How to Grow Sunflowers (Helianthus)

Some favour sowing individual sunflower seeds in 7.5cm pots of fresh new compost and keeping them in a cold greenhouse with the tomatoes – full ventilation. However, you can also plant your sunflower seeds in a sunny and cosy place outside, in a rich soil with plenty of organic matter like compost. Be sure to set mouse traps, we’ve already learned the hard way this season that mouse traps are essential.

Wherever you choose to plant your sunflower seeds, you should first rake the soil into a fine crumbly texture and create holes approximately 12mm deep every 10cm. This will give your sunflowers the space they need to grow and flower.

Sow sunflower seeds individually in small pots of seed compost and place them on a windowsill, in the cold frame of a greenhouse, keeping the compost just moist. A temperature of around 16°C (60°F) is ideal as warmer temperatures can make the seedlings race upwards, becoming weak and spindly. This is best avoided but if it does happen (none of us are perfect!), they can always be supported with a little stick or cane.

When sown individually like this, the seedlings can grow in their pots until the roots begin to appear through the drainage holes in the bottom. At this point, they are ready to be planted into their final growing position outside. But let’s not rush! First, harden them off by gradually acclimatising them to outdoor temperatures. Place them outdoors in a sheltered spot for a few hours every day and then bring them back inside as temperatures fall again in the late afternoon. Extend the time they spend outdoors over two weeks or so, then they will be fully accustomed to outdoor conditions.

It may be tempting to skip this step, but please don’t! The young plants continue growing vigorously at this point, helping them to grow taller and flower weeks earlier than if they stop growing due to the shock of the move outdoors. It is especially important for giant sunflowers that like a long, uninterrupted growing season. Once acclimatised, they can be planted out without being stressed and will grow away strongly.

When to sow sunflower seeds?

Sowing seeds in mid to late spring usually means that they will be ready to plant out when outdoor temperatures are rising, and the risk of late frosts has passed.

If you decide to sow later than this, it has to be June or, if you’re very lucky, July. For late sowing, you may opt for Solar Flash sunflower seeds, Teddy Bear sunflower seeds, Santa Lucia sunflower seeds, or Little Leo sunflower seeds. If you're hoping to grow tall sunflowers and are considering sowing Titan sunflower seeds or Kong sunflower seeds which can reach heights of over 3m, then these will need sowing earlier on in the year, so that they achieve their maximum growing potential.

When it comes to planting sunflower seeds, it’s up to you whether you’d rather plant them directly into the soil, or in pots until they’re around 30cm tall.

If you’re going straight for the soil, be sure to plant your sunflowers into a rich soil, filled with plenty of organic matter and compost in the surrounding area to encourage the roots to spread. Make sure that you give the seeds plenty of space to grow and sow them directly into the ground for the very best results.

It’s also important to think ahead with flower plants like sunflowers and plant them in a spot where they’ll be protected from strong winds.

When to Plant Sunflower Seeds

Really, when looking at when is best to plant sunflowers in the UK you’re looking at March (for indoor planting), April, and May. Rather like their name suggests, sunflowers are partial to a bit of sun and don’t particularly like the cold, so it is always recommended to wait until the last frost is over before you get planting. You can plant your sunflowers indoors from as early as March and take them outside once they are sturdy in April or May. Whenever you plant your sunflower seeds, it’s important to ensure that they don’t dry out. Arguably ironically, sunflowers don’t do well in dry conditions. 

How Late Can You Plant Sunflowers?

June may be here but all hope is not yet lost. If you’ve missed the March to May window you will likely still be able to grow sunflowers in June and, potentially, July. The best way to grow these late starters is either sowing them outside on the border or in a container where they will flower. The advantages here are that you don’t need any special equipment and they never suffer any root disturbance so they will be free to grow steadily. The only real disadvantages of doing this are that you may have to plant them between other plants that will soon become overshadowed by the sunflowers plus the fact that slugs love to munch on sunflowers, so your surrounding plants may suffer.

Whatever your approach, water your sunflowers and sunflower seeds every week with tomato feed and watch them grow – and then, arguably more importantly, watch them flower and bring amazing colour and life to your garden. Sunflowers are easy flowers to grow and take care of, making them ideal for beginners and gardening novices.

Due to most sunflowers’ enviable height, they should be stalked to support their growth and reduce their chances of being blown down, knocked over, or simply being too tall and top-heavy for their own good.