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Northland Nature: Interrupted ferns thrive in summer forests

A close look at the spore growths (sporangia) on a frond of an interrupted fern. Photo by Larry Weber1 / 2
A cluster of interrupted ferns in the summer woods. Note the brown spore growths on the stem interrupting the fronds. Photo by Larry Weber2 / 2

Taking a forest walk in July is a unique experience. The woods is shady and a few degrees cooler than the heat that we feel in the open during this month. Thanks to the thick arboreal canopy, the shade is complete, broken only at a few open sites among the trees.

Larry WeberIn this scene, the spring wildflowers that were so abundant here two months ago can't be found. It's hard to believe that the spring beauties that carpeted the forest floor in May are gone now and don't leave a trace that they even were here.

Some of the spring flora may still have leaves, but wildflowers in July are best found in the roadsides, fields or wetlands.

A few unusual flowers are blooming here and during my walk, I find the white Indian pipe. Without chlorophyll, it does not need sunlight to make food, so it thrives in the shade. Here, too, is the pyrola with white or pink petals. Not blooming in spring, but the green leaves were present at that sunny time and they were able to make food to allow this plant to flower in the shade of summer.

I find fungi too. The bulk of mushrooms are a little later in the summer, but some are present now. Looking like small bushes or trees are the coral fungi on downed logs while the bright yellow-orange sulphur shelf hangs on some tree trunks. But there are plenty of other plants here; the ferns abound in these conditions.

Though they do not get lots of sunlight, they get enough and ample moisture for these green plants to grow tall. Like the nearby trees and flowering plants, ferns do have chlorophyll in their leaves and are able to manufacture needed sugars and nutrition throughout the summer. But they differ in many ways from the flowering plants.

Ferns do not have flowers. The growths that are so obvious now in the woods are leaves. (Ferns have their own vocabulary and leaves are called fronds.) They emerge in spring, unfolding as fiddleheads from an underground stem known as a rhizome. Reproductive spores frequently grow on the undersides of the fronds in small dark structures called sori. Some have a cluster of spores standing in the center of the fronds.

We can find about eight to ten kinds of ferns while walking the woods now and a few more in other sites. Largest and most common are ostrich, lady and interrupted ferns. Each can be 2-3 feet tall, with the ostrich getting more than 5 feet some years.

Smaller bracken along with wood, sensitive, oak, beech and maidenhair ferns are also in the woods. In wetlands, cinnamon and marsh fern grow while rock-cap and fragile ferns are on rocks.

One that I find of interest each summer in the Northland forests is the interrupted fern. Often growing 3 feet tall with many fronds and numerous leaflets (called pinnae). The plant grows well in the deciduous and mixed woods; often along the edge and by trails.

It is called "interrupted" because about halfway up on the fertile fronds, there is a space of no pinnae. Instead, the fern grows a thick spore-producing structure; the sporangia. Therefore, the frond's growth appears to be interrupted. These large ferns often grow in groups and some that are not interrupted. These are the sterile fronds; also quite common.

We may not see many wildflowers blooming in the shade of the summer woods, but lush green ferns make the walk interesting.

Retired teacher Larry Weber is the author of several books, including "Butterflies of the North Woods," "Spiders of the North Woods," "Webwood" and "In a Patch of Goldenrods." Contact him c/o

Larry Weber

Retired teacher Larry Weber, a Barnum resident, is the author of several books, including “Butterflies of the North Woods,” “Spiders of the North Woods,” “Webwood” and “In a Patch of Goldenrods.” Contact him c/o Katie Rohman at