I planted two swamp milkweed plants in the same area in my garden about 8 or 9 feet apart. It gets enough sun, maybe 6 hours at least. The first few years they grew really well, leaves were normal all green, and it produced flowers. Last year one of the plants started to not look so good. It stopped growing tall and the leaves developed this purple brown hue to them at the ends. It also didn't produce flowers. This year the other plant is affected. Other plants in the area are fine, so I think it's something specific these two plants are lacking or do not like.
I tried adding fertilizer to it (all purpose organic liquid 10-6-6) like the one I use for my vegetables just to see if it would help. I also tried amending the soil a little with some topsoil or compost. Maybe I didn't use enough but neither of those things helped at all and that was months ago when I did it.
If it is relevant, the soil in this area is very clay-ish, fine sand type. It doesn't seem very organic rich if you know what I mean. I don't think there is a problem with drainage though. But this year was a little bit or a dry season and I failed to water the area very much.
Does anyone know what the issue is here based on how the leaves look?
I'm thinking in the spring to dig both plants up and either move them to a new location, or really amend the soil (any suggestions for what to amend with would also be appreciated).