Sweet Pea Troubleshooting in Southern Ontario
I tried growing sweet peas for years with minimal success, until last year, when I finally cracked the code for how to grow them in southern Ontario. They were beautiful, went on producing flowers for months and the scent was divine!
This blog post is here to help you with troubleshooting your sweet peas in Southern Ontario.
‘Old Times’ sweet pea by Johnny’s Seeds; one of the varieties I am growing in 2025.
Sweet peas are exceptionally easy to germinate and they are eager to grow! So why are they such a challenge in the garden?
In my experience, growing them inside, they grow so fast and get very leggy even with strong light, and then don’t transplant well. Direct Sowing them gets very sporadic germination results and by the time they bloom it’s too hot and too late for them to be happy.
There are countless blogs and gardening articles covering the topic of how to grow, as Sweet Peas are a classic flower for both the cutting and landscape garden.
However, I find the majority of them are vague “plant in late winter” but is that indoors or outside? Or the seed packets say to start them indoors 6 weeks before last frost or direct sow when soil is cool. Or “start them 4 weeks before planting out” but no indication of when to actually plant out. Or the grower is located in the Pacific North West in America, Coastal BC, England or somewhere with an entirely different climate than southern Ontario.
Most people will default to the May 24 generic weekend of planting and, that often results in plants that grow but then begin dying from the bottom up because of the heat. Been there, done that.
The only date I found for southern Ontario specific sowing, was the Toronto Gardeners Journal. It suggests starting them indoors the last week of February and direct sowing mid March.
I have never had trouble starting sweet peas. They are extremely easy to sprout in my opinion.
The big problem I faced has been timing; keeping them from getting leggy, hardening off, when to plant out and how to do it all without a greenhouse or any fancy equipment.
What growing conditions do Sweet Peas like?
Our springs in Southern Ontario seem to be too cold, too cold, too cold, HOT! And sweet peas really do like it cold, yet they need that full sun/warm temps to finally give you the beautiful flowers in June/July. I promise you they aren’t divas they are just misunderstood.
The trick with growing sweet peas in this sort of climate is to grow them COLD!
Usually seed starting is all about warmth. Heat mats, grow lights, heated greenhouses but with peas, this type of treatment, yea it will get them to sprout, but they will also grow crazy fast and get super leggy. And any leggy vining plant is tricky to work with indoors and it’s much less resilient to that transition outside. A leggy plant is a weak plant.
As with all things seed starting we have to work backwards to get our roadmap.
When do we plant out sweet pea seedlings?
In Southern Ontario, I’m in Durham Region and roughly a hardiness zone 5b, they get planted out 4-6 weeks before last frost. Counting back from mid May (last frost is May 4-11ish in my area), this puts us into the last week of March to early April. The reason we have a date range is because some years March leaves like a lion and it’s cold, stormy, snowy and not at all appropriate for our seedlings. If you are in Toronto, or part of the Carolinian zone of Ontario, you might even get away with mid March. Other years it’s out like a lamb and march has perfect weather for this task. Don’t worry, the way we’re starting the seedlings, they will hold an extra week or two just fine.
We don’t need to worry about the seedlings being snowed on. Snow is actually a great insulator. However if we’re getting below -5°C and there is no snow, then a cover is recommended.
However if there is still significant snow on the ground and the soil hasn’t thawed yet (would be weird for our area but you never know!), then it is not yet suitable for planting.
Now that we know our transplant date it should just be 4 weeks before that right? Afterall, that’s what most of the seed packets and blogs suggest.
Not quite, growing cold means much slower growth. Less leggy, but it really throws off the seed starting timing. 4 weeks just won’t cut it for growing a seedling that is robust enough for planting out in our roller coaster springs.
When do we sow our Sweet Pea Seeds?
We want to double that timing to at least 8 weeks before our transplant date.
This means we are sowing our sweet peas at the beginning of February to the end of January.
I soak my seeds before sowing. Usually for 24 hours. I don’t bother with any germination tests because I don’t plant mine into cell trays but sometimes, life with kids happens and I can’t plant them after soaking them. At this point, a damp paper towel and a baggie will help buy you some extra days before getting them into the soil. And it just so happens to be exactly what you would do for pre-sprouting/germination testing. It isn’t a necessary step but there are often a few seeds in a packet that aren’t viable and this can help weed them out.
If I’m not using cell trays then what am I growing Sweet Peas in?
