|Mickey mouse plant (Ochna serrulata)|
|Digressing from the series on introduced vines I feel the need to address Ochna serrulata (Ochna or Mickey mouse plant), which has to be one of the most serious woody weeds in the Brisbane area. It has been a major problem for bush regeneration in Sydney for some time and the National Trust of NSW has put a lot of effort into it. Naturally from Southern Africa it is extremely hardy and somehow manages to grow even during drought. Where it has become established, it forms dense thickets in which nothing else seems able to survive, and the ground becomes choked with seedlings. Natural regeneration is completely blocked.|
Ochna is a strong-rooted shrub growing up to 3-4 m (usually less) with dark green leaves which have finely serrated edges. Its bright yellow petals fall off after flowering leaving red sepals (some call them bracts). Currently mature specimens of this shrub should be quite recognisable by these bright red displays containing 4-5 glossy black berries, which are very attractive to birds and hence readily dispersed. Regrettably, although listed by the BCC as an environmental weed, Ochna is still cultivated in Brisbane, frequently to form hedges.
The consensus of everyone I've spoken to about Ochna is that this is a very difficult weed to eliminate, but there is no universally accepted way to do it, which is why the following discussion is so lengthy.
i. Manual removal is ineffective once a plant reaches only 10cm because it breaks off in your hand at ground level and then re-shoots vigorously (when the soil is thoroughly soggy I've managed to pull out plants, roots and all, up to about 30cm tall). Chipping will only encourage endless regrowth and digging it out with a mattock is impracticable because its deep taproot will send up new shoots through 30cm of soil.
ii. Cut stump. The NSW National Trust, Rod Woods of the BCC and Jack Green from Greening Australia favour the cut stump method, using glyphosate. Plants are cut as close to the ground as possible below the lowest branch/shoot, and then herbicide applied straight away. Apart from timing and speed (see below), one possible cause of failure with the cut stump method arises from using greased pruning saws which leave enough of an impervious film behind to prevent the herbicide from being absorbed properly. The best way around this is to degrease the saw (with degreaser or kerosene) and then remove the residue this leaves with methylated spirit. Otherwise a surfactant such as Li700 can be used to quickly wipe the stump before painting. Another problem with sawing is that sawdust remnants can stick to the stump, getting in the way of direct herbicide application. Where stems are thin enough loppers or secateurs are preferable. Rod and others recommend cutting at 45? to expose as much of the cambium as possible; however in their Bush Regenerator's Handbook the National Trust advise a horizontal cut to minimise run off. Spraying it on is no good - drops must be absorbed into the wood (a paintbrush helps). Tim Low in Dinkum Gardening suggests scraping 2cm down either side of the remaining stem and painting that as well.
iii. Stem scraping. Another method people use when quick removal of the plants is not required is stem scraping. Here an old knife is used to scrape down the side of the stem in order to uncover, but not penetrate, the sapwood. Herbicide (usually glyphosate) is then painted on immediately. The reasoning here is that the greater length of phloem exposed allows for more herbicide to find its way into the vascular system, but it is more difficult to abrade to the correct depth. There is also disagreement over whether the whole circumference should be uncovered (maximum tissue exposure) or just up to half (so as the plant can continue to grow and mobilise nutrients, assisting in translocation).
iv. Basal bark spraying. In this method the herbicide is mixed with diesel or kerosene and sprayed or splattered right around the lower circumference of the stem from ground level up to 30cm or more. This is undoubtedly faster where large numbers of medium-sized plants are involved. Glyphosate is considered unsuitable: recommended herbicides/concentrations are 10g triclopyr/litre of diesel or 25-50g fluroxypyr/litre of diesel.
The key points to bear in mind when using herbicides are:
(a) Timing. Systemic herbicides work by entering the sap and being translocated throughout the plant (especially to the roots) via its internal system of conduction. The highest rate of movement of water and nutrients inside the plant occurs when it is growing actively: for Ochna this is usually early spring (before fruiting) though spurts can occur after rain. Look for vigorous new light green leaf growth. Herbicide use in winter is almost sure to fail.
(b) Rapid application. Some plants seem able to seal off lacerated vascular tissue in a matter of a few seconds. Apparently speed of application was so important to the National Trust that they would frequently work in pairs so that an assistant could apply the herbicide as soon as the surface was exposed.
Regardless of how diligent you are, however, many people I've spoken to concede that follow-up work on the same plants is often necessary, which is a real pain when you have hundreds of the buggers! What's more, when Ochna comes back, it sports so many new shoots it's hard to know where to begin. Another unresolved issue is the dilution of glyphosate to use. Rod, Jack and others use neat glyphosate (on the assumption that this is more potent). However, Gordon Limburg (National Trust) found 1:3 glyphosate/water to be more effective in both cut stump and stem scrape treatments of two groups of 60 plants (perhaps because the diluted solution is more readily absorbed and transported?).
The herbicide of choice is also open to debate. Others, such as Trevor Armstrong from DNR, prefer using a "stronger" herbicide for Ochna such as triclopyr [eg. Garlon 600; Blackberry and Tree Killer] or fluroxypyr [eg. Starane 200]. I have to admit I felt like giving up on glyphosate after dozens of my carefully treated plants came back with a vengeance after being knocked back initially. The significant advantage of glyphosate is of course its very low toxicity and virtual lack of active residue in both the soil and dead plant material. It is also cheaper and more widely available. Triclopyr carries a higher risk to both personal health and the environment. It is mixed routinely with diesel or kerosene, which makes contact with the skin more of a problem. Triclopyr, while better than "hard core" chemicals like picloram [tordon], does have some residual properties and is probably best avoided around new plantings of natives or close to waterways. Some claim it has no effect on the growth and/or survivorship of plants surrounding the target, others suspect it does.
George Diatloff from the DNR (previously Lands) has recently developed a new mixture at the Alan Fletcher Research Station. He is trialing a promising, novel combination of glyphosate, methylated spirits and the surfactant "Pulse," which may be suitable for basal bark application as well. This concoction allegedly also stores very well, a major advantage for big jobs. More field experiments are presently underway so this formulation can be registered. Any developments will be covered in future newsletters. Given this array of opinions, perhaps you'll have to experiment yourselves. Good luck, and let us know of your own experience because this weed is tough one!