Your barrel probably has a diameter of 25". This means you have 12.5" from the center to the barrel edge to plant in. However, you can't plant right at the edge of the barrel. Depending on how large a container the milkweed is currently in limits how close to the container's edge you can plant. If the plant is currently in a #1 container the plant will be 3 to 4" from the edge. Swamp milkweed can get between 12" and 24" wide, and the barrel is large enough for 4 plants. However, if the plants grow well the container will be quite thick with growth by the end of the season.

As to winter hardiness, Swamp milkweed is recommend for zones 3 to 8, and you are only in zone 6. Nonetheless, plants above ground may not fare as well as in ground plants. Wood does give some insulation against cold, but some articles recommend potted plants be placed on the south side of a building and have bubble wrap encircle their bases, or have leaves piled against their exteriors. I'm in zone 8, so cold damage to potted plants isn't usually an issue, but the last wooden barrel I used rotted within two years. Protecting the inside of your barrel before filling it with soil is advisable. This protection can be as simple as lining the barrel with a heavy-duty trash bag,and poking a hole over the barrel's drain. Then fill the lined barrel with soil and cut off any overhanging liner even with the top of the barrel. Or you could paint the interior of the barrel with an exterior grade paint or sealer. Do a search as there are lots of suggestions.

I also think that they will fit. They don’t have big taproots and it should be fine. They are not 2’ tall tho. They are 4’ or 5’ tall. Good luck!

Here they are today, blooming and pretty happy, I think. Thanks to all for the encouragement.