A successful pathway to eradicating Sweet Briars
Measurements and analysis by Warren Bond
A systematic approach to monitoring and eradicating briars (Sweet Briar,
Rosa rubignosa) across the Pinnacle Nature Reserve between 2012 and 2023 has resulted in a consistent
decline from an average of 44 briars/ha in the 2012-13 season to an average of 3 briars/ha over the 2019-20 to 2022-23 seasons.
Along the way we used a number of indicators to assess our progress.
These included: the decline in the number
of briars requiring treatment each season, the decline in the density of briars overall
as well as at specific hots pots, and the decline in the amount
of time and chemical spent controlling briars.
Our Approach to Briar Reduction
Since 2011, briar control on the Reserve and neighbouring paddocks
(see reserve map) has been carried out primarily by spraying (with the recommended concentration of metasulfuron-methyl). [Some selected cut and dab (with concentrated glyphosate) was trialled in early years but discontinued as being more time consuming and less effective.] Our approach
to removing briars, as opposed to the "sweep search" method for spraying more prevalent weeds, has
been to collect waypoints of briars and then follow up with spraying those targets. All weeders active on
the Reserve collect briar waypoints which are accumulated for several months and then targeted, usually in mid-summer and again in mid-autumn. In later years, as an increasing number of the briars to target were smaller, they were also tagged with flagging tape to make them quicker to locate again. In the follow-up
process, other briars are also generally found and sprayed. After the introduction of briar
density maps (see below) and the availability of GPS devices on which such maps
can be installed (eg the Garmin etrex 20 and 30 series), sweeping of patches mapped as having previously had a high density of briars is also carried out at the
time of spraying (regardless of whether any briars were waypointed in those areas since the last spraying).
This approach relies in large part on all areas of the Reserve being visited by weeders at least twice a year but does greatly improve the efficiency and effectiveness of spraying briars. Furthermore, because each briar
treated is waypointed, maintaining a database of the collected waypoints provides a good means of determining briar
distribution and densities and the changes in these from year to year. Waypoints for briars treated in
other ways (for example by pulling small ones or by cutting and dabbing larger ones) are also added to
This methodology was gradually developed in the 2011-12 season, but not
fully implemented; waypointing was not inclusive, searching not as careful and not all briars were waypointed
when sprayed. Treatment was applied consistently on the Reserve from 2012-13 onwards. For this reason, all
the indicators of briar control show an increase between 2011-12 and 2012-13, and the 2011-12 values are
not included in most of the discussions of indicators below. Briar treatment was started later in the
neighbouring paddocks, and treatment has not been as thorough in all years as on the Reserve.
Note that the average size of briars treated each season (based on
qualitative observation) declined considerably over the years, from something in excess of thigh
height in 2011-12 to less than calf height from 2016-17. A large percentage of briars sprayed in recent years
have been less than 200 mm high.
Total numbers of briars across the Reserve have declined between 2012 and 2022. The chart to the right shows
this decline graphically. As explained above, numbers
increased between 2011-12 and 2012-13 because the methodology was not been fully implemented until 2012-13;
these numbers were therefore excluded from some analyses. The numbers in 2015-16, 2021-22 and 2022-23 defied the downward trend,
most likely because they followed seasons with rainfall well above average and therefore very favourable conditions for germination and growth. [More Blackberry and other
woody weed seedlings were observed in these seasons than in the immediately prior seasons as well.]
Between 2011 and 2023 more than 25,000 briars have been removed from the Reserve, more than 43% of them in the first 2 seasons.
Since 2012 the average density of briars removed across the Reserve has
decreased from 44 per hectare to an average over the past 4 seasons of 3 per hectare.
The change in numbers for each management unit, or paddock, are shown below.
[see the paddock map for paddock locations]
Briar densities (number per hectare) for each paddock can be viewed in a
As expected from the totals, the numbers in most paddocks show a downward
trend in numbers. It can be seen that not all paddocks experienced an increase in numbers in 2015-16. Those
that did have large increases (Dam, Central, Southern, Weetangera and Kama Link) contained most of the densest areas of briars (see
below) and therefore the largest seed bank. Kama Link is also adjacent to areas that have received minimal briar control (notably along the William Hovell Drive verge) and so a large source of seeds. This pattern was repeated in 2021-22 and/or 2022-23.
density version of the above chart shows that by 2018-19 the density in all but one paddock (Dam) was 7 briars/ha or less and by 2021-22 the density in all paddocks was 6 briars/ha or less.
It is also notable that after 4 seasons of values between 13 and 21, the density of
briars in Kama Link paddock declined dramatically in 2016-17, hovering at an average of 3 briars/ha for 5 seasons before doubling to 6 briars/ha in 2021-22. In Hawker paddock a large decline occurred in 2012-13 and took the density to below 5 briars/ha where it has remained since.
