They do need some sun, around half a day, do better in the ground than in pots. If they're Lathyrus latifolius, they don't make an awful lot of growth, it tends to be 3-6 stems which get long rather than lots of bushy growth. They grow easily and well up taller shrubs by looping themselves round small branches, but without something to climb up, they will scramble along the ground. A trellis won't work though, the slats of wood are too broad for the plant to attach itself, you're better fixing some clematis mesh or green chicken wire to the fence. Make sure you pick the right spot - once it's planted, they're next to impossible to remove.