Invasive Plants from a
Horticultural Perspective
Mike Schnelle, OSU
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Please speak up as you have
thoughts/questions during presentation
Origins of Plant Materials
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Both native and non-native plants
have value in the garden and
landscape
Both native and non-native plants
have pest resistance, not just natives
Both will be discussed today
Start With Native Plants
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Growers and consumers could spend a
lifetime just working with plants from the
Lower 48. However, many ornamental
characteristics and “looks” cannot be
satisfied by our natives alone.
So when aesthetics and ornamental
attributes are factored in, it is hard to
overlook plants from other countries.
Mexican Plum
(Prunus mexicana)
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12-15’ tree native to Mexico and much of
U.S. including Kansas
Grow for spring white flowers and sheer
toughness of the tree
Heavy, wet soils will send this species into
a mortality spiral – OKC Bombing
Memorial
Some yellow fall color, attractive summer
fruits
Seed propagation
Mexican Plum
(Prunus mexicana)
Prunus mexicana
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Despite the name, Mexican plum IS native
to many southern and central U.S. states
Serviceberry (Amelanchier)
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Greatly underutilized group of
plants for Oklahoma and
Southern locations
Better known and used north
of us
White flowers in spring
Pest-free summer foliage
Yellow-orange-red fall color
Showy edible fruits range from
red-purple-black
Growth habit varies from shrub
to tree
Many native to U.S.
A crabapple alternative
Serviceberry (Amelanchier)
Non- native plants
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Appropriate when researched and used
responsibly
Pest advantage?
Traits not found in U.S.
Unsupervised Reproduction
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Happens with natives or exotics
Arguments Against Non-Natives
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May not serve as host plants for
pollinators
May not be a significant food source for
native fauna
May reproduce without human
intervention
Other
Invasive Plants
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A cultivated plant that under certain conditions
will reproduce, unassisted by people, and
ultimately displace native flora and occasionally
native fauna
No longer should we consider only exotic or
introduced plants to be the only candidates.
Natives in some instances could be invasive
when moved out of their original U.S. range.,
Eastern redcedar for example.
Challenges
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Most horticulturists want to continue to have the
right to RESPONSIBLY introduce and grow plants
of origins outside the U.S.
Invasive plants are a hot topic among diverse
professionals.
Horticulturists have different training
backgrounds and “ornamental motives” than do
ecologists for example.
Communication and rational discussion amongst
various disciplines is paramount in coming to
agreements that all can live with.
Widespread Condemnation of
Plants
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Because of the sheer size of the U.S., it is
not logical nor appropriate to condemn
any one species for cultivation anywhere
in the continental U.S.
It is questionable whether any species
exists that would prove problematic in all
48/49 states (Hawaii is the exception).
Ornamentals that Bear Fruit
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Strong movement underway to discourage many
female or otherwise fruiting plants in the
landscape
The notion that a fruit-bearing plant will
automatically be weedy IS NOT accurate
Plants have to be screened/observed on an
individual basis to determine their propensity to
reproduce rampantly outside their intended
area.
Tamarix
Pyrus
Ligustrum sinense ‘Wimbei’
(Chinese dwarf privet)
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This cultivar (also found as ‘Wimbish’) has
been around for quite some time. However, it
has taken me years to realize its value in the
landscape!
Its diminutive size (4-7’), short internodes,
variability from plant to plant really make for an
exciting shrub, or extremely small “tree” in the
landscape.
Chinese dwarf privet
Mimosa
Lantana
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In the lower 48, this species is normally
harmless to the environment.
Most breeders are working on fruitless
types more for the reason of toxicity than
for weediness
However, if we lived in Hawaii, for
example, lantana could be a major issue
Breeding for Sterility
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Breeding for sterility, non-fruiting plant,
should be encouraged however when
flowers are the primary focus and the
resulting fruits are known to be
problematic.
Buddleia, for example, is currently being
worked with to develop sterile new
selections
Euonymus alata
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Has reseeded in Eastern forest
Remains essentially innocuous to our
environment here in Oklahoma
Ironwood (Parrotia persica)
Parrotia persica
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One of many exciting plants, from the
Middle East, that so far are not proving to
be invasive!
What About Our Native Plants?
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Eastern redcedar and others are beyond
the scope of today’s presentation.
However, some experts have issues with
this indigenous conifer and others.
Osage orange, hackberry and other
natives are now being watched largely
because setting fires has been
discouraged
Commonly Grown Oklahoma
Ornamentals That are Being
Scrutinized
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Sawtooth oak (Quercus acutisiima)
Russian olive (Elaeagnus angustifolia)
Autumn olive (Elaeagnus umbellata)
Privet (Ligustrum ssp.) – all of them!
