To improve soils to banksia-friendly condition, add gypsum or other soil conditioner or raise the soil level into a mound at least 30-60 cm above the pre-existing level. Banksias appreciate extra water during dry periods, especially during summer. Special care should be taken not to let them dry out until established. They will eventually put roots deep into the ground and find the water table, but the process may take up to two years.

Fertilising with phosphorus should be minimal. A slow-release, low-phosphorous treatment is best. If new leaves turn yellow use iron chelate or iron sulphate according to the instructions. Species that grow from lignotubers, a group that includes Banksia robur, B. spinulosa and B. serrata, may be pruned hard - even back to ground level. Others, including B. ericifolia and B. ‘Giant Candles’ are nonlignotuberous and should be pruned lightly (not below green foliage) after each flush of flowers. This prolongs the life of the plants and keeps them compact.

https://www.gardenclinic.com.au/how-to-grow-article/banksia?pid=44203