Dig a quite wide but shallow planting hole, like a big dish or saucer. Rough up the sides well. Site the holly in the planting hole so that the top of the rootball is several inches above the soil level. Backfill the hole with the clay you dug out, breaking it up as necessary and firm in well. Then mound whatever soil or soil amendments you wish to use in a small berm up to the shoulder of the rootball (do not cover).
The wide, dished planting hole will allow for adequate drainage in the clay yet still provide for root penetration into the indigenous soil. And the amended mounded soil over the top will protect the rootball yet not create soil interface or water penetration issues like a typically dug planting hole would.
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For Castle spire holly and clay soil from gardengal.
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