How to grow great sweet peas

According to my seed-starting spreadsheet seed-starting spreadsheet, which I make every year to tell me when and how I’m supposed to be starting seeds, March 14 was the day to start my sweet peas. But I couldn’t wait any longer so I got all wild and crazy and planted them last weekend. (I’m a seed-starting rebel, I tell you!)

If I could only grow one flower from seed, it would be sweet peas, simply because you cannot find decent sweet pea plants at garden centers. And when you can find them they likely won’t transplant well because sweet peas just don’t appreciate that kind of treatment. To me, sweet peas smell like a garden. It is a scent you can’t find anywhere else and one you’ll never forget. That alone makes the bit of fuss required to grow them worth it to me.

It looks like I first started growing sweet peas in 2015 (based on this blog post) but I can’t believe it hasn’t been longer. I’ve always had moderate to good success with them, but I feel like I’m getting better at it every year. 

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‘Prince of Orange’ was gorgeous last year and made it on my must-grow list.

Here’s how I grow them these days.

I started growing them in root trainers root trainers last year and I feel like this was a positive change over the 4-inch pots I used to sow them in. Root trainers are small but long cells that allow plants to develop nice long roots. They open up like a book so it’s easy to remove the plug when it’s time to transplant them. The idea is that you can sneak these fussy plants that don’t like their roots disturbed into the ground without them really noticing. 

This year’s sweet pea selection that I’ll be growing.

Before I plant anything, I make a label for every single root trainer cell. No matter how much I think I remember that one whole row is one kind of sweet pea, I forget, so now every plant gets its own label with the name on one side and the color on the back along with the date I sowed it on. It’s terrible to have to go back to the plant packets and look up what color a sweet pea is when I’m ready to plant. 

Every plant gets its own label with the name of the variety, color and date sown.

To soak or not to soak: that is the question. Actually that’s only one of the questions when it comes to how to treat sweet pea seeds. Soaking, nicking (or roughing up with sandpaper) or pre-sprouting all have their fans. I did a little trial years ago in which I grew nicked seeds, soaked seeds and plain old seeds and the result as that none of that made a difference. Pretty much everything germinated. 

Sweet pea seeds can be tan to dark brown or even gray.

I see a lot of people pre-sprouting their sweet peas in damp paper towel, but I’ve never done this because it just seems like an extra step that I don’t feel like bothering with.

So I plant seeds—two per root trainer cell—straight into pre-moistened seed-starting mix about a half-inch deep. I always cover my seeds with vermiculite to prevent a crust from forming on top of the soil as well.

Sweet peas like a bit of warmth to get going, so I cover the root trainers with plastic—a humidity dome doesn’t fit on it—then put them on the germination heat mat and throw a towel over the whole thing to maximize the warmth.

Root trainers allow the plants to stretch their roots with little disturbance when transplanting.

Once most of them have germinated—and I find that sweet peas are extremely unpredictable in how long they take to germinate so there are always some laggers—I’ll move them under the grow lights and make sure to keep them moist (as you would with any seedlings).

When they grow to have about four leaves I pinch them all back to two leaves to promote branching for nice bushy plants. 

Sweet peas don’t like a lot of heat so they are one of the first things I move out to the temporary greenhouse. I almost never fertilize seedlings, but sweet peas seem to need a little boost right around the time I pinch them back so I feed them with very diluted fish fertilizer. 

They can be planted out quite early. Earlier, in fact, than I typically do and this year I’m going to try to stay on top of that a bit more. They can even handle a light frost and they certainly prefer to be growing in the ground, so move them on as soon as you can.

One of many sweet pea bouquets of a recent summer.

Sweet peas are particular. They like rich soil (this is true of many vining plants). They don’t like it to get too hot. And they aren’t fond of highly acidic soils (not a problem in my garden). In general my attitude to these demands is something along the lines of, “Suck it up buttercup. You get what you get.” But if I’m feeling particularly generous I will work some manure or compost into the area where I’ll be planting them about a week or two before.

Of course I stay on top the watering and give them a good amount of sun. It just doesn’t get hot enough here to worry about shielding them from hot afternoon sun, but it can help in hotter areas. They do need something to vine on. Some people use plastic mesh (the kind with 1-inch holes) supported on canes, but I prefer to train them up bamboo teepees. I tie in the stems until they are about 18 inches to 2 feet tall and from there they support themselves.

‘Erehwon’ is prominently featured in this quick-pick bouquet.

Beyond that my only sweet pea specific rule is to pick them, pick them, pick them. Sweet peas just keep going if you don’t let them set seed, but if you let even a few flowers to seed the whole plant gets sulky, as though it decides its job is done. So I make it a point to try to cut sweet peas every couple days when they are in flower. This means I have many tiny sweet pea bouquets around the house. And you won’t find me complaining about that.