Every fern grower you speak to will have his or her own favourite method of feeding. The important thing to remember is that ferns grow slowly and have quite limited fertiliser requirements. Don’t overdo it!
Feed potted ferns with controlled release Nutricote (there’s one that’s specifically labelled “Fern & Palm”) or half strength Thrive Soluble. Some indoor gardeners find they get good results by using gentle, organic fish emulsion.
Outdoor ferns appreciate an occasional sprinkling with Yates Dynamic Lifter Soil Improver & Plant Fertiiser, and mulching with old leaves or well-rotted manure.
Some fern experts swear that their ferns enjoy a tonic of vitamin B1. They recommend dissolving one vitamin B1 tablet in 2 litres of water and pouring the solution over the root system