Brushtail possum heaven
Article and photos by John Thompson, Gunns Plains, Tasmania.
On our conservation property at Gunns Plains we are revegetating ~3.0 ha of rank pasture to link existing stands of the threatened Eucalyptus viminalis wet forest community. Our property adjoins lush farmland and suffers from significant overpopulations of pademelon and brushtail possum.
In our first year of plantings in 2011 using conventional 60 cm high corflute plant guards the moment the E. viminalis poked their heads above the corflute guards they were eaten by the taller pademelons, or so we thought!
To combat this we upgraded to 105 cm high wire netting cylinders then went on holiday. A small number of cages had lids on just in case it wasn't the pademelons.
On our return every plant in a netting cylinder without a lid had been stripped of its leaves or snapped at the base. About one quarter never recovered.
Photo 1: Ten month old E. viminalis in wire netting cylinders with a lid.
Photo 2: Ten month old E. viminalis in wire netting cylinders without a lid.
Finding an effective solution
The success of our revegetation program depended on finding an effective, low-risk, scalable solution given that you can't have E. viminalis wet forest without E. viminalis.
There are two broad approaches - constructing a possum-excluding perimeter fence or designing a possum-excluding plant guard. We rejected the former approach as this would provide a single point of failure.
After building a number of prototypes we settled on an ‘inverted top hat' design using wire netting. It emulates the success of floppy fences in excluding possums, while also excluding pademelon browsing and resisting high-speed pademelon collisions.
The design was developed with the aid of a motion-activated night-vision camera to confirm its effectiveness in foiling the wiliest of brushtail possums. When the possum reaches the junction of the cylinder and the floppy lid it baulks at climbing upside down and eventually gives up.
Photo 3: Infra-red photos of brushtail possum climbing into the test cylinder without a lid and subsequently foiled by the floppy lid (Photo 4).
The design has proven 100% successful over an 18 month period as evidenced by the rude health of the 16 and 28 month old plantings in the photo below.
Photo 5: Healthy one and two year old E. viminalis in possum excluding plant guards.
How long are the guards needed?
At this stage we don't know how long the guards will be needed. Our initial plan was to remove the guards when the trees reached 2 to 2½ metres. As an experiment we removed the guard from a two year old 2 metre E. viminalis and within a few days a window shopping possum found the shop door open and helped itself. Since then that plant, shown in the photo below, has been continually browsed. We will repeat the experiment each year until we are satisfied that the tree's growth isn't retarded by possum browsing.
Photo 6: Two year old E. viminalis after guard removal compared with plant still in its guard (Photo 7).
The guard is substantially reusable with the galvanised netting and fencing wire having a 20+ year lifetime away from the coast. If the tree is too tall to lift the guard over the tree then the netting clips securing the lid to the cylinder and the lid itself will need to be cut. The cylinder can be removed by simply extracting the fencing wire knitting the cylinder together. For reuse the cylinder will need to be knitted back together and the lid reattached with new netting clips. New wooden stakes may also be needed.
Plant guard components
1.5 m of 105/4/1.4 galvanised wire netting for cylinder*
1.0 m of 105/4/1.4 galvanised wire netting for lid*
8 x netting clips
1.1 m of 2.5 mm galvanised fencing wire
2 x 1.2 m x 25 mm pointed garden stakes
* Helpful tip - use outer 40% of roll for lids and inner 60% for cylinders
Construction and installation
- Cylinder knitted together with removable fencing wire
- Cylinder fitted over stakes slightly under tension
- Lid attached to cylinder with 8 netting clips
Top cut into six sectors, folded in and tied to wall using the wire at the end of the sector.
Cost per guard is around $9.00 (based on mid-2013 materials purchase price).