How Many is Too Many
Okay this wasn't the literal question, but it's pretty close. A while back, someone asked how many I would plant along an 8 foot trellis. I wish the answer was straightforward, but it's a little more nuanced than that, and if someone's giving you one firm answer, they're trying to sell you something. Choosing how many seeds to sow is like a choose your own adventure book, but at the end you get an incredible garden of sweet peas.
Sweet peas like to start cold. I germinate them (they germinate, I watch) at around 55F. It takes weeks. But when they emerge, they are twice as rooted on the bottom as they are out the top and they are much better prepared because of the slow, steady growth instead of the wild length you'll see at warmer temperatures. A leggy sweet pea is a fraction as healthy as a stocky, slow starter. When they germinate and poke their little heads up, I drop the temperatures even further, growing them on for weeks at between 30-50F. Last year, they dropped to 17 in trays on racks an unheated tunnel. It wasn't their favorite way to be, and I lost a few to damping off, but the rest bounced back. I'd recommend keeping it above 25 if possible. No need to stress them out.
I bring temperature up because it means that I do not have to pinch. When a plant is started slow and low, it branches naturally at the base, sometimes creating 4 vines out of 1 seed. You can see how the math on that is good for a seed producer. It also means that I don't have to go through and cut the top off of tens of thousands of plants a season. They do the work. Smarter not harder, as they say. The fewer touches I can give a plant, the more efficient the whole operation is, which is critical when it's just me and 10 hours a week from a (very competent) 18 year old helper.
(For some reason I had my first grade teacher voice on. I must have been drunk on sunshine that day.)
If I am growing this way, and each plant is branching into 3 or 4 vines, I want to give that plant more space, so I'd probably put it in at 9 inch spacing. Maybe even 12 inches if the area is very damp and doesn't get good air circulation. If I'm keeping it to 2 vines per seed, or even 1 vine on the cordon method, I plant every 6 inches because I'm pulling side tendrils and really pushing for long, single stems on individual vines.
So the answer is, what are you growing for? Production flowers with long stems: 6 inches and the cordon method. Seed production: as many branches as you can naturally get off of each seed and planting according to those branches anywhere between 6 - 9 inches. Tumbledown English-style garden: the world is your oyster. Plant anywhere from 6 - 9 - 12 inches, tie them to the trellis in ways you love to see, don't be shy about pruning entire vines and bringing them in to enjoy, and jump feet first in to the abundance of vines upon vines of the blooms over the season. If I was growing 8 feet of sweet peas in my garden for no other reason than I loved them, I'd be gluttonous. 1 plant every six inches on both sides of the trellis as long as it got good sun, consequences be damned. Would I regret it in August? Possibly. But what if I didn't...
