I was thinking the same as you, ken_adrian. I am in zone 5a NH. The forsythia here *just* bloomed today.

I bought some trough planters to hang on the railings of my front porch. I sowed dwarf sweet peas (Bijou Mix) in them with the idea that they will mass and trail down over the edges of the planter. However, in looking for pictures online, it seems that sweet peas grow fairly vertically, and don't really trail or cascade downwards as I was lead to believe from the description on the seed packet.
Does anyone have any experience with sweet peas? Should I abandon my idea? Would petunias be a better fit for my end goal?

I am curious to see what happens, but at the same time I don't want to waste precious time, especially if I am going to be disappointed.

I live in zone 5B, and the planters are located in a position that gets full sun (from sunrise to about two or three in the afternoon).

I was confused about that too, as I have about 40 started under lights right now. From everything I have read, normal sweet peas want to grow vertically, but the dwarf will cascade somewhat. They grow them in window boxes and hanging baskets in the UK all of the time.

I put in a second planting of High Scent sweet peas about a week ago, and they're beginning to show. A chipmunk ate my first planting, so I sprinkled baby powder on the soil this time. Apparently it worked. It's still very chilly here at night, and I don't imagine the soil is any too warm.

How many dwarf sweet pea plants would you recommend for a twenty-four inch railing planter? I want it to look nice and full. The dimensions of the planter are 24 inches long x 5 inches wide x 9 inches deep.

I sowed eleven seeds in each planter, which are starting to show now. However, I also started some peat pellets to fill in as needed.

Any input is greatly appreciated! Being a novice gardener is fun, as everything is new and exciting, but it is a bit intimidating, too!

I gave in and planted two tidal wave petunias in each planter, along with the snapdragon seedlings. I am hoping for the best of both worlds.