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
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.
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.
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.
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
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.)
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.