When to repot sunflower seedlings to ensure a sunny display this blooming season

Size really does matter

Blooming sunflowers in a green space
(Image credit: Future PLC/Clive Nichols)

The clue really is in the name with sunflowers. These sun-loving blooms can brighten up any outdoor space and add oodles of joy to any garden – but only if you know when to repot sunflower seedlings so you can be sure you're doing it at the right time to ensure the best blooms.

When growing sunflowers from seed, there are a few things that you need to know. Of course, you need to choose the perfect time to plant sunflower seeds and understand how long sunflowers take to grow, but one of the most important steps is knowing when to repot the seedlings. After all, sunflowers are extremely top-heavy, and even the sunflower seedlings will topple over in the wind if they’re not in the right-sized pot.

So, when should you repot sunflower seedlings? During a certain month? When they grow to a specific height? We’ve consulted the experts, and they have all of the answers you need.

When to repot sunflower seedlings

Metal potting bench against a brick wall, with stacked terracotta pots and a blue wooden tray labelled 'seedlings'

(Image credit: Future PLC/Simon Scarboro)

From my own sunflower seed planting experience, I can tell you that sunflowers grow extremely quickly. When sown inside, they can germinate from seed to seedling in just a matter of days. But when they get bigger, sunflowers also benefit from up-potting.

This means that you need to be clued up on when to repot sunflower seedlings to ensure that the growing plants are getting all of the extra nutrients they need to thrive in these larger pots.

However, just like knowing when to repot tomatoes, there’s no definitive day or month that you need to mark in your calendars to complete this job. Instead, it’s better to let the sunflower seedlings take the lead so you can repot them when they reach a certain size.

Morris Hankinson, Director of Hopes Grove Nurseries, explains, ‘Generally speaking, sunflower seedlings should be repotted when they have developed their first set of true leaves (these are the second set of leaves that appear after the initial pair of seed leaves).’

Morris Hankinson of Hopes Grove Nurseries
Morris Hankinson

Morris Hankinson is the founder and managing director of Hopes Grove Nurseries Ltd, the UK’s only specialist grower-retailer of hedging plants. He established the thriving business in 1992, shortly after graduating with a Commercial Horticulture Degree from Writtle College, Essex.

By this point, the seedling should be around two inches tall and already have an established root system. Then, you can repot it into a slightly larger pot and fill it with extra compost (i.e. extra yummy nutrients) to encourage even more growth.

Of course, you need to make sure that you’re choosing the right pot and compost for this process. Morris says, ‘When transplanting sunflower seedlings, choose containers that are at least 6-8 inches deep to allow for growth and provide the right drainage holes at the bottom to prevent waterlogging.’

Harry Bodell, gardening expert at PriceYourJob.co.uk also adds, 'Use a good quality, peat-free, multipurpose compost for the new pots. This provides a well-draining and nutrient-rich environment for your seedlings.' Then, you can pop the pot outside to allow the seedlings to soak up that all-important sun that they love so much. But, unfortunately, the repotting process doesn’t stop there.

Blooming sunflowers in a green space

(Image credit: Future PLC/Camilla Reynolds)

John Clifford, garden expert at Gardenstone, says, ‘Once the sunflowers continue to grow and reach around 30cm, you should then either move them into the ground or into yet another bigger pot. If it's not yet May, then leave the plants to grow for a bit longer.'

‘You don't want to risk any unexpected cold weather damaging them. May should be the earliest you move your sunflowers into their final pot or final growing place.’ From there, it’s typically a good idea to either repot your growing sunflower every month to allow for growth or until you think that the seedling is starting to overpower the pot.

You don’t need to go too big, though. Sunflowers are extremely adaptable to the pots that they’re grown in, which means that they don’t necessarily need a huge amount of space. They just need a pot with the right nutrients.

That's all there is to it, really. Just don’t forget to propagate your sunflowers when they’re fully grown so you can enjoy them time and time again.

What you'll need


How big should sunflower seedlings be before transplanting?

Whether you want to transplant them into the ground or simply repot your sunflower seedings, you should wait until they are around two inches tall before moving them. Some experts even suggest waiting until they have at least four leaves, as this is a sure sign that the seedling is well-established and will survive the move.

What month should you plant out sunflower seedlings?

Sunflowers are the ultimate sun worshippers, which is why you should only plant out sunflower seedlings when the risk of frost has gone, and the sun is shining. Normally, this will be around May-June time.

What to do with leggy sunflower seedlings?

Leggy sunflower seedlings are a common problem for those growing sunflowers from seed. The stems are often too tall and spindly, and the small leaves just don’t seem proportionate.

One of the best ways to avoid developing leggy sunflower seedlings, to begin with, is repotting them and popping them outside or in the greenhouse as soon as they reach two inches tall. This way, they’ll have more access to constant sunshine - which is what they don’t get when they’re inside and on the windowsill.

If you already have leggy sunflower seedlings, however, you can simply plant them deeper into the compost when repotting to compensate for this extra length. Just make sure that you don’t plan them so deep that they’ll be smothered.

So, is it time to repot your sunflowers seedlings yet?

Lauren Bradbury

Lauren Bradbury is a freelance writer and major homes enthusiast. She graduated with a Bachelor’s degree in English and Creative Writing from the University of Chichester in 2016, before dipping her toe into the world of content writing. After years of agency work, writing everything from real-life stories to holiday round-ups, she decided to take the plunge and become a full-time freelancer in the online magazine world. Since then, she has become a regular contributor for Real Homes and Ideal Home, and become even more obsessed with everything interior and garden related. As a result, she’s in the process of transforming her old Victorian terraced house into an eclectic and modern home that hits visitors with personality as soon as they walk through the door.