Sunflowers are one of the easiest flowers to start from seed, and they're a particularly satisfying choice when children want to see a plant change quickly. A small seed can become a towering stem in a single growing season, although dwarf varieties are better suited to pots and small gardens.
The main decisions are when to sow, how much space to leave and whether to start indoors. The Royal Horticultural Society recommends sowing annual sunflowers indoors from late March or directly outside during mid-April and May, depending on conditions. In a colder spring, the outdoor seedbed may need a little longer to warm.
Choose a variety that fits your space
Sunflowers come in much more than one size. Giant cultivars may reach several metres and need strong support, whereas dwarf types can remain around 30–60cm tall.
Check the seed packet for expected height and whether the plant has one main flower or several branching stems. A tall single-stem sunflower isn't the best choice for a small balcony pot.
If you want flowers for cutting, consider a branching variety that produces more than one head. If your aim is a children's height competition, choose a genuinely tall-growing cultivar.
Flower colours also vary, so check the packet.
Find a sunny, sheltered place
Annual sunflowers grow best in full sun with fertile, well-drained soil. A site exposed to strong wind can cause trouble as taller plants become top-heavy.
Clear weeds and large stones from the ground before sowing. Rake the surface to make a fine, crumbly seedbed.
If the soil is poor, incorporate suitable garden compost before planting. Don't simply bury seeds in fresh uncomposted kitchen waste or a waterlogged patch of ground.
For a very tall variety, consider positioning it near a strong wall where it can eventually be tied to substantial support.
Sowing sunflower seeds outdoors
Wait until spring conditions are suitable and the soil has warmed. The RHS suggests sowing directly from mid-April through May for annual sunflowers, although local weather matters.
Make shallow holes or a drill following the depth printed on your packet. The RHS children's growing guide demonstrates holes around 12mm deep, while another school guide gives around 2cm. Use the depth specified for the seed you're planting.
Place a seed in each hole, cover lightly with soil and water gently with a watering can fitted with a rose.
Mark the row or position with a label so you don't mistake the seedlings for weeds when they emerge.
How far apart should the seeds be?
You can sow relatively closely and thin later, or put seeds straight into their intended final positions. Dwarf, medium and giant cultivars need different space.
One RHS children's guide suggests sowing seeds around 10cm apart and then thinning stronger seedlings to approximately 45cm apart. This is an example for garden-grown sunflowers, not a requirement for all varieties.
Follow the packet's final spacing, especially if your chosen variety branches widely. Leaving every seedling in a crowded row can produce weak stems and smaller flowers.
If several seeds germinate in a single pot, keep the strongest suitable seedling or transplant only when the roots can be handled carefully.
Starting sunflower seeds indoors
Indoor sowing gives plants a head start and protects emerging shoots from some pests. Fill individual small pots with fresh peat-free seed compost.
Sow one or two seeds per pot at the stated depth, water gently and place the pots somewhere warm and bright. The RHS describes late March onward as a reasonable period for starting annual sunflowers under protection.
After germination, move seedlings into strong light. Plants kept in a dim corner may become tall, thin and weak before they are ready for the garden.
Don't start them so early that they're growing into oversized indoor plants while outdoor nights are still freezing.
Hardening off and planting seedlings outdoors
A seedling raised indoors needs time to adjust to outdoor sun, wind and temperature. Harden it off gradually over a week or two, bringing it back into shelter when the weather becomes unsuitable.
Plant out after the danger of damaging frost has passed. Carefully ease the rootball from the pot and put it into prepared soil at approximately the same depth.
Water the plant after setting it in place and protect it from strong wind until the roots establish.
Avoid pulling a seedling out by its stem. Hold the leaves gently or support the rootball as it comes free.
Planting dwarf sunflowers in containers
A compact variety can grow well in a large pot with drainage holes. Use suitable peat-free multipurpose compost and position the container in full sun.
Choose a container stable enough that the plant won't topple when it becomes taller and the foliage catches the wind. An exposed balcony may need a heavier pot than a sheltered patio.
Water when the compost begins drying rather than relying on a rigid daily schedule. During hot weather a full container may need attention frequently.
A giant sunflower in a tiny decorative pot is unlikely to perform like a plant with ample root space in garden soil.
Protect seedlings from slugs and snails
The RHS warns that young sunflowers are attractive to slugs and snails. Outdoor seedlings can disappear almost as soon as they emerge.
Inspect the bed regularly, especially in damp weather. Use appropriate barriers or other wildlife-conscious controls if damage is severe.
Raising seedlings indoors until they are larger may improve their chances. However, plants grown in protection still need gradual hardening off before being exposed to outdoor conditions.
Keep the area weeded so the seedlings receive light and are easy to recognise. Avoid using unapproved chemicals or excessive treatments around children and pets.
Watering and feeding sunflowers
Water new plants well while their roots establish. Established sunflowers can tolerate some dry periods, but tall fast-growing annuals may need extra water during prolonged hot weather.
Aim the water at the roots rather than soaking every leaf. Check the soil below the surface before adding more.
The RHS suggests that container-grown and actively growing annual sunflowers can benefit from a suitable potassium-rich feed. Follow the product label instead of increasing concentration to encourage extra height.
A healthy plant in reasonably fertile garden soil may need little additional feeding. Plenty of light and enough space are the more important foundations.
Supporting tall stems and collecting seeds
As a sunflower becomes taller, tie the stem loosely to a strong cane or other suitable support. Check the ties as it grows so they don't cut into the expanding stem.
Giant cultivars may need support attached to a substantial structure. A thin cane that bends in the wind won't protect a heavy flower head.
After flowering, the central disc can develop many seeds. Some gardeners leave the heads for birds, while others collect mature seeds to sow the following year.
If you keep seeds, dry them properly and label the variety. Seed saved from hybrids may produce plants that differ from their parents.
Why some sunflower seeds don't germinate
Cold or waterlogged soil may delay germination or cause rot. Birds and rodents can remove seeds, and young shoots may be eaten by slugs.
Check the packet's date and storage history. Very old or badly stored seed can germinate poorly even when you prepare the soil well.
Don't dig up every seed after two days to see whether anything has happened. Give the plants a reasonable period under suitable warmth and moisture.
Start a few spares in pots if you need a reliable display. Sow, label, protect and then allow the sunflower to do its growing.
Sources checked
RHS: growing sunflowers and RHS: growing sunflowers with children.