In this issue:
Seeking black walnut
Curing the no-bloom hoya
Understanding color-shifting gladiola
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What will do
well in the general vicinity of a black walnut? - B.L.
The roots of black walnut (Juglans nigra) produce the
chemical juglone, which suppresses the growth of other plants. It
even kills some species that are very sensitive to this chemical.
Juglone can also leach into the soil from decaying leaves, bark,
roots and fruit.
This toxic effect is called allelopathy. Other plants are
allelopathic -- quack grass, mums, California pepper tree, and some
yarrows -- but black walnut may be toughest to deal with because it
has such strong, wide-reaching effects.
We call the problem black walnut wilt. Many gardeners have
learned the hard way that tomatoes, potatoes, rhododendrons,
azaleas, Pieris, mountain ash, lilac, burning bush, white
pine, potentilla and blueberry are among the most susceptible. The
first effect may be a general stunting, as the walnut's roots reach
the sensitive plant. It may gradually or very quickly advance to
the point of sudden wilt and death.
Like you, we tired of seeing the list of what dies near
black walnuts, so started searching for the opposite lists. Based
on our own experience and a West Virginia University inventory of
species that occur naturally under black walnuts, here are some
plants that should thrive.
Shrubs, vines and trees: arborvitae (Thuja), barberry,
beech, bittersweet, black cherry, blackhaw (Viburnum
prunifolium), clematis, dogwood (Cornus florida),
elderberry (Sambucus), elm, hawthorn, hazelnut (filbert),
hydrangea, maple (sugar and red), maple-leafed viburnum
(Viburnum acerifolium), oak (white and red), red cedar
(Juniperus virginiana), spice bush (Lindera),
sycamore, Virginia creeper, witchhazel.
Perennials: Anemone species, aster, bee balm
(Monarda), bellwort (Uvularia grandiflora), blue
mist flower (Eupatorium species), Christmas fern, coral
bells, geum, Geranium species, ginger, goldenrod
(Solidago), hosta, St Johnswort (Hypericum),
impatiens, ironweed (Vernonia species), Lobelia,
marginal shield fern (Dryopteris marginalis), mayapple,
Rudbeckia (blackeye susans and coneflower species) sedum,
Walnut toxicity info update, April 2014
In our original article we shared two links, but even university
Extension publications slip out of publication sometimes and that's
what's happened to two we found quite valuable regarding black
walnut allelopathy, one from West Virginia University (Bulletin 347
Occurrence of Plants Under Black Walnut) and one from
Illinois' Morton Arboretum
The good news is that the Morton Arb didn't let the information
slip away from us. What they did there is to put the information
about walnut tolerance into individual plants' descriptions.
(As at right.) Looking through the Morton's plants pages nets us
this list of species resistant to or tolerant of black walnuts
nearby adding juglone to the soil:
White oak (Quercus alba)
Swamp white oak (Quercus bicolor)
Pawpaw (Asimina triloba)
Fringe tree (Chionanthus virginicus)
American hornbeam (Carpinus caroliniana)
River birch (Betula nigra)
Sycamore (Platanus occidentalis)
Ohio buckeye (Aesculus glabra)
Redbud (Cercis canadensis)
Red maple (Acer rubrum)
Tulip tree (Liriodendron tulipifera)
New Jersey tea (Ceanothus americanus)
Black haw (Viburnum prunifolium)
Koreanspice viburnum (V. carlesii)
Mapleleaf viburnum (V. acerifolium)
Arrowwood (Viburnum dentatum)
Probably the Morton staff has this in common with us: They are
adding to their website all the time. So if you go to the site,
Search walnut toxicity and get the most recent list.
If you go to the site use the link below (or if it stops
working, go to the Morton and type walnut toxicity into
their Search field):
West Virginia University's bulletin was unique. The researchers
did a survey of several hundred black walnut trees, recording
everything growing within the each tree's root zone. That meant the
list had specific perennials and grasses as well as woody
companions. Better, they compiled the data as a list with percent
of the time a given species ocurred under a black walnut. Thus by
looking at other sources' woody plant lists we can see there are
exceptions to every rule but also be able to play it safe by
choosing plants that occur with black walnuts most often (every 4th
or 5th tree or mroe frequently, reported as 20% of the time),
rather than species that were under only 5% or 10% of the
In the group gleaned from West Virginia University's findings
are these perennials/groundcovers that have worked well for us,
too, under black walnuts:
Geum species (27% occurrence)
woodruff/bedstraw (Galium species), (30%)
Aster species (54%)
blue mist flower (Conoclinum coelestinum) (21%)
ironweed (Vernonia species) (40%)
Virginia creeper (Parthenocissus quinquefolia) (41%)
We'd love to take our paper copy of West Virginia's bulletin and
put the information here. Sponsor us and mention this topic in
your pledge if you want that addition!
