When should I plant sweet pea seeds?

If you love the sight and smell of fragrant sweet peas delicately weaving themselves around wigwams, then you’re in luck. That’s because there are two windows each year – one in spring and one in autumn – when you can start sowing sweet peas. 

Though the perceived planting window for sweet pea is wide (October-March), gardeners should avoid getting started in the darkest months of December and January due to the lack of light. That means there are essentially two smaller planting windows – October to November and February to March. 

Horticultural adviser and organic kitchen gardener Michael Thurlow told us sweet peas can be sown as early as February. And though we know spring is an ideal time for sowing sweet pea seeds, they can also be sown in October and November too, according to the RHS.

But does it matter what type of sweet pea you are sowing? Well, not really. Despite there being different types of sweet pea available (see below), the general consensus is that they can be sown at the same times of year. Guy Barter, chief horticulturist at RHS told us, however, that perennial varieties are often sown in April.

Barter also explains that sweet pea seeds need warmth. If you’re sowing any sweet pea seeds in the autumn, they’ll only survive the cold winter and any frost if they have somewhere warm, bright and dry to grow until they’re ready to be planted outside in spring. This can be a heated propagator or a warm, bright windowsill with a radiator nearby. 

The upside of sowing sweet peas in autumn is that surviving plants will be much sturdier going into the ground when the time comes.

There are two distinct ‘types’ of sweet pea: an annual (lathyrus odoratus) that needs sowing from seed each year, and perennial lathyrus (such as lathyrus latifolius) that will keep growing back and appreciates a little maintenance to flourish. 

Their characteristics can vary, depending on the type you buy. Perennial lathyrus tend not to be fragrant (except Lord Anson’s blue pea) and have fewer colour choices than annual sweet peas. But as we’ve explained, they can be planted at the same times of year. 

All sweet peas will prefer a sunny spot in your garden, like to climb around a support and grow best in peat-free compost that holds moisture, but still drains relatively well. This makes them a good option for growing in tubs and containers, but they can also flourish in raised beds and borders, too. 

Sweet pea seedlings have long, narrow roots that need room to stretch their legs properly. That’s why specially designed root trainers were made for them.

If you don’t want to buy a root trainer or would prefer to reuse something from around the house, then head to your recycling bin. The cardboard from a loo roll can be just as effective (or from a kitchen roll, cut in half).

Plastic trays with holes that hold your strawberries and grapes will also do the trick, as will a clean food tin, as long as you’ve drilled drainage holes into the bottom.  

Knowing when to plant sweet pea seeds is one thing. Ensuring their successful germination is another. 

Sweet peas, like many leguminous (pea family) plants, have a very tough seed coat. This means it can take a while for water to penetrate the seeds after sowing, leading to slow germination and losses due to rot, Barter told us. 

Some techniques to avoid this include people scraping seeds with sandpaper or chipping the seeds with a knife. Others soak the seeds overnight. Some place the seeds in a heat-proof container, cover them with boiled water from a kettle and leave them to cool.  

“I prefer to put the seeds in an airtight plastic box on some wet kitchen towel and leave in a warm place, such as the airing cupboard,” says Barter. “When they sprout (chit), I plant them into pots of peat-free potting compost.”

“Having said that, I know of many people who simply sow sweet pea seeds in the soil and get good results. The truth is, I suppose that seed lots vary with cultivar and length of storage.” 

Thurlow advises that the best time to plant your sweet pea seedlings is April or May, though this is frost dependent.

According to Sam Gallivan, team leader at RHS Wisley’s nursery and propagation unit: “Sweet peas are exceedingly tough. They will survive outside in frosts with absolutely no problems whatsoever.”

But a sweet pea plant’s nemesis is a combination of both frost and rain together, so they shouldn’t be planted into the ground if this is still a risk. You could, however, keep them under a cold frame.  

Gallivan recommends staggered planting from late spring through to early summer for a longer flowering season.