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Indicators of a successful pathway to eradicating Sweet Briars

Measurements and analysis by Warren Bond

Summary and Conclusions

A summary of indicators are presented on this web page to show that fotpin is successfully reducing the number of briars (Sweet Briar, Rosa rubignosa) on the Pinnacle Nature Reserve.

Indicators include: the decline in the number of briars requiring treatment each season, the decline in the density of briars overall as well as at specific hotspots, and the decline in the amount of time and chemical spent controlling briars.


Since 2011, briar control on the Reserve and neighbouring paddocks has been carried out largely by spraying, with some selected cut and daub from time to time. Our approach to briars, as opposed to the "sweep search and destroy" spraying of 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. In the process of targeting other briars are also identified and sprayed. Since the refinement of the briar density maps (see below) and the implementation of GPS devices on which such maps can be installed (eg the Garmin etrex 20 and 30), sweeping of denser patches is also carried out at the time of spraying. This approach greatly improved the efficiency of spraying. Furthermore, because each briar treated is waypointed, maintaining 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 daubing larger ones) are also added to the collection.

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) has declined considerably over the five seasons, 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.

Briar Numbers

briar declineTotal numbers of briars across the Reserve have declined between 2012 and 2019. The chart to the right shows this decline graphically, together with a best-fit trend line fitted to the data. As explained above, numbers increased between 2011-12 and 2012-13 because the methodology had not been fully implemented in 2011-12; these numbers were therefore excluded from the analyses. The numbers also increased between 2014-15 and 2015-16, possibly because the conditions for germination and growth were very favourable. [More Blackberry and other woody weed seedlings were observed in 2015-16 than in previous seasons as well.]

Between 2012 and 2019 the average briar density across the Reserve has decreased from 44 to 5 per hectare. The rate of decline has slowed in the last few years to a steady 370 per year, considerably less than the decline after the first full season of over 3000 in a year.

The change in numbers for each management unit, or paddock, are shown below. Briar densities (number per hectare) for each paddock can be viewed in a pop-up chart.        [see the paddock map for paddock locations]

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 and Weetangera) contain the densest patches of briars (see below) and therefore the largest seed bank.

briar decline

The density chart shows that by 2018-19 the density in all but three paddocks was 5 briars/ha or less. The exceptions were Dam and Central paddocks (which as mentioned above had the densest patches of briars and therefore the largest seed banks) and the Extension (which is close to a rich source of uncontrolled briars along William Hovell Drive). It is also notable that after 4 seasons of similarly large values, the density of briars in the Extension declined dramatically and has remained at a relatively constant, but well above 5 briars/ha, since. A similar observation can be made about Hawker paddock where the decline took only one season and took the density to below 5 briars/ha where it has remained since.

Briar densities

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 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 2019-20.



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 density observed in the Reserve was 25 per 100 square meters, whereas in 2019-20 it was less than 1.8 per 100 square meters. Average density across the whole Reserve has declined from 27.4 briars/ha to 3.0 briars/ha.

The images also show that the basic distribution pattern has remained more or less the same but that areas of high density in 2012-13 have become much less dense and contracted by 2019-20. This latter observation is also demonstrated below through examples of the change over time in briar numbers at several hotspots.

Although briar control has also been occurring in the neighbouring paddocks, briar density maps are presented here only for the Reserve where, because it takes 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, which is also ungrazed but 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 on the Reserve, however, has no trees whatsover.

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 decline, although with slightly different patterns, from 2012 to 2019, consistent with the data above showing that numbers have declined. Note that the volume sprayed tracks briar numbers closer than the effort because as the density of briars decreases 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.


Annual effort (left) and chemical volume (right) expended on spraying Briars on the Reserve. The Extension is excluded because spraying of briars was started later there.


Hip collection

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 reproduce and increase the seed bank than on overall numbers. For a weed with a quoted seed longevity in the soil of ~ 5 years, this is an important sign that a long term impact is likely being made.

Analysis of briar numbers in specific high density areas

Inspection of the cumulative briar density map ( 2011 - 2019) and comparison with an aerial photo 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 and the change in briar numbers across the 4 seasons for which data has been collected.

1. A tree on the edge of the Forest paddock

forest briar tree

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.

The source of these briars is expected to be from bird droppings. Whether or not the numbers remain negligible in the next few years will be determined by whether the birds' sources of briar seeds have been eliminated within a sufficient distance from the tree.


Season Number
2012-13 64
2013-14 17
2014-15 48
2015-16 20
2016-17 5
2017-18 9
2018-19 3
2019-20 0


2. A tree at the track crossroads west of the dam

dam briar tree

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.

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.


Season Number
2011-12 64
2012-13 20
2013-14 6
2014-15 11
2015-16 4
2016-17 0
2017-18 1
2018-19 0
2019-20 0


3. The bare area downslope of the rabbit-infested rocky outcrop in the Central paddock

bunny dell


Season Number
2011-12 197
2012-13 313
2013-14 166
2014-15 78
2015-16 58
2016-17 12
2017-18 13
2018-19 11
2019-20 2

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 has harboured a major infestation of rabbits. 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 numbers have been tracked. 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 that was 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 2019-20 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 in the background section above.


4. The south-west corner of Weetangera paddock

weetangera patch

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 hunting 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.

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 near zero (an average of 3 over the last 3 years). There is hope that they have been all but eliminated from this area that was once the densest briar patch on the Reserve.


Season Number
2011-12 81
2012-13 138
2013-14 16
2014-15 36
2015-16 45
2016-17 27
2017-18 1
2018-19 7
2019-20 2



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