Purple Loosestrife
Lythrum
salicaria
The purple loostrife is
a controversial invasive plant. For hunters it makes trotting through wetlands
havoc, but does it really pose a biological threat to the ecosystem here in
Newfoundland, and indeed the rest of Canada and around the world?
It is an herbaceous perennial plant, generally
growing to 1.2-3m in height. It has a woody stem with whorled leaves, square in
cross-section and is reddish-purple. Lance-shaped leaves are common but
heart-shaped or rounded leaves can be present at the base of the stem. Its
flowers are magenta-colored and have 5-7 petals. It reproduces by seed and
through vegetative growth. It can produce more than two million seeds annually
per plant.
Full size adult Purple
Loostrife plant
Purple loosestrife prefers moist soil and is
therefore capable of invading a wide variety of wetland habitats such as
marshes, stream and river beds, lakes, ditches and reservoirs. Once
established, it can tolerate a wide variety of conditions and has even moved
into some drier areas such as pastures and cropland. This invasive plant easily colonizes disturbed habitats with
exposed soil which provide ideal conditions for seed establishment (Project
UFO).
It was originally introduced to North America along
the eastern seaboard in the 1800's from Europe as a decorative plant. Purple
loosestrife now ranges across Canada, from Newfoundland to British
Columbia, and is located in much of the United States.
Purple loosestrife has also been intentionally
introduced for medicinal purposes, including treatment for diarrhea and
dysentery. Dried purple loosestrife leaves have also been used to treat
ulcers, sores and even heal wounds.
Another method this invasive species has been
spread to new areas is by the action of beekeepers. This plant is sometimes
used for its quick establishment and high nectar yields.
Accidental introductions have also occurred due to
contaminated ballast water transport.
Purple Loostrife flower
(Photo credit: John Maunder)
The plants are also very aggressive and quickly
crowd and out-compete many native species. Purple loosestrife is of special
concern in wetland, where it can push out the local vegetation. These large stands
of purple loosestrife may completely alter the physical habitat of the
wetlands, and their ability to support a high diversity of insects, birds and
other animals.
Seeds of this invasive species can remain viable
within soil for several years and in water for up to twenty months. Their small
seed size also contributes to its easy dispersal and success as an invader. These
seeds are very small, less than 1mm round and survival rates as high as 60-70%. Even
though this invasive plant species is of concern, it can still be purchased
locally as an ornamental plant and if strains of non-sterile plants are
available, they can quickly escape from local gardens.
Small populations of purple loosestrife can be controlled by physical cutting and uprooting, and chemical
measures such as herbicides. For larger populations it is often necessary to
have biological control such as the introduction of predatory insects.
Field of Purple
Loostrife. (Photo Credit of the Department of Ecology, State of Washington)
In
comparison to the mainland, Mann (2012) stated that “Although still rather
uncommon provincially, more sightings are being noticed across the Island....
The species has been known from the Corner Brook area since at least the
1940’s, but still shows no indication of being an overly aggressive invasive as
it has in the more moderate climates on the mainland”. For much of the 20th
the negative and invasive threat of purple loostrife has been considerably
promoted by the hunting and trapping lobbies “who noted that concentrated
stands of loosestrife somewhat deceased desirability of the aquatic habitat for
waterfowl, muskrats, beaver, and other wildlife of direct economic interest to
humans”. From more recent literature it appears that in some instances Purple
Loosestrife actually increases biodiversity of vegetation, birds, insects, and
no doubt other groups not so easily observed and measured. “Recent research
indicates that as Purple Loosestrife moves northward in Ontario it rapidly
loses it vigour and fecundity. The same can probably be expected for
Newfoundland where vigorous southern CFA’s often meet their ecological limits
in the climate and hardy adapted native vegetation”.
Although it is
seen as an invasive species for most, the debate still goes on, as seen in a
very interesting article by Henry Mann (2012). In this article he states “it is
difficult to find a site promoting any virtues of this species. Yet its beauty
as a wildflower and a garden perennial is unquestionable, and if we dig deeply
into the literature we find that it is a superb honey plant producing copious
nectar and premium honey for the bee industry....When its detesters claim that
it has no redeeming qualities, only destructive ones in wetland habitats, do
they not consider the insect and small bird community of any value in the
ecosystem?”
This
again makes us rethink on what our criteria must be to list something as a
truly ‘invasive species’ or not, and whether it should also depend on the
locality and fecundity of where it can be found.
See our next blog on the Canadian Thistle.
References:
I think you should have a look at the research paper by Hagar and McCoy. Hager, H.A. and K.D. McCoy, 1998. The implications of accepting untested hypotheses: A review of the effects of Purple loosestrife (Lythrum salicaria) in North America. Biodiversity and Conservation, 7(8): 1069-1079.
ReplyDeleteIn it they discuss several experiments that test whether loosestrife is as big an ecological threat as people think. Part of the hysteria is because it is a showy flower but more recent research suggests it only dominates in disturbed areas ( along roads, ditches) and that in undisturbed wetlands it may establish, but isn't able to completely take over. You references are not very scientific and come more from advocacy groups, I suggest you look at the scientific research to get a bit more diversity of views.
Thank you Yolanda. The article was very helpful.
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