Totoket Times June 25, 2005
Joy Of Gardening
By ML. Blue

Black Walnut Tree Toxicity to Plants

Black walnut trees can be found all over this area. I have several growing
on my property. These trees are very tall (up to 80 feet) with an open canopy.
The leaves are compound; they are made up of 13-23 leaflets, each about 1-3
inches long. The nuts on the tree are covered by smooth round green husks and
are the size of golf balls. When the nuts ripen, usually after falling, the husks
turn brown. The brown husks will ooze a dark fluid, which will stain everything
it touches. Black walnut trees bud out late in spring. When there is a dry summer,
the trees will shed leaves all summer long, so that there are few leaves left
by fall. Some people claim that you cannot plant under or near black walnut
trees, because these trees produce a toxin called jug lone.
Juglone is released from black walnut trees through its roots and also from
its leaves and the husks of its nuts. It can inhibit the growth of nearby plants and
may even kill juglone-sensitive plants. It does this by inhibiting the plant's respiration
and thereby depriving the plant of needed energy for metabolic activity.
The leaves of juglone-sensitive plants will yellow and wilt. Cutting down a black
walnut tree will not help right away, because jug lone can persist until the roots
are dead and decayed. This can take 5 years or more.
I have beautiful shade gardens under my black walnut trees. That is because
not all plants are affected by juglone. Daffodils, bleeding heart (Dicentra),
Brunnera with its big heart-shaped leaves and forget-me-not-like blue flowers,
forget-me-nots (Myosotis), sweet woodroff (Gallium odoratum), and Vinca with
its lovely blue flowers all bloom under the trees in early spring. It should be
noted that Vinca, often used as groundcover for shade, will become invasive
here in Connecticut and should only be grown in restricted spaces. Iris, daylilies,
Serbian bellflower (Campanula poscharskyana), and daisies bloom later. Ferns
and hostas thrive under these trees. A dwarf oak-leaf hydrangea and dwarf
Alberta spruce are also very happy. Two arbovitaes were doing well until deer
got to them this winter. I fill in the spaces between the perennials with annuals
such as pansies, alyssum, blue lobelia, and impatients.
Plants that are known to be susceptible to juglone toxin are some peonies,
azaleas, potentilla, mountain laurel, rhododendron, lilac, saucer magnolia, white
birches, pine, and vegetables such as tomatoes and potatoes.
Juglone is also known to be toxic to earthworms, which is bad news for
gardeners. On the other hand, juglone is a natural insecticide. It is known to
even reduce fleas of cats and dogs, if branches of the tree are placed near their
bedding. It is okay to compost walnut tree leaves, because the toxin breaks
down in 2-4 weeks when exposed to air, water, and bacteria.