Is there anything I can plant on the inside base of a cedar hedge to make it appear more dense without damaging the cedar ie., ferns?
Cedar trees make difficult growing conditions. They tend to create dark, dry, acidic soil. So you need a plant that can not only tolerate those conditions, but thrive in them, producing dense, bushy growth.
I looked for plants that can grow under cedar trees, and got these suggestions (source). Choose one(s) that will grow to the height you want.
- Ground covers: Wintergreen (Gaultheria procumbens) grows 6 inches tall; several cultivars of Japanese spurge* (Pachysandra terminalis)
- Perennials: “Big Blue” lilyturf (Liriope muscari “Big Blue”) grows 12- to 16-inch-tall; Bergenia (Bergenia cordifolia) grows 1 to 3 feet tall; hostas grow 1-3 feet tall, but you may need to water them.
- Shrubs: dwarf blueberries will tolerate the acidity and shade, but you won't get many berries; shade-tolerant dwarf rhododendrons will grow to about 2-3 feet, varieties include Snow Lady and Ramapo. Regular-sized rhododendrons are fine if your lowest cedar branches are above 5 ft.
- Ferns: (most ferns don't tolerate drought, but the few that do will enjoy the shade and acidity): Lady fern (Athyrium filix-femina) grows up to 3 feet tall; Christmas fern (Polystichum acrostichoides) grows 1 to 2 feet tall.
*Don't plant Japanese spurge / pachysandra unless you're sure you want it forever, because once established it's quite difficult to get rid of. Make sure it's surrounded by a barrier like regularly mowed grass or pavement, otherwise it will invade nearby areas and become a pest. A weed edge barrier will help, but you'll need to regularly monitor the border and dig out any escapees. If you mix it with other plants such as ferns, you will need to constantly defend those plants from the spurge or it will slowly devour them.
Have a look at the space under your cedar hedge, paying attention to how dense the shade is, how dry the soil is, and if the soil is very acidic (you can test soil acidity inexpensively with pH strips). If one or more of those conditions is not too severe, then you can choose from a wider range of plants.
SF has a nice guide to plants that tolerate shade and high-acidity soil. Here's a similar guide from Gardening Know How. Here's a guide from Better Homes and Gardens about perennials that tolerate dry shade. If you do a search for "plants that tolerate shade, acidic soil and drought" and include the name of your state, region or hardiness zone, you can find similar guides that are more tailored to your area.
A landscape is a dynamic process - plants grow, then die, to be replaced by other species that suit the new growing conditions. While it is possible to find temporary colour to fill a gap that arises, the problem remains in that shade was created and will get worse until the canopy dies off. It's a problem particularly acute in historic landscapes where a goal might be to freeze a landscape to represent a particular era. Irrational emotional attachments develop towards particular specimens that do not represent the era or the original function.
So the problem might not be the shade that the cedars create, but the cedars themselves. It invites the question "Should the cedars be there?" At which point many factors arise such as privacy and boundary delineation. This leads to a more drastic solution, tear out the cedars and either replant with young replacements or choose a more suitable replacement and prune it appropriately to ensure that the base is filled for the life of the individuals.
No doubt your particular circumstances dictate many more constraints than you have indicated in the question. This answer is intended as an encouragement to think more abstractly about the issue and where the cedars themselves stand in the landscape.