If you love beautiful, old-fashioned sweet pea flowers (Lathyrus odoratus), these tips will ensure a successful, bloom-filled growing season. Be sure to choose annual dwarf or vine varieties and watch out for the invasive, perennial species.
Another magnificent flower to grow at the same time are these giant sunflowers.
How to Grow Sweet Peas
1 Know What You’re Growing
A There are two basic types of sweet peas: annual and perennial.
The annual ones (Lathyrus odoratus) are the single-season beauties we know and love.
In many areas of North America, the perennial sweet pea (L. latifolius) is an invasive problem, clogging open fields and roadsides. It looks pretty until you realize it’s crowding out other plants and acting like a thug.
When buying your seeds, be sure the package says they annual seeds suitable for your growing zone.
Annual Sweet Pea Seeds
B The other decision is, dwarf flowers or vines?
If you want climbers that grow up a trellis, or hang down from a hanging basket, choose annual vines. Many grow 5 to 6-feet tall.
If you want short, stout (but beautiful) flowers, choose an annual dwarf variety. Heights vary from 6 to 18-inches.
C Is fragrance important?
Love the fragrance? The older, heirloom open-pollinated varieties may be just what you’re looking for. They are best known for their fragrance. The invasive perennial ones have no scent.
2 Get Your Timing Right
Overall, sweet peas prefer cooler spring-summer temperatures and don’t do well in intense heat.
You can start your plants from seed or buy starter plants and transplant them after last frost.
There are a few options for timing your seed starting:
1 Start indoors late winter / early spring (8-10 weeks before last frost).
2 Sow directly outdoors early spring (6 weeks before last frost).
3 Sow directly outdoors in autumn: recommended for mild winter climates where the ground does not freeze.
The optimum soil temperature for sowing sweet pea seeds is 55-65°F (13-18°C).
3 Prepare the Seeds Before Sowing
Sweet pea seeds have a tough coat. To speed up germination, we ‘scarify’ the coat, which means nick it or rough it up to enable water penetration.
We do this for indoor and spring sowing. There is no need with fall sowing: nature gradually does it for us.
There are a few ways to prepare the seeds:
1 Rub seed coat with sandpaper or a small garlic grater, or nick with nail clippers. You want to score the brown seed coat without damaging the inner seed.
2 Soak seeds in damp paper towel 12-24 hours or a jar of warm water for 8 hours max.
Soaking the seeds causing the seed to swell and the seed coat to soften. But don’t overdo it or the seeds turn to mush.
4 Choose a Prime Growing location
Here in Ontario, Canada, we have hot, humid summers. The best location for sweet peas provides approximately 6 hours of morning sun, and dappled shade for the afternoons when the sun is quite fierce.
The planting area needs well-draining soil amended with lots of compost or composted manure. Sweet peas do not like damp or dry conditions and are heavy feeders.
5 Sow Seeds at Right Depth and Distance
This is simple but it can make a difference. Be sure to sow at the correct depth. Your seed packet will confirm the recommended amount.
Generally, sweet pea seeds should be sown just one-half to one-inch deep and spaced 4 to 6-inches (10 to 15 cm) apart.
Sown too deep, they may not germinate. Sown too close to the surface, the critters will get them or they will dry out without germinating.
Germination typically takes 2-3 weeks and the plants will have mature flowers 75-85 days after that.
6 Protect Seeds from Critters
You’ve got this far. Now take the extra step and protect those seeds and seedlings from all the wild things. Birds, snails, slugs, and more all want them!
My favourite option for protecting newly-sown seeds is wire cloches that sit right over the planting area and allow room for growth inside.
Here’s what they look like:
The critters stay out but sun and rain still get in.
Once the sweet peas are about 4 to 6-inches tall, they are less vulnerable to pests and you may be able to remove the cloches.
If pests are a big problem (e.g. you have relentless slugs or snails), growing in large containers may be a better option.
I offer no solution for deer or ambitious rabbits.
7 Add Trellis Right Away
It’s much easier, and less likely to damage your plants, if you get your trellises in place before the vine grows.
Use trellis, hardware cloth, netting, or stakes.
In the first month of growth, the vine will need your help to guide it onto the trellis with twine or clips. Check every other day.
