What you need is a ground cover that spreads like crazy or you could try an Oso Easy Rose Bush. They do require lot's of sun but they can spread 5 feet or more so you could plant them near the needles and they will cover them in theory. My Rose bush grew so rapidly that it began taking over my flower garden in the first year. When the snows melted this year it was still green and getting ready to leaf out with many little buds. Even after pruning it twice I had to get rid of it because it grew to fast. Be warned rose bushes has thorns.
If the cedar loses it's needles for the winter time you could try Spring bulbs that would get sunlight in the Spring before the needles came back and would then go doormant in the Summer/Fall. They may come up through the needles.
You could try plants that like partial to full shade and grow fast. Lily of the Valley is one option and another is Foxglove. The problem with Foxglove si they last 2 years and the needles may prevent them from reseeding. You could try Columbine to. They love partial shade areas. There is some ornamental grasses that like partial to full shade but I don't know how they would do with the needles. Ajuga is a ground cover that may work. Ajuga do not need alot of water so dry shady areas work for them. Some people say they are invasive. Ajuga is a ground spreading thug. It has lot's of pretty flowers though. Thymus is another spreading ground cover but it requires more sunlight. There is another ground cover I think called Vinca that suposedlly grows like crazy and takes over the area it is planted in but it looks more like a weed to me.
Your biggest problem is plants need sunlight to grow. Shady plants may be ok but the needles may prevent some plants from getting the needed sunlight thus keeping them from growing.
I hope that gives you some ideas you could try.
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