Answer: Your Hetz Midget Arborvitae have reached their mature size. You can prune them back to generate new growth in the center of the plant by carefully cutting back to an inward-facing bud. Don't prune off more than one-third of the live plant tissue, though, or you'll stress your plants. It may take two or three years of annual pinching and pruning to get your little shrubs back into shape, but you can keep them looking neat and tidy in years to come by pinching them back each spring.
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