Answer: Hollies (and most evergreens) grow best in soil that is organic and evenly moist yet well drained, and in at least a half day of direct sun. In deep shade they will grow slowly, and they may be relatively thin.
Hollies, as would any plant, will need several years to become rooted and fully established before they grow their best. In the meantime, it is important to keep them mulched with several inches of organic mulch year round and water if needed to supplement rain. The soil should be evenly moist like a wrung out sponge, not sopping wet/saturated and not dried out. Watering is probably the most important thing you can do to help them establish. You can also top dress with compost and fertilize each spring with a slow release fertilizer for acid loving plants such as Hollytone -- read and follow the label directions.
If you plant something else in between them or next to the hollies, the new plants will create root competition for the hollies. The crowding will cause the hollies to grow even slower. And, most evergreens are slow growers to begin with. If you feel the hollies are growing poorly, I would suggest you work with your retailer to make sure they are healthy, and check that they are watered correctly and were planted correctly (not too deeply, no encircling roots, suitable soil). Or, you might bring in a horticulturist as an on site consultant to help evaluate their condition and suggest ways to help them along.
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