Briar density distribution maps
As mentioned above, all briars are given a geographic location (geo-referenced);
GIS software is used to calculate briar density maps from the waypoints. The density analysis calculates the number
of briars per 100 square meter circular area around each briar waypoint and then combines and colour-shades the
densities as shown in the images below. The base area of 100 square meters was chosen because it approximates the
notional worst-case GPS error of ±5 meters (a 100 square meter circle has a radius of 5.64 m).
A collection of briar density maps for different individual seasons and sets of
accumulated seasons can be found on the briar density page. Just two are presented
here to indicate progress in controlling briars between 2012-13 and 2022-23.
Click each image above to view a larger version;
both can be opened at once and positioned on the screen to allow comparison.
These images show that the overall density has declined considerably,
in keeping with the decline in total numbers. In 2012-13 the maximum spot density observed in the Reserve was 25
per 100 square meters, whereas in 2020-21 it was less than 1.6 per 100 square meters. This trend was broken in 2021-22 by a new patch with a maximum density of 19 briars per 100 square meters, as described below. In 2022-23 the maximum spot density was 9 briars per 100 square meters Nevertheless, average density across
the whole Reserve has declined from 44 briars/ha in 2012-13 to an average over the 2019-20 to 2022-23 seasons of 3 briars/ha.
The images on the briar density page also show that the basic distribution pattern remained more
or less the same up to the 2015-16 season but with the areas of high density in 2012-13 becoming much less dense and contracting in size.
Many of the dense areas in 2012-13 had disappeared by 2020-21 and since then the areas of highest density each season are largely unrelated to those in 2012-13. These observations are also demonstrated below through examples of the change over
time in briar numbers at several density hotspots.
Although briar control has also been occurring to some extent in the neighbouring paddocks,
briar density maps are presented here only for the Reserve where, because it has taken priority, coverage has been more
complete. A density map for the whole area may be found at the bottom of the
briar density page. This allows a comparison of briar densities in Bottom Pinnacle and North Kama with those on
the Reserve. It may be seen from this map that briar densities are much less in the neighbouring paddocks.
One factor affecting this is suspected to be cattle grazing. The density of
briars increases as the time without grazing increases (from North Kama to Bottom Pinnacle to the Reserve). The two
northern paddocks of North Kama have a lower density that the southern North Kama paddock (now the extension), which is also ungrazed but
has been for less time than Bottom Pinnacle. This observation is consistent with cattle indiscriminately removing small
briar seedlings in their grazing.
The density differences could also be associated with the tree density, which
varies in the same way. In many instances dense briar patches are associated with trees. The
densest patch that has existed on the Reserve, however, has no trees whatsover and judging from historical aerial photos hasn't had trees for at least 80 years.
Analysis of briar numbers in specific high density areas
Inspection of the cumulative briar
density map ( 2011 - 2023) and comparison with aerial photos shows that high density areas can occur in
two very different kinds of locations: around specific trees and in areas devoid of trees. This section shows
two examples of each (patches 1 to 4) where high briar densities were found in the first two seasons of monitoring showing the change in briar numbers since then. Patch 5 is a high density patch that appeared in 2021-22, illustrating the need to remain vigilant across the whole Reserve even if overall numbers are decreasing.
1. A tree on the edge of the Forest paddock
This tree is located on the walking track
that defines the bottom of the Forest paddock, about 50 m south of the Macrorhyncha Track.
The limits of this 270 square meter high density patch are shown in white in the accompanying
photomap. The cumulative density at the centre of the patch is 46 briars per 100 square meters.
Small briars were first sprayed under this tree in 2012. After a sharp decline in 2013-14 followed
by a large increase the following year, numbers have been steadily declining and reached zero in
2019-20. It is hypothesised that the increase in 2014-15 may have resulted from the favourable summer
rainfall which encouraged greater germination and growth in competition with the surrounding trees.
The subsequent decline in 2015-16, also a very favourable year for germination of briars) suggests a
depletion of the soil seed bank, confirmed by the lower numbers in subsequent seasons. A total of 172 briars have been removed from this patch.
The source of these briars is expected to be from bird
droppings. The numbers have remained negligible for the last 5 years hopefully suggesting that the birds' sources of briar seeds have been eliminated within a sufficient distance from the tree.
2. A tree at the track crossroads west of the dam
This tree is located on the Valley Track
about 150 m due west of the dam. The limits of this 680 square meter high density patch
are shown in white in the accompanying photomap. The cumulative density at the centre of
the patch is 18 briars per 100 square meters. Briars were first sprayed under this tree in
2011. Their numbers have been declining since then and have been effectively zero since 2016-17. A total of 107 briars have been removed from this patch.
It is again expected that the source of these
briars is from bird droppings. Given that the numbers have now been zero since 2016-17, whether
or not further briars are found in the future will be a guide to how effectively the source of hips
has been eliminated on the Reserve.
