Select - Department of Plant Sciences

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BY BILL KLiNGEMAN1,
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TOM HILL2, GARY MCDANIEL3, and STEPHEN
quascapes and watergardens represent one of the
fastest growing and most popular components of
our changing Green Industry. Opportunities for
innovative planting plans are boundless. Creative water garden
enthusiasts can blend aquatic and marginal plant species with
bog plants. Berms and borders can be used to showcase novel
spetimen plants with woody garden favorites that prefer dry
soil. With very li~le effort, a weekend gardener can assemble a
landscape that has as much biological diversity as a tropical rain
forest simply by shopping mail order catalogs and visiting a
well-stocked retail nursery.
In addition to aesthetic harmony, habitat stability is a crucial
objective of a successful water feature. Plants selected for the
water garden and its border should complement each other.
Textures and colors should be pleasing to the eye, of course. But
plants should also suit the maintenance and management objectives of the gardener or client. Just as important, plants should
be compatible with other species and with the habitat created.
This is not always the case in the developed landscape.
Through time, plants have developed biological strategies
that enable them to compete for available resources: water, air,
~utrients, and sunlight. Chemical byproducts of plant photosynthesis and metabolism can either be stored or used as needed for defense against herbivorous animals and arthropods.
Several examples of these "secondary metabolites" have been
used as medicines. Examples include aspirin from species of
willow (Salix sp.), quinine from the bark of Cinchona ledgeriana trees, digitoxin from foxglove (Digitalis purpurea), and
pest control agents like nicotine from Nicotiana tabacum,
pyrethrum from Chrysanthemum cinerariae and C. coccinum,
and rotenone from Derris elliptica and Lonchocarpus sp. roots.
It is not surprising that many of these same plants have been
used to stun (stupefactants) or kill (piscicides) fish.
Documentation of worldwide ethnobotanical explorations has
provided records of how indigenous peoples have used plants to
harvest fish. Close to home, Native Americans used mullein
GARTON4
(Verbascum thapsus), yaupon holly (Ilex vomitoria), and jimsonweed (Datura stramonium) to either stun or kill fish that
were then collected for food or fertilizer.
The pursuit of horticultural novelties and aesthetic treasures
has brought many plants that are known to be "ichthyotoxic"
(poisonous to fish) into our landscape (see Table 1). Water acts
as a solvent and can leach defense chemicals from plant tissues
even before decomposition starts. These compounds may occur
in leaves, bark, roots, or fruits and seeds and several different
chemicals may be produced. Leaves and seeds of buckeye
(Aesculus sp.) release saponins, which are soapy compounds
that contribute to the gel-like consistency of Aloe vera. Pecan
(Carya illinoensis) and walnut (Juglans nigra) roots, leaves and
nut hulls leach tannins and juglone, an allelopathic compound
that limits competition from other plants and can kill fish.
Leaves, stems, and fruits of peach and cherry (Prunus sp.) generate cyanide derivatives. Shrubs including butterfly bush
(Buddleia sp.), tea (Camellia sp.), rhododendrons and azaleas
(Rhododendron sp.), and yew (Taxus sp.) are all reported to be
toxic to fish. Callicarpone, produced by some beautyberry
shrubs (Callicarpa sp.) is reportedly more toxic than rotenone,
an insecticidal plant extract that is also poisonous to fish.
Numerous annuals and perennials that are common in landscapes have also been reported to kill fish. Weedy plants that
are related to tobacco and potato, such as nightshade, jimsonweed, and ornamental' Angel's Trumpet' (Datura = Brugmansia
sp.) produce alkaloids (similar to belladonnin and atropine) and
cardiac glycosides (the same group that includes cocaine and
digitalis and protects monarch caterpillars from hungry birds).
Global explorations have also transported new species of plants,
which are related to ichthyotoxic genera, into American gardens. Very little may be known about these species, including
their potential impact on aquatic habitats and their occupants.
For example, leaves, roots, seeds, or fruits from certain species
of Styrax, Ternstroemia, Vitex, Smilax, Solanum, Illicium,
Gardenia, Ligustrum, Eucalyptus, and Yucca have all been
The Tennessee Nursery & Landscape Association
Table1. List of OrnamentalPlants Known or PotentiallyPoisonousto
Fish (afterAcevedo-Rodriguez,1990). In manycases,very little informationis known aboutthe amountof plant tissue or durationof exposureneededto injure fish. Somereportshavenot documentedthe parts
of the plants that are toxic and most do not include the type of chemi-
TREES
Buckeye'(Aesculus
glabra & A. pavia)
Pecan(Carya illinoensis)
Silky dogwood(Comus amomum)
Persimmon*(Diospyrossp.)
Walnut (Juglans nigra)
Chinaberry,Beadtree(Melia azadirach)
Peach(Prunuspersica)
Willow (Salix sp.)
