Effect of Deep Planting On Landscape Shrubs

Six shrubs commonly planted in southern California landscapes, Nandina domestica, Pittosporum tobira 'variegata', Ligustrum japonicum, Acacia redolens, Acacia redolens 'Desert Carpet' and Photinia farserii were planted from 3.7 L containers at four depths: at grade and below grade at -2 cm, -4 cm and -9 cm. The experimental design was a randomized complete block with five replications. Shrubs were irrigated three times per week with drip emitters to simulate typical over-irrigation found in many landscapes. All shrubs were established in the first year before measuring growth in the second year. Six acacias, both A. redolens and ‘Desert Carpet,' died in the second year: two were planted at grade and four were planted below grade. No other shrubs have died thus far in the third year after planting. Shrubs were pruned annually to maintain dimensions of 1 m³ or 1 m² for acacia groundcovers. Shrub height and yield of clippings were species-dependent: nandina grew the least and acacia and photinia grew the most. Growth was not affected by planting depth, however, indicating shrub tolerance to deep planting. These results suggest wide latitude in planting depth for successful young shrubs, except for acacia, which may be more sensitive to below-grade planting.