Both hosta and button fern (and other ferns) are quite happy in a soil with an acid pH. Are you using a special fertilizer for your holly?

What is the pH of the soil under normal conditions?

Most often it is not the soil acidity that does not allow for plants to thrive beneath trees. If you are in western Oregon...the pH of any soil is more affected by our heavy rains which leach out the alkalinity and leave us with pH of about 5.5 to 6.0. Trees as large as holly, fir, maple and others suck up so much water and deplete the soil of nutrients to a degree that it can be challenging to grow much under them unless you water and feed them regularly. This is most important while the plants are getting established.

Here are some other options for plants that grow well beneath trees in shade.

*Hakonechloa macra(Japanese Forest Grass): comes in green, variegated ('Aureola') green and gold striped(my favorite look) and gold.
*Epimediums
*Hellebores

Hope this helps

Thanks for your responses. Currently i am not feeding my holly tree. I was planning on mixing in some soil conditioner and a little bit of compost before i plant the hostas. I will only be working on one side, as the tree is on a fence line. It is an old sturdy tree; hopefully this attention to its soil will do more good than harm.