic 12+ foot plants with dinner plate sized yellow flower heads. A classic sunflower variety.
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Sunspot – Unique red and yellow bicolor blooms on 5-6 foot stalks.
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Elf – Dwarf 1-2 foot plants perfect for containers. Bright yellow single blooms.
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Italian White – Elegant 5 foot plants with soft, creamy white petals and dark centers.
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Chocolate – Rich mahogany flowers bloom 6-8 feet tall.
Use Seed Starting Mix and Containers
Sunflowers have large seeds and seedlings. Give them enough room to spread their roots by using deep seed starting pots or trays with drainage holes. 3-4 inch pots or cell packs work well.
Use a commercial seed starting mix instead of potting soil to avoid disease. The lightweight, soilless mix provides good drainage and air flow around young roots.
Plant Seeds
Plant sunflower seeds 1 inch deep in your starting container. Place 1 seed per pot or cell, or sow seeds 2 inches apart if using a tray.
Lightly cover the seeds with seed starting mix and gently water. Place containers in a warm (65-75 F), bright location.
Sunflowers sprout in 5-14 days indoors. Provide ample sunlight once they emerge to prevent leggy, weak growth.
Grow Seedlings
Once sprouted, sunflowers grow quickly! Monitor soil moisture and fertilize weekly with a diluted liquid fertilizer. When the first set of true leaves open, thin tray-grown plants down to one per cell if needed.
If leggy, provide supplemental grow lights for 16 hours per day. Gently brushing over the seedlings daily can strengthen stems too.
Hardening Off & Transplanting
About 1 week before transplanting outdoors, begin acclimating or “hardening off” the young sunflowers. Move them outside to a sheltered spot for a few hours a day, gradually increasing time outside over the week.
Once night temperatures stay above 50 F, transplant the sunflowers into sunny garden beds 12-18 inches apart. Dig holes the depth of the root ball and bury the stem up to the lowest set of leaves. Water well and enjoy your head start on giant sunflower blooms!
Why Start Sunflowers Indoors?
Starting sunflowers inside offers several advantages:
Earlier Blooms
Sowing seeds indoors 4-6 weeks before your last frost date allows the plants a head start on growth. This results in earlier, faster flowering compared to direct sowing seeds outdoors after danger of frost has passed. You can enjoy those iconic sunflower blooms sooner!
Longer Growing Season
Starting early indoors extends the sunflower growing and blooming season. First year plants normally only bloom for about 6 weeks. Staggering indoor planting dates spreads out the harvest window for months of flowers.
Higher Germination Rates
Seeds germinate better in warm, stable indoor conditions than temperamental outdoor spring soil temperatures. You can also protect delicate sprouts from wind, cold, and pests. The result is higher germination success.
Larger Plants
Getting a 1 month jumpstart on growth means your sunflowers will be more mature and larger by summertime. When seeds are sown right into the garden, the short growing season limits plants’ potential size.
More Variety
Because you can start growing earlier and transplant on schedule regardless of outdoor conditions, starting inside allows you to successfully grow more sunflower types and varieties. Cool season loving or large/tall cultivars perform better when started indoors.
Year Round Gardening
Who says you have to wait for summer to garden? Starting sunflower seeds indoors lets you enjoy growing plants even during the winter and early spring. It’s a great cure for gardeners’ impatience!
Troubleshooting Common Sunflower Seedling Problems
Sunflowers are generally easy to start from seed, but occasionally you may encounter issues. Here are some common challenges and how to address them:
Seeds Not Germinating
Check soil moisture – irregular watering causes erratic germination. Keep planting mix consistently damp but not soaked.
Increase warmth – sunflowers prefer 70+ degree soil. Use a heat mat or grow lights to warm the area.
Plant fresh seeds – sunflower seeds can lose viability after 1 year of storage. Buy new seeds annually.
Leggy, Spindly Growth
Leggy seedlings with weak stems often occur from lack of sunlight. Move to the sunniest window or provide supplemental grow lights to strengthen plants.
Damping Off Disease
Overwatering can cause fungal damping off disease. Allow containers to partially dry between waterings and provide good air circulation.
Insect Pests
Fungus gnats and other soil-dwelling insects can attack seedlings. Use sterile starting mix and allow it to fully dry between waterings to deter pests.
Enjoy Months of Brilliant Blooms
Now that you know when and how to start your sunflower seeds indoors, you’ll be prepared to grow these summer icons for an extra long season brimming with cheerful color. Watching those seedlings grow from potting mix into towering, dazzling blooms is one of gardening’s greatest rewards. The radiant flowers will turn their faces to follow the sun across your yard. Get ready to enjoy the sunshine-filled world of sunflowers!
What advice do you have for growing sunflowers to be ready for an event?
We’d like to grow different kinds of sunflowers, like Burpee Mammoth, Santa Lucia, and smaller ones with more than one stem that we can put in vases at an event. We’d like the flowers at the stage where their yellow ray petals are in best appearance (i. e. , ornamental flowers rather than in seed production).
When should we plant our seeds to make sure they bloom in mid-September? Should sunflowers be grown from seeds that are sown directly or from seedlings that are started in 6-packs? What kinds of sunflowers should we try?
Horticulture Professional Pamela Hargest: Sunflowers usually grow quickly from direct seeding in the garden after the last frost. But you can start them indoors two to three weeks before the last frost if you’d rather transplant them. Remember that deer love to eat sunflowers, so putting up a fence to keep them out is always a good idea.
No matter how many days it takes for sunflowers to bloom, you should start them when they are ready. This will be listed on the seed packet as well in seed catalogs. It can take anywhere from 50 to 95 days for different kinds of sunflowers to mature. Flowers on branching types of sunflowers last longer in the season. To make sure the sunflowers are ready when you want to pick them, I suggest planting two sets a few weeks apart and choosing a pollenless (hybrid) branching variety. Remember that their growth will slow down if the weather gets cooler in September. This is something you should think about when you plant them. Depending on the days to maturity (dtm), I would suggest planting a few times between late June and early to mid-July. A few types I’ve grown and like are Gold Rush (hybrid, 60–65 dtm) and Soraya (85–95 dtm).
How to Plant Sunflower Seeds Indoors – With Kelly Lehman
FAQ
What is the best month to plant sunflowers?
Do I need to soak sunflower seeds before planting?
Should I start sunflowers in pots?
How long does it take for sunflower seeds to germinate?
When should you plant sunflower seeds indoors?
Plan to plant sunflower seeds indoors about 4 weeks before your area’s last frost date. If you wish to direct sow sunflower seeds, you should wait to plant them until after the threat of frost has passed. Where do you start sunflower seeds indoors?
Should you plant sunflowers indoors?
It’s really important to start Sunflowers indoors from seed at the right time – more-so than most varieties – because of their taproot. Sunflowers are fast growers and extra sensitive to becoming root bound. However, if you plant them too late you might miss out on the early harvest advantage of starting indoors.
Can you plant sunflower seeds early?
Starting sunflowers early also gives you the option to plant the flowers as a natural trellis near climbing plants like pole beans. When can you plant sunflower seeds? Plan to plant sunflower seeds indoors about 4 weeks before your area’s last frost date.
How to grow sunflower seeds indoors?
All you need are your sunflower seeds, some paper towels, water, and a bag or container (ziploc-type bags work as well as take out food containers- no need to get fancy). By starting your Sunflower seeds indoors, you are able to extend your growing season, especially if you are in one of the cooler growing zones.