My mailbox is set on concrete. I am thinking of building a wattle basket around the base of it, lining it with coir and planting sweet peas in it. Am I right in thinking that the sweet peas will not be able to cling to the metal post but will need some kind of obelisk built around the post in order to climb?
Also, I was thinking that having some trail down the outside of the basket would look nice too, but will sweet peas trail attractively?
And thirdly, I haven’t done much container gardening, but I’ve noticed that they tend to cram plants a lot closer together in containers than in open ground. How many pea plants per square whatever, should I plan to plant in this basket?
Sweetpeas won’t be their best in a container because they want deep cool soil. You will probably want a minimum of 18″ of soil.
I would also be concerned about growing them on concrete, as the concrete may hold too much heat if part of it is exposed to the sun. If you decide to go ahead, I would plant just one or two plants as sweet peas branch heavily. For supports use some garden twine tied on one end to the mailbox and the other to some earth staples pushed into the soil. I have successfully grown Thunbergia alata (black-eyed susan vine) this way around a mailbox, which might be a good alternative for you.