As part of our savanna restoration work, we have been removing a large number of black walnuts (Juglans nigra). Although black walnut is considered a cash crop in forestry, it is an invasive species as far as we are concerned.

Walnut was absent from the original land surveyor's records for our area, indicating that it was only a minor component of the original vegetation. Because it is relatively fire sensitive, it would not have been a major component of the savanna. Black walnut is primarily a tree of the mid-continent of the United States and is at the northern edge of its range in southern Wisconsin. Here it is usually found in mixed forests as single isolated specimens, although in some restricted areas it may become dominant or subdominant.

Black walnut toxicity Black walnut produces a toxic substance called juglone (5-hydroxy-1,4-napthoquinone), which occurs in all parts of the black walnut plant. Juglone inhibits the growth of many other plants, a phenomenon called allelopathy. However, tests have shown that many native plants are resistant to juglone and are thus able to grow in the presence of black walnuts. Resistant plants include all grasses, and a number of woodland forbs, including jack-in-the-pulpit, bellflower, bellwort, dutchman's breeches, wild geranium, mayapple, solomon's seal, bloodroot, and trillium. Also, spiderwort, a common prairie plant, is resistant to juglone. Honeysuckle, an invasive shrub, is also resistant.

Walnut removal at Pleasant Valley We had two areas at Pleasant Valley Conservancy that had large populations of black walnut, Unit 18 and Unit 21 These two units are on opposite-facing hills that had the same geology. Each of these hills had been in pasture until the mid 1950s, after which they remained as open land. Both hills had some very large bur oaks which had been unaffected by grazing. After grazing ceased, these two hills gradually filled in with woody vegetation, of which black walnut was the most common tree.

The walnuts on Unit 18 were removed in the winter of 2000-2001, leaving behind a handsome stand of mature bur oaks that had survived years of grazing pressure.

The residual selective toxicity of juglone after walnuts are removed may influence the kinds of plants able to become established. It is interesting that in Unit 18, spiderwort, a species known to be tolerant to juglone, developed extensively on the lower slope after the walnuts had been removed, and has remained in large numbers ever since.

Heavy seeding of Unit 18 with native species was done in the fall of 2001, and again in 2003. Also, Unit 18 has been burned four times, including two very successful burns in 2003 (in both spring and fall) and one in the fall of 2004. Although there are still some major problems (brambles, in particular), we anticipate that this hillside will eventually become a fine oak savanna. After each burn, the unit has been seeded with forbs and grasses.

Unit 21, on the hill opposite Unit 18, is in the same geological formation and has had the same land-use history. When restoration began in January 2004, the principal trees were black walnut and slippery elm, with a number of small- to medium-sized oaks. The understory was almost pure honeysuckle.

A period of cold weather and moderate snow cover was favorable for walnut removal, and all of the trees were off the site within a two week period. Many of the walnut specimens were fairly large, and were good candidates for saw logs (lumber). We took advantage of the solidly frozen ground and had these logs brought down to the town road by a log skidder. Care was taken to avoid disturbance of the soil. There was also a smaller area of large walnut trees in the ravine between Units 12A and 20 and these were cut and skidded to the road at the same time.

In total, we had 25 logs ranging in length from 9-10 ft and diameter from 14-25 inches. They were taken by a logging truck to a sawmill. (The logs have been donated to a nonprofit organization to use for making furniture.)

The Resprout Problem. For most tree species, treatment of the cut stumps with 21% glyphosate prevents resprouting. However, after treatment extensive resprouting of cut stumps of walnut occurred. By the summer of 2002 the walnut stumps on Unit 18 (cut in the winter of 2001) had resprouted extensively. From each stump as many as 20 new shoots had developed, most from the root collar but some from the side of the stump. In January 2003 all of these resprouts were cut and treated again with herbicide. (See photos below) However, many of these stumps resprouted again in the summer of 2003.

Inquiries on the list server of the Invasive Plants Association of Wisconsin (IPAW) elicited the information that in contrast to most trees, glyphosate may not be completely effective with walnut. The recommendation was that triclopyr (Garlon) should be used instead. In January 2004, all of the resprouts were cut again and treated with Garlon 4 in oil. We plan to continue monitoring walnut resprouts until the problem has been eliminated.