Alligator weed (Alternanthera philoxeroides (Mart.) Griseb) forms dense infestations in aquatic environments and is the focus of intensive management programs in many jurisdictions within Australia, including Victoria. A critical component of weed biosecurity programs is surveillance to find the location and extent of the target weed so that control strategies can be implemented. Current approaches within Victoria rely heavily on ground surveys and community reporting. However, these methods do not provide a systematic approach to surveillance across landscapes, resulting in undiscovered infestations. The aim of this study was to detect alligator weed from aerial photography and demonstrate the potential use of remote sensing data to support existing ground surveys and monitoring programs. Two random forest algorithms were trained based on data from 2010 and 2016. Both classifiers had high levels of accuracy, with an overall pixel-based classification accuracy of 96.8% in 2010 and 98.2% in 2016. The trained classifiers were then applied to imagery acquired annually between 2010 and 2016. The classification outputs were combined with class probability and water proximity data to produce a weighted, normalised alligator weed likelihood data layer. These datasets were evaluated by assessing alligator weed patch detection rates, using manually delineated areas of weed for each year. The patch detection rates for each year ranged from 76.5% to 100%. The results also demonstrate the use of this approach for monitoring alligator weed infestations at a site over time. The key outcome of the study is an approach to support existing biosecurity monitoring and surveillance efforts at a landscape scale and at known infested localised sites.

Lake drawdown, or water withdrawal, is often used to control invasive submerged macrophytes; however, regrowth of the target species often occurs rapidly. A mechanism proposed to explain such unsatisfactory results is that as water recedes and submerged aquatic weeds become exposed to the air, mounds of stranded vegetation form on the dewatered lakebed. These mounds may insulate underlying weeds, creating an environment protected from desiccation and frost. This study tests this mechanism by comparing the viability of the submerged aquatic weed Egeria densa Planch. from within mounds to the viability on the surface of these mounds during a winter drawdown of Lake Mulwala, Australia. After 22 days of exposure, no stems on the surface of weed mounds were viable, but 22% of stems on the bottom of mounds and 76% of crowns under the mounds were viable. Further, after 34 days, 12% of stems and 32% of crowns collected from the bottom of the weed mounds were still viable. We conclude that, in areas of lakes that are exposed during drawdowns, regeneration from in situ stem fragments and crowns following refilling are an important potential source of reestablishment for E. densa.

Imazapyr is a herbicide that can be used in irrigation canals to control a range of aquatic weed species, however, its residual nature, combined with its phytotoxicity to crops at low concentrations, means that the water in canals must be carefully managed following imazapyr application. Residues of the herbicide imazapyr (isopropylamine salt) in irrigation water were analysed and modelled after application to irrigation canals in south-eastern Australia. A treatment program to control delta arrowhead (sagittaria; Sagittaria platyphylla (Engelm.) J.G. Sm.) in over 400 km of irrigation canals was enacted by applying imazapyr to dewatered canals during winter. Following imazapyr application, canals were left dewatered for a period (up to eight weeks) and then refilled. After refilling, canals were ponded for a period (up to 28 days) to allow degradation of imazapyr in the water via photolysis. Upon refilling canals, ~650 water samples containing imazapyr were collected across the treatment area and data modelled to measure the extent of water contamination and to guide efforts to reduce the subsequent irrigation hazard to crops. Modelled data demonstrates that imazapyr behaviour in irrigation water following canal refilling was predictable when 1) amount of imazapyr applied, 2) the dewatered period following herbicide application, 3) the water ponding period, and 4) solar exposure during water ponding were taken into account. Minimising the amount applied (g imazapyr per km of canal) and maximising the time between spraying and refilling (dewatered period) reduced the initial concentration in the water following canal refilling. The amount of imazapyr in the canal water following refilling was reduced by half for every 16 days (confidence interval = 10–38 days) that the canal remained dewatered after imazapyr application. Imazapyr dissipation during the ponding period following canal refilling occurred at a rate that depended on solar exposure. Dissipation did not occur when solar exposure was 10 MJ m⁻², imazapyr concentration in the water reduced by half for every 4.4 days of ponding period (confidence interval = 2.9–9.5 days). Our two models, combined with local climate data on solar exposure, can be used by canal managers to determine the optimal time to refill canals so that imazapyr dissipation is maximised, and thus risk of damaging irrigated crops is minimised.