Answer: Viburnums tolerate most soil conditions, including alkaline, acid, and heavy clay soils. They take full sun, but appreciate some protection where summers are extremely hot. This wouldn't seem to apply to your location. They are susceptible to aphids, thrips, spider mites, or scale, all of which could influence its ability to thrive. Have you examined it carefully for any sign of infestation? They also need regular watering. Is it receiving water through the entire root zone area, both to a depth of about two feet and as wide out as the dripline? Finally, is it possible that the roots were rootbound before planting? In that case, they might continue growing in a spiral, wrapping around themselves, not spreading out into the surrounding soil.
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