When to Plant Sunflower Seeds

Why grow sunflowers?

Sunflowers are bold, cheerful and surprisingly easy to grow. With their tall stems and large, sunny blooms, they make a strong visual impact in borders and gardens, and they’re also great for wildlife—bees love the flowers and birds feast on the seeds in late summer. From towering giants to dwarf types suited to pots, sunflowers suit all kinds of growing spaces.

When to plant sunflower seeds

In the UK, sunflower seeds can be sown indoors from March to April and directly outdoors from late April to early June, once all danger of frost has passed. Starting them off indoors gives a head start and is ideal for colder areas, but they grow fast once temperatures rise. Soil should be around 10°C or warmer for best germination. Avoid sowing too early outside, as cold soil can delay or prevent sprouting.

Choosing the right sunflower variety

There’s more to sunflowers than the classic yellow giants. Tall varieties like ‘Russian Giant’ can reach 3 metres, while dwarf types such as ‘Little Leo’ stay compact and flower early. Multi-headed sunflowers like ‘Valentine’ offer a longer bloom time, and red or orange varieties like ‘Moulin Rouge’ add unique colour. Consider your space, height preferences and whether you want cut flowers, pollinators or seed heads for birds.

How to sow sunflower seeds indoors

Start seeds in 9cm pots filled with seed compost. Sow one seed per pot, about 1.5cm deep. Place on a warm windowsill or in a propagator—sunflowers germinate quickly, often within 7 days. Keep compost moist but not soggy. Once seedlings are about 10–15cm tall and have at least two true leaves, harden them off by gradually introducing them to outdoor conditions before planting out.

How to sow sunflower seeds outdoors

Choose a sunny, sheltered spot with well-drained soil. Sow seeds 1.5cm deep and 30cm apart for dwarf types, or 45cm apart for tall varieties. Water in well and cover with fleece if nights are still chilly. Protect seedlings from slugs and birds, which may dig them up or eat young shoots. Thinning is usually unnecessary if spacing is correct from the start.

Soil and site preparation

Sunflowers love full sun and rich soil. Dig in compost or well-rotted manure before planting to boost nutrients. Avoid shaded or exposed, windy spots, especially for tall varieties, which need protection and sturdy support. In very poor soils, a general-purpose fertiliser can help, but too much nitrogen can lead to tall plants with fewer flowers.

Supporting tall sunflowers

Giant sunflowers often need staking to keep them upright. Use bamboo canes or wooden stakes and tie stems loosely as they grow. This is especially important in windy sites or if the soil is soft. Dwarf or branching types generally don’t need support, making them easier for containers and small gardens.

Watering and feeding sunflowers

Sunflowers grow fast and need regular watering, especially during dry spells. Water deeply to encourage strong root systems. Once buds form, a weekly liquid feed high in potassium (like tomato feed) can promote bigger blooms. Avoid feeding too early with high-nitrogen fertilisers, as this encourages leafy growth over flowering.

When do sunflowers flower?

Most sunflowers bloom between July and September, depending on when they’re sown. Early indoor sowings flower sooner, while direct sown plants catch up quickly in warm weather. Some multi-headed varieties will flower for several weeks, especially if deadheaded regularly.

Harvesting and saving sunflower seeds

Let the flowers fade and dry on the plant if you want to collect seeds. Once the back of the flower head turns yellow or brown and the seeds are plump, cut the head and dry it indoors. Rub out the seeds and store them in a paper envelope somewhere cool and dry. You can use them for sowing next year or leave them out for birds.

Common issues when planting sunflower seeds

Slugs, snails and birds are the main early threats—use collars, netting or grit to protect young seedlings. Poor flowering usually comes from too much shade, overcrowding or excess nitrogen. Tall plants may snap in strong winds if not supported, and dry soil can lead to stunted growth or drooping.

Succession sowing for longer flowering

You don’t need to plant all your sunflower seeds at once. By sowing in small batches every couple of weeks from April through early June, you can extend the flowering period well into late summer or early autumn. This is especially useful if you’re growing them for cut flowers or want a steady stream of colour.

Companion planting benefits

Sunflowers can serve as living supports for climbing plants like beans or cucumbers. They also attract pollinators and beneficial insects, making them great neighbours for fruiting vegetables. Avoid planting them right next to potatoes though—they can compete for nutrients and water.

Not all sunflowers produce edible seeds

If you’re growing for seed harvest, choose varieties bred for it, like ‘Sunzilla’ or ‘Giganteus’. Ornamental types may still produce seeds, but often smaller, fewer, or less palatable ones. Be mindful that birds often beat gardeners to the harvest, so consider covering heads with netting if seed-saving is your goal.

Sunflowers can improve soil—but also deplete it

Sunflowers are deep-rooted and can help break up compacted soil, but they’re also heavy feeders. After they’re finished, it’s a good idea to replenish the soil with compost or rotate the area with low-demand crops like legumes the following season.

Beware of allelopathy

Sunflowers release natural chemicals that can suppress the growth of some other plants, particularly lettuce and some legumes. This isn’t always a problem in home gardens, but it’s something to keep in mind if you’re planting densely or using sunflowers as a backdrop in mixed borders.

Tracking the sun

Young sunflower buds exhibit heliotropism—they turn to follow the sun across the sky. Once mature, the flowers usually settle facing east. This means planting them in open east-facing areas often gives you the most visually impactful displays.