After that time, tendrils start to form and grab hold. You do still need to make sure the plant is growing as desired and not wandering off or getting entwined on itself.
Left unattended it just takes days to have a tangled mess. Mind you, it will still flower even if muddled so not all is lost.
8 Pinch and Remove Tips for Bushier Growth
Congrats! Your sweet peas are on their way.
If you’re growing vine varieties, it’s time to decide if you want tall, thin vines, or bushier plants with more flowers. The difference is regular maintenance.
Once the plant has at least 3 sets of leaves on each stalk, you can start pinching it back to encourage side shoots.
Snip off central vines just above leaf joints and continue this throughout the growing season. The plant will be fuller, and, combined with deadheading, will produce more flowers than it would if left unattended.
9 Keep Soil Moist Not Damp—and Fertilize if Needed
Sweet peas don’t like to dry out: keep the soil consistently moist without allowing it to get soggy or dry. Super hot days can be a killer. If you can’t tend to them daily, you might want to use slow-release water feeders or soaker hoses. I’ve got a list of self-watering garden options here.
I find fertilizing is not necessary if the seeds are started or transplanted into a garden bed amended with composted manure. If you do fertilize, keep in mind that it could trigger leafy growth but not flowers.
10 Deadhead Flowers
Sweet pea flowers are fairly long-lasting but eventually they will begin to dry out and turn to seed.
Deadheading is the process of trimming back the old flowers, which encourages new growth right into late summer or early fall.
To deadhead, find an old flower, follow the stalk down to a set of leaves, and trim just above the node (where the leaves meet the stem).
Taking cut flowers achieves the same thing: the plant is stimulated to continue growing rather than producing seeds (until the reduced sunlight and cool temperatures of fall win).
If you want to save seeds for next year, leave your best flowers on the plant. The best flowers provide the best seeds.
Frequently Asked Questions
1 Can you grow sweet peas in pots?
You can, but bigger growing space is better. The size of the flowers is proportionate to the size of the roots, and these plants like lots of room—both width and depth.
Containers at least 18-inches deep are recommended: a smaller size can inhibit growth. If this doesn’t matter to you, use smaller containers. They are beautiful flowers either way.
Some sweet pea seeds are marketed for container growth so look for those.
2 Will sweet peas self-seed?
Yes. If you leave your sweet pea plants to turn to seed in fall, and allow those seed pods to pop open and disperse the seeds, it is absolutely possible for some to self-seed next year.
But, if you want to better odds, also sow seeds intentionally at the right time and protect them with fine wire mesh cloches.
3 Do sweet peas need deadheading?
Sweet peas will grow more abundantly with pinching back and deadheading but it’s totally optional.
4 How do I grow a sweet pea trellis or arch.
See How to Grow a Sweet Pea Garden Arch for step-by-step instructions. The trick is to start early and train the plant all along the way so it grows where you want it to.
5 Can I save seeds from sweet peas?
Yes. If they are an open-pollinated variety there is always a chance they have cross-pollinated with another variety but you’ll still have viable seeds. It just means they may not be identical to the parent plant.
To save seeds, pick the seed pods on a dry day in fall when they are fully-formed. Be sure to do this before they pop open! Allow pods to dry out for another month (in a cool, dry indoor location) before removing and storing the seeds. They should be viable for 3-5 years.
Sweet Pea TV
- Grow annual sweet peas. Beware of invasive perennial sweet peas.
- Sow seeds at the right time. Early spring is best.
- Prep the seeds before sowing either by nicking the coat or soaking in water.
- Pick a morning sun growing location with rich soil and good drainage.
- Sow seeds at right depth (usually 1/2 to 1-inch). Plant at least 4-inches apart.
- Cover your newly-planted seeds with wire cloches to keep critters away.
- Add trellis early on for vines. And add twine or clips to help the plant climb.
- Pinch back main shoots if you want a fuller, bushier plant with more blooms.
- Keep soil evenly moist. Best to start with rich soil (compost, manure) rather than fertilizing.
- Deadhead flowers to extend blooming season. Save seeds from your best flowers for next year.
There you go! You’re all set.
~Melissa the Empress of Dirt ♛