Hariliku kilpjala taimed on kõrgekasvulised, ühekaupa risoomile kinnituvate lehtedega; kolmnurkjas lehelaba kinnitub tugevale leherootsule mõnikord peaaegu rõhtsalt. Lehed on ühekujulised – steriilsed ja fertiilsed sarnased, soorused paiknevad reana sulgosakese serval, on kaetud kahehõlmalise loori ja enne eoste valmimist ka allakäändunud leheservaga.
Bracken has a perennial black rootstock that creeps extensively underground and at intervals sends up fronds. Individual rhizomes have been documented as spreading up to about 400 metres (1,300 feet) in length, making bracken one of the largest plants in the world. The fronds may reach a height of 5 metres (16 feet) or more and, despite dying in autumn, often remain standing throughout the winter, affording cover for game in some regions. The minute spores are incredibly lightweight, which has enabled the plant to spread nearly worldwide.
