How to Grow Perfect Sweet Peas
If you’re not growing sweet peas yet, you should be.
I’m not sure how a little climbing flower from Sicily evolved to smell like a cotton candy dream, but somehow the humble sweet pea has managed it. If a seagull pecked out both your eyes and flew off with them, my advice would be to grow sweet peas. You can forget vision! When you live in the same universe as the Sweet Pea, olfaction is king!
These heavenly-scented flowers bloom in almost every colour - mottled pastels, striped reds and purples, salmon pink, pure white with ruffles - set atop long thin stems that make them perfect for picking. Sweet peas epitomise spring, ushering in seasons of cloudless skies and warm weather. They’re easy to grow (although trickier to perfect) and will reward your care and diligent watering with weeks of blooms and a garden that smells like a parfumerie.
Clearly I’m a bit obsessed. So no one could blame you for assuming that I’d be a pro at growing sweet peas.
Unfortunately, not so much. They’ve always done OK in my garden, but I’ve never had the kind of tumbling onslaught of blooms I’ve hoped for. Sweet peas are easy to grow, but to really flourish they require a couple of things that have not traditionally been my strong suit: planning and preparation. Fortunately, the lovely thing about gardening is that we improve one season at a time, and every year is a new blank slate and a chance to try again. This year I’m ready for it - I’m going to perfect the sweet pea.
After plumbing the depths of the internet for old wives’ tales, science-backed tips and random pearls of wisdom, I think I’ve got it sussed. Here’s a roundup of the best tips and advice the internet has to offer about how to grow sweet peas, I hope it helps you to perfect them too!
A mass of fragrant sweet pea blooms in spring
1. When buying sweet pea seeds, look for ‘highly scented’ varieties
Originally all sweet peas were highly perfumed but now some of the modern varieties are not particularly scented at all. In my opinion growing a sweet pea that has no perfume is about as criminal as baking a cake that has no sugar. I refuse to buy a sweet pea variety that isn’t scented and strongly recommend checking on the packet, where it will almost always say ‘highly fragrant’ or ‘perfumed variety’ if the sweet peas carry that beautiful scent from yesteryear.
2. Ignore the old saying that ‘you must sow sweet pea seeds before Anzac day’
Wondering when to plant sweet peas in Perth? Worried you might have already missed the seed sowing window? Don’t stress! Although many websites recommend that you plant sweet before a specific day (some say Anzac Day, some swear by St Patrick’s), the truth is that sweet peas have a nice long growing season (especially for those of us growing them in mild climates like Perth where there are no winter frosts).
So, if you live in a mild climate in the Southern Hemisphere, you can sow sweet pea seeds anytime in March, April or May.
In fact, if you miss the autumn sowing window, you can even plant sweet pea seeds in spring. Just note that sweet peas planted in spring may not grow quite so large or flower quite as prolifically, and will likely bloom a little later than seeds planted in autumn.
The bonus good news is that because the window for seed sowing is actually way bigger than people assume, you can also sow seeds in batches throughout autumn and spring (also called ‘succession planting’ - keep your eyes peeled for my blog post all about succession planting which is coming out next week!). Each batch will come into flower at a different time, meaning you can have flowers for a much longer period than if you’d simply planted all your seeds in one go.
Sweet pea seedlings starting to emerge - you can start sweet peas in punnets or direct in your garden beds.
3. You can also ignore advice to ‘scarify’ or soak sweet pea seeds before sowing
People like to make things more complicated than they need to be! There’s plenty of advice online saying that you should either ‘scarify’ (scratch) the hard outer shell of sweet pea seeds prior to planting, or soak the seeds overnight before you sow them. I came across this advice numerous times in my researching, but it was contradicted by a few people who I regard as more trustworthy than Joe Blogs and his gardening blog (namely, the inimitable Monty Don and Dr Keith Hammet; a sweet pea cultivating expert from New Zealand who is responsible for breeding many of the beautifully perfumed cultivars sold across the world today). They said not to bother with all that fiddly nonsense, so I won’t!
Also soaking and scarifying the seeds can do more harm than good; soaking seeds can deprive them of necessary oxygen and cause them to rot before they germinate, while scratching the wrong part of the seed can damage it irreparably.
4. Seeds can be started in pots or in the ground
This year I’ve decided to follow Monty Don’s advice and start my sweet peas in pots. Monty recommends planting into deep pots (~12-15cm deep) that are long and thin so the roots can stretch out long and deep. Fill these pots with a combination of good quality potting (or seed raising) mix combined with a bit of garden soil and some well rotted manure.
Sow three seeds to a pot and plant them out into your garden beds when you can see the roots emerging from the bottom of the pot. There’s no need to split the plants up when planting them out, just pop all three in the same hole at once (planting them more densely like this leads to a bigger wall of colour).
You can also keep growing your sweet peas in pots throughout their entire growing season if you like, just transfer the seedlings into a larger pot once they’ve outgrown their punnets. Make sure the new pot is nice and big and filled with good quality potting mix and remember to provide a study climbing frame for the sweet peas to grow up.
If you decide to plant your seeds direct in the ground, make sure to plant them next to a climbing frame so they have something to attach to as they grow. It’s also a good idea to really enrich the soil first, as sweet peas are heavy feeders (more on that in a minute). Seed packets will recommend a planting distance of 20-30cm, but if you want a densely packed flowering display you can plant them a little closer (15-20cm).
