Pippi: Sweet peas will climb on anything that is vertical. If your mailbox is mounted on a post, it will climb on that. I usually grow mine in a pot, in which I form a trellis with a few twigs from the garden. This year I also made a tripod with twigs. on which a few vines grew. I prefer vines to grow on wood, rather than metal. I feel that in very hot sun, when the metal gets very hot it would stress the plants (don't know if this is true). The pic. below shows my sweet peas which are growing in a pot, but does not show the setup. Be aware that these plants can grow quite tall, as much as 5 feet, so you need something to accommodate that. Hope this helps.
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