I know that cardinal flower is OK in shade, but isn't happy if it's too dry. Blue Lobelia IME self-seeds weedily. Although it was only in the garden by my choice for 2 years, 5 years later I am still weeding out seedlings in the lawn, the veggie garden, all the perennial beds, etc. Be sure to deadhead your plants religiously. Which fern? Some ferns are large enough to overpower the lobelias.

It works as a look if you like the way it looks. It is your garden. Sometimes you don't know if it works until you try it.

However, lobelias (both cardinalis and siphilitica) actually perform best in full sun with consistently moist soil. This can be hard to achieve in hot, dry summers, but they are not shade loving plants like many claim -- they prefer several hours of sun each day.

I was thinking of more delicate ferns like Lady Fern. I only have experience growing ostrich ferns. Hopefully, they are not too hard. The area I am thinking of planting is in the yard between two houses and also trees, so lighting is going to be on the low side. I have successfully planted some celandine poppies and they do really well there. I then attempted to add some hepatica, but for some reason the animals there kept destroying it.

I have Cardinal flowers growing in full sun, but the get crispy in the summer. The guy at the arboretum said both plant should be in full shade, but I am not sure that's correct.

Paul

They definitely aren't happy in full shade. I have tried it and failed. They could probably manage in dappled shade. They naturally grow in sunny marshy lowlands and at water's edge -- so they like to get some sun. In gardens, they typically don't get enough moisture to take the dryness of full sun exposure, so it is best to provide some shade to preserve the moisture. But, they do perform best in full sun with moist soil.

We have cardinal flower growing wild on the floodplain of the river behind our house where it gets sun all day except for the very earliest and latest part of the day when the trees further up the bank provide a bit of shade, so they have a long day of sun. However, I have seen plants growing and blooming along a stream in full all-day shade. I was so surprised at the flash of bright red I had to go investigate, but since it was right across from my house at the time, I do know that it was full shade, so it might be worth trying in your spot, with the realization that they might not be happy.

I have lobelia cardinalis (the red one) that seeded itself in the stream down the hill from my house. It's in the middle of the stream and on the edge of the stream, and it's in full shade all day.

Across the road where the stream continues in half day sun, it's growing at the edge of the stream, about 10 plants. I think the common factor is the stream! Shade or sun, they like it wet.