5. Give them great soil to grow in
Sweet peas like to eat and, if you feed them well, will grow strong stems and many blooms. So it’s important to enrich the soil before planting them. Dig a few bags of sheep or cow manure into your garden beds before sowing the sweet pea seeds. If you’ve got a compost tumbler, add some compost as well. Once they’ve started to grow, give them an occasional water with an organic fertilising liquid (if you’re growing comfrey, try making your own comfrey tea, otherwise you can use diluted worm juice from a worm farm or a diluted seaweed-based fertiliser).
6. Keep them well watered
Sweet peas don’t like to dry out, so once they’re big enough, spread a good quality mulch around the base of the flowers. I like to use lupin mulch as it adds structure and nitrogen into the soil (and encourages worms), which the plants love! Mulching around the base of the flowers will help stop the soil drying out on hot days, but it’s important to also make sure the sweet peas get plenty of water too.
Use a climbing frame with thin wires so the sweet pea tendrils are able to cling on
A monarch butterfly visits a pink and purple sweet pea
7. Give them ‘cool feet and warm heads’
Do sweet peas ned full sun to flower and thrive? Absolutely! But (like clematis) sweet peas actually enjoy shade on their feet and their faces in the sun. This means that ideally the base of the plant will be shaded and kept cool, while the climbing tendrils will be growing up into bright, direct sun. To achieve this, you can plant small annuals (e.g., calendula, marigolds, vincas) at the base of the sweet pea plants to shade their roots while their heads stretch towards the sunlight.
8. Give them something to grow up
Sweet peas need something to grow up, so it’s important to know how to support your sweet peas as they grow. My biggest tip is to get your climbing frames ready early. So often I’ve been caught off guard by sweet pea seedlings that grow way too tall before I’ve sorted out something for them to grow up. This year I’m more organised and I’ve got the climbing frames ready even before getting the seeds in the ground. It’s also easy to underestimate just how tall the climbing frames need to be, and a lot of the shop-bought frames aren’t nearly tall enough to give your sweet peas enough room to grow.
Sweet peas can grow to over 1.8m tall so they need a really strong and tall support to enable them to reach full height. They also need a fairly fine climbing frame that their little tendrils are able to grab hold of, so chunky lattices or thick metal trellises won’t work well. If your trellis is made of wire thicker than about half a centimetre, the sweet peas will have trouble getting a good grip. Something finer - like chicken wire - is great, or you can use a soft netting like this one, attached to a stronger support structure.
9. Pinch them out (even though it feels wrong!)
Pinching out sweet peas means pinching the tip off each plant once they’ve reached about 15cm in height (or when the stems have grown long enough to have four sets of leaves). Pinching out sweet peas encourages them to grow more lateral side shoots instead of sending up a single vertical shoot. This leads to dense and bushy plants. It might be hard to bring yourself to nip the tip off the little seedlings you’ve been so diligently caring for, but most growers recommend it if you want to grow dense and lush sweet peas with plenty of flowers.
10. Pick pick pick!
I always struggle to pick flowers out of my garden because I feel like I’m committing them to death in a vase and depriving the bees of pollen. BUT (like many annual flowers) the thing about sweet peas is: the more you pick, the more they flower.
By not picking the flowers, you are telling the plant that it’s time to set seed - basically that the season is over and their job is done. This means the fewer flowers you pick (and the more you leave on the plant) the fewer flowers the plant will produce. So everyone loses. Instead, enjoy your sweet peas!! Pick big bunches about every 9-10 days and your plants will go on flowering for weeks on end - a bonus for you, your garden and the bees.
Sweet peas and fairy lights in the spring garden
11. Don’t make these common mistakes when growing sweet peas
Lastly, here’s a list of everything Past Casey has done wrong when growing sweet peas. Avoid my mistakes and hopefully you’ll have a garden bursting with these beautiful flowers by spring!
I’ve left my sweet pea seedlings in seed raising trays for way too long before planting them out.
This meant they got leggy and sat in thin, dry soil before I planted them out. Letting your plants get weak and struggle like this before planting them out makes them more susceptible to the bugs that want to eat them. Instead, feed your sweet pea seedlings really well and plant them out just as their roots begin to fill the pot they’re in. This way they will be maximally strong and will be off to a great start when they’re moved into your garden beds.
I planted them in average soil and didn’t feed or water them often enough.
Sweet peas need nutrients to thrive and they don’t like dry or sandy soil, so make sure to enrich the soil before you plant them. Add animal manure, compost and - if your soil is very sandy - clay. Once your sweet pea seedlings are big enough, spread a generous layer of mulch around the base of each plant to help keep the soil around their roots moist.
I planted them in pots that were too small.
If you want to grow your sweet peas in pots, remember that they need really big pots to thrive. Despite their delicate appearance, sweet peas have long, deep tap roots and will rapidly exhaust their growing space and start to struggle if you plant them in small or shallow pots. Without room to grow and without enough soil to feed them well, your sweet peas can end up sickly and undernourished. So, if you want to grow sweet peas in pots, plant them in nice big pots (with a depth of at least 30-40cm), use good quality potting mix (plus some extra manure) and feed and water them well.
I used climbing frames that were too small or not sturdy enough
Sweet peas that are well fed grow tall and prolifically and they need adequate support. Little trellises from Bunnings aren’t going to cut it - you need something sturdy and tall, with a fine mesh that the sweet peas can grab hold of. Pick your growing frames carefully and don’t cobble something together at the last minute - sweet peas that are well supported thrive, while those that sprawl around on the ground or in nearby shrubs do not.