The ones that you saw that were floppy in the middle may have needed dividing. They like full sun, so they probably won't do as well as you want in the partial sun border... other than that, they're quite easy to grow and tolerate less-than-ideal situations (dry soil, etc.). They can spread and reseed quite a bit so they can become a nuisance. You'd probably be better off finding a plant that fits your partial-sun area better and/or planting the sundrops somewhere where they get full sun and you don't care if they spread.

Happy gardening!

They were invasive in my garden, so they no longer reside here.

This was the most obnoxiously invasive plant ever in my garden. I may have gotten rid of it all... Yikes! Don't recommend planting it.

There are three species of Sundrops that are commonly found at nurseries: Oenothera macrocarpa (Missouri Primrose); Oenothera speciosa (Pink Evening Primrose); Oenothera speciosa (White Evening Primrose). Depending on supplier will depend on who calls what an "Ozark sundrop". The first one is a large showy yellow flower and is well behaved in the garden. The other two - just difference in color - they have smaller flowers that are pink or white. This species can get invasive and the one I think other posts are warning to stay away from? I have several O. macrocarpa planted in my garden. They die back in the severe heat of August unless they have some shade and/or moisture.