Size Matters
Had a great question about what kind of trays or pots to use for sweet peas and this one is pretty straightforward, thank goodness!
Size (specifically depth) does matter.
Let's break down why really quickly. Sweet peas have a root system that plunges deep for both water and nutrients. It starts this almost immediately after it germinates. The very first little germ you see when a sweet pea finally opens up is the root that will head straight down, followed closely by the sprout that heads to the surface for all that good sunshine.
When you start your sweet peas the way they prefer (cold and slowly), you'll see exponentially more growth below the surface of the soil than above it. By the time you have an inch up top, you likely have 4 to 5 inches down below and you want to do everything in your power to encourage this behavior. In times of heat, drought, or in poor soil, those deep roots are going to keep your plant cool, hydrated, and it will access a net of microbiology at that depth that will keep it fed if you can't.
If you start those seeds in a shallower pot or tray, the root system will still grow that long, but it will quickly hit the bottom of the container and begin to wind itself around the exterior of the tray looking for a way out. A week of that and you have what we call a root-bound plant. For sweet peas in particular, this isn't their favorite. They can pitch fits at transplant anyway, and if you plant a root bound start, it will just continue to wind itself around the plug for a while before it reorients itself downward. Alternatively, if you try to tease the roots apart at transplant, you will effectively shut the plant down for a while. They hate having their toes touched.
So the best option is always to give them as much depth as possible as early as possible and then disturb them very little at transplant.
Good options are deep trays, deep pots with multiple seeds in each, big yogurt or to-go soup containers with holes cut in for drainage, or biodegradable pot slips. I've used all of these and had great success. Root trainers specifically are fine, but spendy, and I don't think the clamshell opening on the roots is necessary. I like 50 cell forestry trays for my operation because it's easy to know how many I have for each row if I start one or two trays, and they can be purchased in bulk.
One thing I don’t use, contrary to a lot of DIY recommendations, is toilet roll tubes. The cardboard in toilet rolls will wick away moisture requiring a lot more watering, or get so soggy that they create environments for molds and bad fungus to form. I also never trust the chemicals used in making those cardboards and the glues that bind them in the roll, so I’m not interested in introducing them to my garden soil.
