Answer: As long as the new growth looks healthy, I don't think there's a serious problem with your holly. Broadleaf evergreens do eventually lose their oldest leaves. These are replaced in due time. With hollies, unless the branches are pruned back to force new growth, the centers of the plants can remain bare. However, the foliage is usually so dense at the tips of the branches, the bare centers are rarely noticed.
Your hollies will appreciate a deep soaking once each week during the growing season. You can build a watering basin beneath each by mounding up a few inches of soil in a circle, radiating out about 12" from the trunks. Fill the basin with water, allow it to drain, then fill it a second time. This method of watering concentrates moisture over the roots and allows it to trickle down, effectively saturating the entire root mass.
Best wishes with your landscape!
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