3. The bare area downslope of the rabbit-infested rocky
outcrop in the Central paddock
This 7800 square meter area is
located northwest of the above location. It runs from the trees just above the grass
experiment enclosure near the Valley Track upslope to the rocky knoll which after many years of treatment still harbours
the major infestation of rabbits on the Reserve. The peak cumulative density, at the point shown by the red
dot just below the knoll, is 43 briars per 100 square meters. Briars were controlled by
grubbing before 2011; since then it has been by spraying and a total of 864 briars have been treated.
Their numbers have been declining since thorough spraying began in 2012.
It is unknown what the original source of
these briars was, but it is expected that in recent years the source has been the seed bank
created by a heavy infestation among shrubs at the top of the knoll, and that seeds have been moved
downslope by water and dispersed by rabbits and birds.
Given that the local source of seeds has been eliminated,
the steady decline in numbers to near zero in 2020-21 should be permanent and is consistent with the
estimate of seed viability of about 5 years.
Note that the increase between 2011-12 and 2012-13 is
attributed to the bedding down of the targeting methodology as discussed above.
4. The south-west corner of Weetangera paddock
This 1000 square meter patch is located in
the southwest corner of the Weetangera paddock adjacent to the ACTEW Pump Track. The cumulative
density at the centre of the patch is 55 briars per 100 square meters, making it the densest patch
on the Reserve. After peaking in 2012-13 (as a result of intensive searching for them) the numbers
have declined considerably, with the increase in 2014-15 and 2015-16 attributed to the favourable
summer rainfall compared with the previous year. A total of 356 briars have been removed from this patch.
The source of these briars is unknown, though it is
expected that there was in the past a large infestation in the area that built up a considerable
seed bank. As for the previous patch, the absence of a local seed source has seen a decline in the
numbers to zero in the last 2 seasons. There is hope that they have now been
eliminated from this area that was once the densest briar patch on the Reserve.
5. Trees west of the rabbit-infested rocky
outcrop in the Central paddock
This 300 square meter patch is located in
the southwest corner of the Central paddock west of patch 3. Until 2020-21, only one briar had been found in that area, but in 2021-22 it was the densest patch in the Reserve with a
density at its centre of 19 briars per 100 square meters. In fact this is the densest patch found on the Reserve for many years, as can be seen by looking at the annual briar density maps. This patch accounted for 30% of the increase in briar numbers on the Reserve between 2020-21 and 2021-22.
It was a surprise to see such a dense patch after many years and a reminder that birds can bring such seeds in at any time. Continual vigilance is required to eliminate such patches while the briars are still small. After spraying the briars (all less than about 30 cm tall) in that patch in April 2022 none were found in 2023, although the densest patch on the Reserve in 2022-23 (8.8 briars per 100 square meters) was only a short distance away.
For the first few years of control, other indicators that provided evidence of progress included the effort and volume of chemical sprayed required for control each season and the quantity of briar hips removed each season. These have now been discontinued because as briar numbers become fewer both effort and volume of chemical required become less reliable indicators, as discussed below, and because there are now not enough hips removed each season to make it worthwhile monitoring.
Time taken and volume of spray used
Two other useful indicators of the long-term trend in weed control are the
time spent and the amount of chemical spray mixture used on particular weeds in particular areas. These are shown
below for Briars on the Reserve. Both show a significant decline from 2012 to
2021, consistent with the data showing the decline in numbers. Note that volume sprayed correlates more closely with
briar numbers than effort. There was a 48-fold decrease in volume used between 2012-13 and 2020-21 compared with a 40-fold decrease in briar numbers and only a 7-fold decrease in time spent. The poor correlation with effort arises because as the density of briars decreases from season to season more time is spent walking
between briars, so the effort reaches a point where it doesn't decrease very much with a decrease in numbers.
The volume of spray used was a good surrogate for briar numbers in assessing the extent of briar reduction. It should be noted, however, that all spraying was done by the same person applying a similar amount of spray commensurate with briar size; spraying by multiple sprayers, particularly different sprayers in different seasons, may not provide such a reliable correlation. By 2020-21 the volume of chemical used was so small as to be difficult to measure reliably.
Annual effort (left) and chemical volume (right) expended on spraying
Briars on the Reserve.
While there is no quantitative data to support this, the number of
briars reaching the mature hip stage on the Reserve has drastically reduced over the period that treatment has
been intense. In the first two seasons of spraying briars, several kilograms of hips were collected each season,
whereas since 2014-15 there have been no more than several handfuls collected. Thus, the briar reduction program
is probably having a greater effect on the ability of briars to increase the seed bank and reproduce than on
overall numbers. For a weed with a quoted seed longevity in the soil of ~ 5 years, this was a useful early sign
that a long term impact was likely being made.