Dryopteris x australis

Dryopteris × australis is a fern that cannot, strictly speaking, reproduce, and is all the more vigorous for it. It is a natural hybrid, thrown wherever two Southern wood ferns grow within a spore's reach of one another: the Log Fern, Dryopteris celsa, and the Southern Wood Fern, Dryopteris ludoviciana. The cross comes out sterile. It sets spores that never amount to anything, and so it cannot seed itself across a bed the way a large fern usually will. Every plant in cultivation traces back by division to a wild clump found somewhere between Virginia and Louisiana, the greatest number of them in Alabama.

The pedigree is better than it has any right to be. The Log Fern parent is itself a hybrid, a fertile cross of that same ludoviciana with the northern Goldie's wood fern, Dryopteris goldieana, which puts ludoviciana in the family tree twice and tucks a cold-country fern into the third corner. Whatever the arithmetic is doing, the result is the largest fern native to eastern North America. They grow rather as a Christmas fern might if you scaled it up and stood it on end: lance-shaped, lustrous dark green fronds, finely cut and twice-divided, held bolt upright in a dense vase four to five feet tall and half as wide. There is nothing floppy in it. The fronds rise and stay risen, which is uncommon at this size, and the effect in a shaded bed is frankly architectural, a green exclamation where most ferns manage a comma.

Southern by birth, it is unbothered by cold. The fern man John Mickel, who knew the genus as well as anyone, counted it among the great garden ferns and grew it strongly in New York, and it holds reliably from zone 5 through 9. Give them part to full shade and the rich, moist, humusy ground their parents keep along streams and in swampy hammocks. Once established they will take the dry rooty shade under a canopy without complaint, but a damp footing is where they show off. The one real caution is wind, since fronds this tall can tatter in the open, so site it somewhere sheltered. Semi-evergreen, glossy and green through a mild winter, bronzing and falling after a hard freeze, best cut back before the new fiddleheads climb. Sterile, so it stays precisely where you put it, and increased only by dividing the short creeping rhizome.

A sterile giant with a borrowed pedigree, too well-bred to spread and too handsome to overlook. The tallest native fern in the East, standing at attention in the shade while everything around it leans.