According to the U.S. Forest Service, more than 20 percent of the total forested area of the state of Pennsylvania suffers from invasive, debilitating fern-dominated understory. That likely includes sections of your woodlot.

Estimates by the U.S. Forest Service’s Northern Forest Research Station indicate the 3.7 million acres of Keystone State land with fern understory is typical of that found across the Northeast and up into New England.

New York fern, hay-scented fern and bracken ferns are the major culprits in this woodlot management issue. These are single-frond ferns (because the fronds or leaves grow singly without the circular clumps found in clump ferns). Single-frond ferns spread aggressively via rhizomes, pushing their shoots underground and popping them up through the soil at random, but frequent, intervals.

The clump ferns in our area are the Christmas fern, interrupted fern, sword fern and wood fern. While they dot the understory, they spread slowly and rarely create the dense blankets that the single-frond ferns create. Since they do not create shade competition with trees, the clump ferns are generally seen as more benign.

None of the common single-frond species are new to the Northeast. Yet their aggressive competition with valuable tree species is recent. With the shade these ferns spread on the forest floor, valuable species like oak, maple and black cherry cannot develop. In one study by the U.S. Forest Service, the number of desirable sugar maple, red maple and black cherry seedlings that were found after five years competing with hay-scented fern numbered 19,000 per acre. In the fenced experimental area kept free of ferns, there were 106,000 desirable tree seedlings per acre.

Fern stunts the growth of seedlings that manage to survive the shading. In a separate U.S. Forest Service study where deer and fern were kept out, the typical black cherry seedling grew nearly 5 feet – 55 inches – over the five-year study period. In the presence of hay-scented fern, those seedlings eked out less than a foot – just 11 inches – of growth in a five-year span.

Ferns are not the only species that compete with timber. Striped maple is a major weed pest. Despite being the Pennsylvania state plant, mountain laurel is another. Species like American beech, ironwood and spicebush are probably best excluded from woodlots.

Competition can inhibit regeneration of desirable plants like native wildflowers, forbs and herbs.

Any time ferns cover 30 percent or more of an area, they are likely to dominate the understory and interfere with reestablishment and growth of tree seedlings.

Researchers with the forest service say there are two likely culprits. One is natural, the other human-induced.

Oh, deer

Virginia white-tail deer are “Forest Enemy No. 1” when it comes to fern proliferation. Where deer impact is high, fern and other less-preferred browse species dominate the forest understory.

Many feel that fern understories are a legacy effect of decades of overabundant deer herds.

Before you suggest you’ve never seen a deer chow down on fern (and fern is, indeed, not a top choice for deer), keep in mind that deer are selective browsers. They do eat native hardwood seedlings, blackberry and plants like Canada mayflower, trillium, lady slippers and even woody species like devils club. By selectively browsing preferred plants, the deer allow nonpreferred plants – single-frond ferns, for example – to proliferate and spread.

While the game commission loves to see scads of deer out in the forest, all of those nonresilient plants are cowering when they see deer coming. Browsing pressure can completely eliminate those plants in the understory. The result is a forest floor that is dominated by plants deer avoid: fern and huckleberry, for example. Another indicator in a woodlot is presence of a “browse line” – typically about 5 feet above the ground where deer have eaten everything desirable from that level down. It is easy to see on species like oak, maple, ash and poplar. It does not take long. A deer can pack away 4 to 8 pounds of browse per day for seven months of the year.

While down from all-time highs in the 1960s and 1970s, deer populations in many areas are well above carrying capacity all across the Northeast.

Even in areas where deer numbers are reasonable, poor-quality areas will continue to decline as deer feeding removes desired browse species and ferns spread their footprint.

Since ferns can grow in areas where there is shade and full sunlight, they quickly out-compete other vegetation including tree seedlings.

Human impact

As a woodlot owner or manager, you frequently roll across your land for many reasons. Travel on tracked vehicles with metal cleats or sharp-edged new rubber tires are one of the big ways ferns get a jumpstart on proliferation.

Remember that single-frond ferns spread aggressively via rhizomes. A tracked vehicle will break all those rhizomes up into shorter pieces. Instead of one rhizome popping up on the woodland floor, there now are a half dozen for each one snapped and resnapped by the equipment.

Sometimes these places are easy to spot. Foresters will look for “fern tracks,” those foot-wide, parallel lines where ferns seem to be positively dense. Most likely, they are caused by equipment.

Ironically, those fern tracks might be caused by the very vehicles deployed to control ferns and other woodland weeds: the spray rigs that typically treat for understory weeds.

The best materials for control of ferns are sulfometuron-methyl (sold as Oust), glyphosate (sold as Roundup) or the two in combination. The materials are approved for use in ground-based air-blast sprayers or backpack mist blowers. Note that Oust has no herbicidal effect on beech or striped maple.

Roundup usually is applied at 1 to 2 quarts per acre and Oust at 2 ounces per acre. Both can be applied as broadcast foliar applications at those rates – but check label specifications first.

Early July, once there is full leaf expansion, that is the best time to apply either material. Quit by mid-September to early October.

Addition of Oust to Roundup seems to handle the fern track problem as well as grass and sedge re-invasion. Oust is soil-active and readily taken up by fern rhizomes. It has some pre-emergence activity as well.

Producers who shun herbicides can attempt mechanical control. However, in a forest setting, cutting and pulling are not as effective at controlling pest species. Too often, competing plants are broken off or the root is not fully pulled and they resprout. In some cases, the pest species is set back enough that the desired species can get sufficiently ahead of them and thrive.

To decide whether to treat, set out a half-dozen or so plots. The area covered by a circle with a 37.2-foot radius represents one-tenth of an acre. Be sure to spread the plot areas out throughout the woodlot.

If you find fern cover is 30 percent or greater on 30 percent or more of the plots, take steps to remove fern cover throughout the woodlot.