Q.What Can I Use To Fill In An Arborvitae Border In My Back Yard That Has Become Sparse At The Bottom?
Plants are 10 ft. high, run east to west on the southern border of my yard and get plenty of sun? Bottom 3 feet have become very thin and no longer act as a border.

Certified GKH Gardening Expert
It can be difficult to find a companion for this one. The Rose of Sharon is a good one to consider for this space, though. Here is an article that will help:
https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-sharon-companion-plants.htm
You can try other options, but some plants will have a harder time growing near Thuja.
Here are some more articles that will help:
https://www.gardeningknowhow.com/ornamental/trees/arborvitae/grow-green-giant-arborvitae.htm
https://www.gardeningknowhow.com/ornamental/trees/arborvitae/growing-arborvitae-trees.htm
https://www.gardeningknowhow.com/special/shade/shade-tolerant-wildflowers.htm
https://www.gardeningknowhow.com/ornamental/groundcover/gcgen/ground-covers-under-trees.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/planting-garden-beneath-trees.htm