Japanese barberry (Berberis thunbergii)
Japanese spirea (Spiraea japonica)
Chinese/Japanese wisteria (W. sinensis/floribunda)
Asian bittersweet (Celastrus orbiculatus)
Wintercreeper (Euonymus fortunei)_
Barberry a Problem in
New England
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Again, most fruiting non-natives in OK are
not problematic
More “Bad” Plants
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Periwinkle (Vinca minor)
Porcelain berry (Ampelopsis
brevipedunculata)
Japanese akebia (Akebia)
Ajuga (Ajuga reptans)
Saltcedar (Tamarix spp.)
Beach vitex (Vitex rotundifolia)
Many more as well as grasses and aquatics not
discussed today!
Possible Solutions
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Horticulturists must implement screening
processes before introducing new plants to the
market. This would demonstrate our concern
for the environment and in some cases squelch
a small no. of plants from ever becoming
widespread.
There may come a time when growers may
want to consider voluntary stop sales on a few
species. This has already transpired on the
West Coast. For example, some people are no
longer growing/selling Hedera helix (English
ivy).
Proposed Code of Ethics
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Do no harm to plant diversity and natural areas
Perform risk assessments for introductions
Remove invasives from plant collections
Control invasives in natural areas
Develop noninvasive and native alternative plant
material
Do not distribute plants and seeds that will be invasive
elsewhere
Educate the public
Become partners with conservation organizations
Drought/Heat/Cold
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Up until about two years ago, many of us
considered a number of our “staple”
landscape plants to be essentially bulletproof. However, given extreme drought,
few record-cold nights and also record
heat, some of these “bullet’proof” plants
did indeed succumb or were badly
damaged beyond aesthetically acceptable
levels. Many of the best survivors are
exotics!
Native vs. Exotic
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It is interesting to note that many of our
affected plants were/are native to the U.S.
and often including the State of
Oklahoma. Still, even with their native
origins we have all been reminded that all
living organisms have limits to what they
can survive!
One of the Few Unscathed
Species from 2011-2012
Vitex
(Vitex spp.)
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Genus as a whole taken for granted
White, pink, purple or blue flowers on long
spikes (for most spp.)
Marijuana-like foliage
Can grow to be a tree sometimes to 20’
Normally winters will take it back down
Vitex
Aralia elata ‘Variegata’
Variegated aralia
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Has been scorned by most because it is
coarse and considered too overwhelming
with it’s color patterns
Should be used more – see Lendonwood
Gardens or Sunshine Nursery for
specimens to view
Aralia spinosa
(Devil’s walkingstick/Hercules club)
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Native Aralia sometimes goes
unnoticed
Large shrub – often a small
tree to 20’+
Tropical looking leaves, white
creamy flowers, club-like
stems great for winter interest,
purple black fruits
Not a plant for the timid
I’ve grown weary of people
rejecting this genus solely
based on the presence of
thorns
Staghorn sumac (Rhus typhina)
Tiger Eyes sumac
Rhus typhina Tiger Eyes
Don’t Lose Sight of Sheer Size
of U.S.
Put Popular Articles Into
Perspective
Prioritizing Invasives
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Work could be done between ornamentals
industry and OkIPC to rank “biggest”
offenders in order to have a realistic
starting point.
Plants of Concern
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Ornamental pears (Pyrus spp.)
Privet (Ligustrum spp.)
Honeysuckle (Lonicera spp.)
Tamarix (Tamarix spp.)
Russian olive (Elaeagnus angustifolia)
Other
Species of Less Concern
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Paper mulberry (Broussenetia papyrifera)
Mimosa (Albizia julibrissin)
Common mullein (Verbascum spp.)
Siberian elm (Ulmus pumila)
Other
Planned “Talks” are Underway
for 2013
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I am hopeful that a representative of
OkIPC will come to an ONLA board
meeting yet this year to begin a dialogue.
Summary
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1) All of us desire the same end result in
leaving the land as good or better than
what we inherited. However, our
approaches to doing so will differ.
2) Many horticulturists will always desire
to grow non-native plant materials in a
responsible fashion of course.
Summary (continued)
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3) Plantspeople must continue to educate
themselves about alternative plant
materials, when warranted, that are safer
to the environment, i.e. male selections,
triploids, etc.
4) Solving or at least mitigating the
invasives dilemma will necessitate a
multidisciplinary approach meaning that
horticulturists, ecologists, botanists, etc.
come to the table and begin discussions
Summary (continued)
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5) “Talks” will occur yet this year between
OkIPC and the Oklahoma Nursery and
Landscape Association (ONLA). This is a
step in the right direction.
6) In the end, concessions and
compromise will be required from all
parties involved.
Thanks for Your Attention
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Questions?