Chinesetallow tree (Sapiumsebiferum)
Hemlock (Tsugasp.)
r-
SHRUBS
Butterfly bush (Buddleia asiatica,B. davidii & B..,
Beautyberry* (Callicarpa candicans)
Tea(Camellia sp.)
Daphne(Daphnesp.)
Oriental Paperbush(Edgeworthiachrysantha)
Holly (Ilex verticillata & I. vomitoria)
Oleander(Nerium sp.)
Rhododendron& Azaleas(Rhododendronsp.)
English yew(Taxus baccata,1: X intermedia)
Sweetviburnum (Viburnum odoratissimum)
ANNUALS
I PERENNIALS
I VINES I AQUATICS
Monkshood(Aconitumsp,)
Jack-in-the-Pulpit(Arisaematriphyllum)
Dutchman'sPipe, Birthwort (Aristolochia sp.)
Crossvine(Bignonia capreolata)
Spiderflower (Cleomesp.)
Grand Crinum Lily (Crinum asiaticum)
Cyclamen(Cyclamensp.)
Foxglove(Digitalis sp.)
Carolinajessamine(Gelsemiumsempervirens)
Hellebore(Helleborus sp.)
Elecampane(Inula sp.)
GermanIris, Flag Iris (Iris X germanica)
Lobelia sp.*
Mandevilla (Mandevilla illustris)
Nicotine (Nicotiana tabacum)
Castorbean (Ricinus communis)
WEEDY PLANTS
Jimsonweed(Datura stramonium)
Gripeweed*(Phyllanthussp.)
Pokeweed(Phytolaccaamericana)
Smartweeds(Polygonumsp.)
Mullein (Verbascumsp.)
Goat'sRue (Tephrosiavirginiana)
FERNS, LICHENS, MOSSES
None werereportedto be toxic to fish.
I
calor modeof actionthattakesplace. Asterisks(*) indicatethat ichthyotoxic activity wasreportedfor relatedplant speciesthat are not native
andare not commerciallyavailablein theU.S. Toxicantactivity has not
beentested in native or commerciallyavailablerelatedspeciesthat are
commonin easternU.S.landscapes.
reported to kill fish. Although the species that were reported are
not commercially available, few or no studies exist to document
the ichthyotoxic potential of related plant species that are
already abundant in the landscape. Very few, if any, of the
reports that document plant effects on fish indicate either how
much plant material (or which plant part) is toxic or the duration of exposure that is required to injure fish. Instead, the speed
at which we find and distribute new plants has greatly outpaced
our ability to document their potential impacts. In short, little
research has been conducted in these areas and much remains
to be learned.
It is important to realize that plant tissues do not have to
include stored or defensive chemical compounds to be detrimental to fish. Introduction of a large amount of plant tissue to
a closed system (like lawn clippings spilled into a koi pond or
pruned leaves and stems dropping into a water garden) can disrupt the balance of the water chemistry by placing a large
demand on the amount of dissolved oxygen in an aquatic habitat. As plant tissues are decomposed, dissolved oxygen in the
water may be depleted and fish can become stressed. A fish
under stress may look and act normally, but can burn up energy
reserves that are used to maintain the immune system. In turn,
stressed fish become more susceptible to infectious diseases.
When dissolved oxygen concentrations fall below about 3 parts
per million, fish may become severely stressed and even begin
to suffocate. Death of fish usually results when oxygen concentrations drop below I ppm.
Early planning and some simple management practices for
aquatic habitats can reduce the likelihood that either tissues or
chemical defenses of plants will stress, injure, or kill fish.
When selecting a site for a closed-system water feature, avoid
locations that include poisonous canopy trees like buckeyes,
pecan, hemlock (Tsuga sp.), or walnuts. Take particular care, if
they are used, to place plants that are reportedly toxic to fish
(Table 1) on the garden borders and maintain them so that
leaves, roots, stems, and fruits are kept away from the pool of
water. Better still; select alternative plants that may include
Tennessee-nativewetland species. Be especially careful when
pruning and mowing around ponds and watergardens, and in the
fall when trees are dropping leaves. Routinely inspect the water
feature and remove leaves, stems, and branches before they
decompose. A long-handled skimmer net will simplify this
chore and can be purchased or made from hardware screen.
Whether they are $15,000 champion koi or $4 fantail goldfish,
these preventive steps are important contributions to the health
and well being of your aquatic companions. I
Noteson theAuthors:
IAssistant Professol; Plant Sciencesand Landscape Systems,ULI.A., University of
Tennessee
2Professol;Foresty,Wildlife,and Fisheries,ULI.A., University ofTennessee
3Professol;Plant &iences and LandscapeSystems,ULI.A., University ofTennessee
4AssistantProfessol; Plant Sciencesand Landscape Systems,ULI.A., University of
Tennessee
The Tennessee Nursery & Landscape Association m