Hoya hangs around, bloomless
About 30 years ago, my sister-in-law gave me a cutting
of her hoya plant. It has grown to an enormous size, is beautiful,
but has never bloomed. The plant the cutting came from bloomed. A
few years back, that sister-in-law moved to Florida and took a
cutting of my plant with her. That cutting-grown plant bloomed. Why
doesn't mine bloom? - M.B. -
Based on its age, your plant is probably Hoya carnosa,
called wax vine or honey plant. The other popular houseplant hoya,
Hoya bella, lives for only about five years. Hoyas are in
the milkweed family - when you finally see their waxy, cream
colored flowers in hemispherical clusters, you'll recognize the
At Michigan State University recently, we were lucky enough to
catch up to Gregory Goff, MSU's Plantscaping Technician. We admired
one of his hoyas, in bloom, while we discussed your question.
Goff feels that if it's healthy, large and growing strongly, any
hoya will bloom. In warmer climates, it might bloom off and on all
summer, but seems to blooms in winter for us in Michigan.
So take a critical look at the growing conditions and size of
your plant versus your sister-in-law's. Maybe yours has too little
sun. Like Michigan's native outdoor milkweeds, Hoya
species need direct sun - about 4 hours a day - to bloom, but they
appreciate midday shade.
Perhaps you're pruning too much. Even in the sun, hoya vines
won't bloom on stems less than three feet long. Once a branch
blooms, if you deadhead it, remove flowers but leave the flower
stalk (peduncle), because the next crop of flowers will form from
growth that originates there.
What kind and how much fertilizer do your each give your plant?
Over-fertilizing can cause hoya, and many other plants, to grow
lush foliage at the expense of flowers.
Goff gave us a tour through his extensive, labeled collection of
interior plants in the MSU teaching greenhouses. A great learning
experience, and one you can have, too, at the annual MSU
Horticulture Club Spring Shows. They are free and include seminars,
demonstrations, informational displays, a plant sale, and lush
plant displays within the huge greenhouse Watch the club's
website for the current information.
I planted and grew some gladiolas two years ago, a mix
of colors. They bloomed nicely and so I dug them at the end of the
year, saved the bulbs over winter, And replanted in spring. They
bloomed that second year, but all of them had turned to yellow - no
more mixed colors! The same thing happened to my neighbor. What's
going on? - B.G. -
This stumped a lot of people I talked to, but not Sam Fisher,
the Executive Secretary of the All-America Gladiolus Selections
Committee. Sam advises us that gladiolas can change color, but this
happens only about 1% of the time. More likely, fusarium disease is
This fungus disease often kills gladiolas while in storage. Some
of yours may have developed it during their first year and though
the corms looked okay when dug, they died during storage or in the
second season before they could bloom. They may even have been
infected when you got them, but it took more than a year for the
disease to run its course. Since lighter color gladiolas - white
and yellow - are usually more resistant to fusarium than the
others, it would make sense that you'd end up with a lot of yellow
and not many other colors.
The answer, according to Sam, is to start with healthy bulbs.
The best source is a nursery that specializes in gladiolas. Such
specialists use preventives on corms going into storage, such as a
30 minute soaking in hot water with 5% alcohol, or a solution of
the fungicide Benlate.
Fusarium can live over a winter in the soil, so don't plant
gladiolas into the same place where you grew the ill-fated
Special thanks to Sam Fisher with the All-America Gladiolus
Selections Committee, who was a great help on this puzzle. In a
note of thanks we sent to Fisher, we told him we, "included the
notes about hot water dip and Benlate based on Pirone'sDiseases and
Pests of Ornamental Plants, once you'd pointed us in the direction
of fusarium. ...people here may not try it themselves, but
certainly readers say they love this kind of additional
First published 4/15/95 and updated 4/21/13
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