Clear storage tubs! I saved and stored all of my kids clothes in these bins for hand me downs so I had a bunch stacked up and no longer in use. I fill up the bottom with 1.5-2 inches of pre moistened potting soil and scatter my seeds; I don’t worry about spacing specifics but I do move any that are touching and cover with a bit more soil. I sow more then I plan to need to account for some not germinating or other possible failures along the way. I fit approximately 100 seeds per bin and use these bins from Canadian Tire.
If, like this year, I’ve pre-sprouted them, I use my finger to poke a deep hole into the soil and drop the sprouted seed into the hole. I tuck them in with a bit more potting soil.
Because sweet peas like it dark in order to germinate, I usually tuck the bin into a black garbage bag. You could pop it into a basement room, cold cellar, closet etc. The perfect space should be a cool, dark one. I place mine onto the exposed concrete floor in my basement. Check on it every few days and once you notice sprouts, remove the bag. From now on they need it bright! But still cold.
Therefore growing them in your house is too warm.
So if we’re growing Sweet Peas cold, but we’re buried in snow outside… what does this look like?
I don’t have a greenhouse… yet. And I don’t have cold frames either. I do have a cold cellar but getting power in there for grow lights is tricky and I’m not wanting to invest in more grow-lights for that area. If you have lights, a cold cellar is actually a perfect location to grow sweet peas in. It typically stays around 5°C in mine.
What I do have is an unheated garage with a big south facing window.
The beautiful thing about growing them in these storage bins is that it’s like a mini greenhouse. So they get the protection of the garage and the bright light of the south window, and the added protection of the bin with lid. If you have an unheated garage but not a window, some grow lights will be needed. LED’s give off very little heat so you’ll maintain that cold growing environment. I grow mine without any supplemental lighting and just rotate the bin each week to account for any reaching to the light growth.
If you have a window, but it doesn’t face due South, I would again suggest you add grow-lights.
In Durham Region, we are getting around 10 hours of daylight at the beginning of February and it only increases from there.
But February and March can still get some really deeply frigid days (& nights!), won’t my Sweet Peas freeze?
Sweet peas are ok down to -5°C, but if it’s going to be colder than that, having some sort of a space heater on a thermostat can be really beneficial. Last year I simply did a lot of moving. Looking back I definitely played it too safe. If it even got down to 0°C I brought them inside for the night and put them back into the garage the next morning once the sun was up. Winter of 2023-2024 was unusually mild so the effort didn’t feel like too much.
This year I have a thermometer so I can gauge the actual temperature and a space heater at the ready should we need it. This year, the space heater has been needed even during the days as it has been consistently much colder. We use a space heater that has an anti-freeze setting. It keeps the temperature hovering right around 1°C even as the outside temperatures plummet.
I keep the lid closed the majority of the time but on mild days I do pop it off for both air flow and temperature regulation. Even though it’s not a tall tub, because they are growing so slow it is more than ample.
Sweet Peas Sown in Storage Tubs. Location: a garage in early February
There is a lot of information online about Sweet Peas not liking root disturbance and recommendations of soil blocking, toilet paper tubes, peat pots etc to prevent this. In my tub method I simply scoop up the plant with my hands, grabbing a bunch of the surrounding soil, occasionally teasing apart seedlings from each other, and pop that into my hole in the garden. I haven’t found them to be overly fussy on this point. Perhaps because they have grown nice robust roots from the slow, long growth and aren’t the quick grown houseplants that many are dealing with.
The week before I plan to plant them I start moving them outside with the lid off to harden them off. And just to recap, we’re planting them outside, 4-6 weeks BEFORE our last frost date.
This gives them a long period of cool temperatures to get some great root growth before the temperatures climb. And once they climb, the plants are mature, established, not leggy, and ready to bloom!
They will need a trellis or some form of support to climb.
Pinching early on is recommended to encourage branching and therefore more flowers but sometimes a cold snap will do that for you. So when and if you do it is up to you.
To keep them blooming continuously, make sure to keep cutting the flowers. Do not let it go to seed or else the plant begins to refocus its energy from blooming to seeds and it doesn’t recover after that.
Sweet peas are a favourite of mine because grown this way, they are so hands off and don’t require any fancy equipment or even indoor real estate. If you’ve been struggling to get a successful crop of sweet peas then I hope something we covered here today troubleshoots your situation and that this year is the year you crack the code!
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