How to Fertilize hard shield fern?
Diluted low-strength foliage fertilizer every four weeks spring to fall

hard shield fern · Polystichum aculeatum
Also known as: prickly shield fern
Grow in fertile, humus-rich well-drained soil in deep or partial shade; protect the crowns from excessive winter wet and provide a mulch around the crown to prevent crown drying out
Diluted low-strength foliage fertilizer every four weeks spring to fall