Introduction
During the summer of 2006 a survey was undertaken
in the Tay Salmon Fishery District to identify
the distribution and abundance of three invasive
plants, namely Japanese knotweed, giant hogweed
and Himalayan balsam, with the purpose of guiding
an eradication project of these species in the
coming years.
The work was conducted by staff of the Tay District
Salmon Fisheries Board as a contract for the
Tay Foundation. Most of the fieldwork was performed
by Chris Brodie, a student from the University
of Stirling during the summer vacation.
The above mentioned plants are aggressive and
spread rapidly, crowding out native plant species
and have a general detrimental effect on biodiversity
and river ecology. A survey of selected sites
conducted by Scottish Natural Heritage in 2003
showed that Himalayan balsam especially had considerably
expanded its distribution over the previous decade.
Therefore, before these species become even more
dominant than they currently are management is
urgently required. For a control programme to
be successful however it must be conducted in
a systematic and coordinated manner, starting
from the top of the catchment and working downstream,
otherwise these plants will merely re-colonise.
Before any treatment will take place it is therefore
crucial to identify all areas where these plants
are found in the Tay catchment, and to determine
their upstream boundary.
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Methods
The survey commenced in July, by which time Himalayan
balsam was in flower and easily identifiable. The
approach taken was to commence surveying the lower
reaches of major tributaries where some of these
plants were known to occur and then to proceed
upstream following the plants until upstream limits
were detected. Where possible river banks were
walked, but because of the large extent of the
Tay catchment and the difficulties of access in
some areas, surveys were generally done from road
bridge to road bridge on many of the smaller streams.
The survey was only conducted in the Tay upstream
of Perth Harbour, in the Earn from Bridge of Earn
and the Eden from the tidal limit at Guardbridge.
The Firth of Tay was not surveyed, although it
is known that invasive species are common in some
localities.
At each site the presence or absence of the
three species was noted and an approximate estimate
of the area of plant stands was made. Grid references
of each site were obtained by a hand-held GPS
and the data were then inserted into a Geographical
Information System (G.I.S.) program (Arcview).
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Results
The 2006 survey showed that these three invasive
plant species have a widespread distribution
and there are now in the region of 200,000m2 of
these
plants in the Tay District, upstream of Perth
harbour (Figure 1).
Himalayan Balsam is by far the most widespread
plant and it is estimated there is approximately
175.000 m2 of Himalayan Balsam throughout the
Tay District (Figure 2). Himalayan balsam plants
were practically ubiquitous along the main stem
of the Tay from Perth up to the junction of the
River Lyon just upstream of Aberfeldy, although
their incidence was sparser for several miles
below Aberfeldy. It is also profuse on the Earn
almost up to Loch Earn, the lower Isla and the
Eden. As yet, balsam is absent from the highland
tributaries. While it is widely found on the
larger lowland tributaries it has still to colonise
many of the smaller lowland tributaries. For
example, it occurs on very few of the tributaries
of the Isla, but judging by the situation on
the lower Earn, it is only a matter of time before
it spreads into every ditch in Strathmore.
Japanese Knotweed has only really formed extensive
stands on the Earn and in the Tay downstream
of Perth Bridge and in Perth Harbour (Figure
3). Elsewhere it largely occurs in small
pockets. The most frequent of these were in the
Loch Tay
area. A number of the records shown merely consist
of individual plants. In total there is approximately
25,000m2 of Japanese Knotweed within the Tay
District.
Giant Hogweed is the least common invasive plant
species within the Tay District (approximately
500m2) and was only found in the lower reaches
of the District (Figure 4). Of any site, it was
most abundant at Perth Harbour, a notable hotspot
for all three species.
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Factors helping spread
During the course of the survey a number of observations
were made which may help explain the distribution.
Knotweed and, for example, often tended be associated
with properties, indicating that the sporadic
distribution still reflects it’s introduction
as a garden plant. Its introduction to the Tummel,
for example, has undoubtedly occurred as a garden
escapee in Pitlochry, where it is present along
a stream in the middle of the town, backed on
to by gardens. Similarly two isolated pockets
of giant hogweed were traced to gardens at Farleyer
(near Aberfeldy) and Meikleour.
In most instances knotweed did not show extensive
signs of spread, but on parts of the Earn its
spread has been rapid and extensive. At Kinkell,
for example, the ghillie reports that four years
ago there was none and now there are stands up
to 15 m wide extending for hundreds of metres.
Similarly, on the Bervie Water, near Stonehaven,
it is noticeable from the A90 road that thick
stands of knotweed now extend for several miles.
We speculate that the rapid spread on the Earn
has been caused inadvertently through inappropriate
attempts at management. For example, evidence
of strimming of knotweed was apparent at the
village of St Fillans at the outlet of Loch Earn.
Perhaps floating stems then root further downstream.
This is a well known method of transmitting knotweed
and failing, slashing, strimming or mowing is
not recommended.
Balsam, however, spreads rapidly as the seeds
disperse very readily. It was noticeable that
balsam was mostly present in areas where river
banks were either fenced or usually where arable
crops grew up to the water side. Where river
banks were grazed balsam was scarce, although
there were other issues relating to bankside
ecology at some such sites. Balsam is a very
succulent plant and is clearly eaten readily
by livestock. It has very shallow roots and is
very easily uprooted.
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Proposed management
To prevent the further spread of these plants
and eventually eliminate them the following course
of action is recommended.
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A campaign of public awareness is required.
Those who are actively involved in riparian
management – fishery
owners, anglers, bankside householders – must
be targeted and made aware of the dangers.
They must receive appropriate management
information. For example, those who currently
cut knotweed
must
be made aware this is only exacerbating the
problem.
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In collaboration with land owners, fishery
owners, Perth and Kinross Council, Scottish
Natural Heritage, SEPA and other interested
bodies, a plan
must be formulated to tackle all the knotweed
in the district and to commence to manage balsam
from
its upper limits on the various tributaries.
To this end, the Tayside Biodiversity Partnership’s
Water and Wetland’s group have been
awaiting the production of this report.
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Meetings with the above parties are being
arranged. But in the meantime, a proposal has
now been submitted
to the Esmee Fairbairn Foundation, through
Rivers and Fisheries Trusts of Scotland (RAFTS)
for a
grant to eradicate these invasives from the
Tay District. If successful, the eradication
work will
start in early 2008, continuing through 2010.
The aim is to treat all areas containing Japanese
knotweed
and Giant hogweed during this three-year period.
Due to the extensive labour and costs associated
with Himalayan Balsam eradication it will not
be feasible to clear large areas and control
will
therefore be targeted to avoid colonisation
to any new areas.
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