When to plant out sweet peas — if you sowed the seeds indoors this year, it's almost time to move them outside

We love sweet peas - doesn’t everyone? They’re beautiful, they smell great, and they’re easy to grow. If you’ve already sown them indoors, you’re probably wondering when to plant out sweet peas.

Knowing when to sow sweet peas indoors is one thing, but it won’t be long before you’ll need to start thinking about the best time to plant your seedlings outside. In fact, the transplanting stage is one of the most important steps in growing sweet peas from seed.

Well, it's almost time — but you’ll need to avoid any hard frosts. We consulted a panel of trusted garden experts, including Sarah Raven, to figure out exactly when to plant out sweet peas and let your plants hit the ground running this spring.

What you'll need

• Some farmyard manure like the RocketGro Peat Free Farmyard Manure from Sarah Raven
• A sweet pea support structure like this bamboo plant pot trellis from Amazon

When we say it’s nearly time to plant out your sweet peas, we mean it: in fact, if the weather is in your favour, you could be adding the task to your list of jobs to do in the garden in March.

‘Plant sweet pea seedlings outside during a mild spell between March and May,’ advises gardening expert and writer Sarah Raven.

According to Sarah, the earlier, the better. ‘I always recommend getting them in the ground early so they have more time to establish strong roots,’ she explains.

If you’re a little wary of the weather in your area, that’s fine, too — especially if you’re set on caring for your sweet peas in the best possible way.

‘I’d suggest planting sweet peas outside in April,’ says David Glass, head gardener at Bowood House & Gardens. ‘They can deal with light frosts, so don’t be too concerned if temperatures are relatively low.’

So, a light frost doesn’t necessarily spell disaster for our sweet peas. But before you plant your seedlings outside, you'll need to harden them off.

What's hardening off? It simply means slowly acclimatising your plants to outdoor conditions. If you do it all at once, you'll risk landing your plants with transplant shock.

'Once the weather warms up, typically from April onwards, start hardening them off on sunny days by placing them outside during the day in a sheltered spot,' explains Jane Westoby, creative director at The Hampshire Seed Company.

'Start with a few hours at first, gradually increasing the length of time each day. Do this over the course of a few weeks to help them acclimatise to temperature fluctuations. This process strengthens their stems and reduces transplant shock.'

You’ll also need to ensure your soil is in the right condition.

‘Before planting, enrich the soil with plenty of organic matter,’ Sarah advises. ‘I’d recommend using farmyard manure as it retains moisture well and gently feeds hungry plants with nutrients.’

It’s a good idea to put plant supports in place, too.

‘Sweet peas need support, so plant them next to a teepee, arch, or frame, tying them in early so they grow strong and upright,’ says Sarah.

FAQs

How tall should sweet peas be before planting out?

There's no hard and fast rule, but generally, sweet peas will be around six to eight inches tall before it's time to plant them out in the garden. By this time, they'll have developed into established young plants, ready to take on the outside world (once you've hardened them off).

If you missed the indoor sowing window, you're probably wondering when to sow sweet peas outdoors — and you can do that soon, too.