Search

Gamba grass (Andropogon gayanus Kunth.) is a highly invasive, naturalised Weed of National Significance in Australia due to its economic, environmental and social impacts. It outcompetes native pastures and fuels intense fires in northern Australian rangelands. To aid management of current infestations and to better understand its potential distribution, this study determined the germination response of gamba grass under a range of constant (13°C−48°C) and alternating (11/7°C–52/42°C) temperature regimes and quantified the potential longevity of soil seed banks. The effect of different soil types, levels of pasture cover and burial depths on seed longevity was investigated in the Dry Tropics of northern Queensland. Germination of gamba grass occurred under a wide range of both constant (17°C−39°C) and alternating day/night temperatures (16/12°C–47/39°C), although the level of germination declined at the lower and higher temperature ranges. At the cooler temperatures, seed viability was not affected, but seeds went into a state of dormancy. The highest level of seed viability was recorded at the lowest constant temperature regime (13°C) and at the two lowest alternating temperatures (11/7°C and 16/12°C). A gradual but variable decline in viability occurred thereafter with increasing temperatures. At the higher temperature range (e.g. constant temperatures of 39°C−43°C and alternating temperatures of 47/39°C) both dormancy and loss of seed viability were occurring, but once alternating and constant temperatures reached above 47/39°C and 43°C all seeds were rendered unviable after 9 and 6 weeks respectively. In the Dry Tropics of northern Queensland, viability of seeds was

Stevia ovata Willd. is an invasive weed that has become naturalised in northern Queensland, Australia. To aid management of current infestations, this study evaluated seed germination under a range of constant (13−48°C) and alternating (11/7 to 52/42°C) temperature regimes and quantified the potential longevity of soil seed banks. The effect of different soil types, levels of pasture cover and burial depths on seed longevity was investigated in both the dry- and wet-tropics of North Queensland. Germination of S. ovata occurred under a wide range of both constant (13−39°C) and alternating day/night temperatures (16/12 to 52/42°C), but optimum conditions ranged between 24 and 27°C and 24/20 and 37/31°C respectively. As temperatures declined below the optimum, an increasing proportion of seeds went into a state of enforced dormancy. In contrast, higher than optimum temperatures caused a proportion of seeds to lose viability. Differential responses in seed longevity of S. ovata occurred between the two experimental sites. In the wet-tropics, seed viability was

Understanding the reproductive biology of Calotropis procera (Aiton) W.T. Aiton, an invasive weed of northern Australia, is critical for development of effective management strategies. Two experiments are reported on. In Experiment 1 seed longevity of C. procera seeds, exposed to different soil type (clay and river loam), pasture cover (present and absent) and burial depth (0, 2.5, 10 and 20 cm) treatments were examined. In Experiment 2 time to reach reproductive maturity was studied. The latter experiment included its sister species, C. gigantea (L.) W.T. Aiton, for comparison and two separate seed lots were tested in 2009 and 2012 to determine if exposure to different environmental conditions would influence persistence. Both seed lots demonstrated a rapid decline in viability over the first 3 months and declined to zero between 15 and 24 months after burial. In Experiment 1, longevity appeared to be most influenced by rainfall patterns and associated soil moisture, burial depth and soil type, but not the level of pasture cover. Experiment 2 showed that both C. procera and C. gigantea plants could flower once they had reached an average height of 85 cm. However, they differed significantly in terms of basal diameter at first flowering with C. gigantea significantly smaller (31 mm) than C. procera (45 mm). On average, C. gigantea flowered earlier (125 days vs 190 days) and set seed earlier (359 days vs 412 days) than C. procera. These results suggest that, under similar conditions to those that prevailed in the present studies, land managers could potentially achieve effective control of patches of C. procera in 2 years if they are able to kill all original plants and treat seedling regrowth frequently enough to prevent it reaching reproductive maturity. This suggested control strategy is based on the proviso that replenishment of the seed bank is not occurring from external sources (e.g. wind and water dispersal).

Cascabela thevetia (L.) Lippold (Apocynaceae) is an invasive woody weed that has formed large infestations at several locations in northern Australia. Understanding the reproductive biology of C. thevetia is vital to its management. This paper reports results of a shade house experiment that determined the effects of light conditions (100% or 30% of natural light) and plant densities (one, two, four or eight plants per plot) on the growth, time to flowering and seed formation, and monthly pod production of two C. thevetia biotypes (peach and yellow). Shaded plants were significantly larger when they reached reproductive maturity than plants grown under natural light. However, plants grown under natural light flowered earlier (268 days compared with 369 days) and produced 488 more pods per pot (a 5-fold increase) over 3 years. The yellow biotype was slightly taller at reproductive maturity but significantly taller and with significantly greater aboveground biomass at the end of the study. Both biotypes flowered at a similar time under natural light and low plant densities but the yellow biotype was quicker to seed (478 versus 498 days), produced significantly more pods (364 versus 203 pods) and more shoot growth (577 g versus 550 g) than the peach biotype over 3 years. Higher densities of C. thevetia tended to significantly reduce the shoot and root growth by 981 g and 714 g per plant across all light conditions and biotypes over 3 years and increase the time taken to flower by 140 days and produce seeds by 184 days. For land managers trying to prevent establishment of C. thevetia or to control seedling regrowth once initial infestations have been treated, this study indicates that young plants have the potential to flower and produce seeds within 268 and 353 days, respectively. However, with plant growth and reproduction most likely to be slower under field conditions, annual surveillance and control activities should be sufficient to find and treat plants before they produce seeds and replenish soil seed banks. The most at-risk part of the landscape may be open areas that receive maximum sunlight, particularly within riparian habitats where plants would consistently have more favourable soil moisture conditions.

Bellyache bush (Jatropha gossypiifolia L.) is an invasive weed that poses economic and environmental problems in northern Australia. Competition between pasture and bellyache bush was examined in North Queensland using combinations of five pasture treatments (uncut (control); cut as low, medium, and high pasture; and no pasture) and four bellyache bush densities (0, 2, 6 and 12 plants m−2) in a buffel grass (Cenchrus ciliaris L.) dominated pasture. The pasture treatments were applied approximately once per year but no treatments were applied directly to the bellyache bush plants. Measurements of bellyache bush flowering, seed formation, and mortality were undertaken over a 9-year period, along with monitoring the pasture basal cover and plant species diversity. Maximum flowering rates of bellyache bush occurred after 9 years (97%) in plots containing no pasture, with the lowest rates of 9% in uncut control plots. Earliest flowering (322 days after planting) and seed formation (411 days) also occurred in plots with no pasture compared with all other pasture treatments (range 1314–1393 days for seed formation to occur). No seeds were produced in uncut plots. At the end of 9 years, mortality rates of bellyache bush plants initially planted averaged ≥73% for treatments with some pasture compared with 55% under the no-pasture treatment. The percentage of herbaceous plant basal cover in uncut plots was increased 5-fold after 9 years, much greater than the average 2% increase recorded across the low, medium, and high pasture treatments. The number of herbaceous species in uncut plots remained largely unchanged, whereas there was an average reduction of 46% in the cut pasture treatments. Buffel grass remained the species with the greatest basal cover across all cut pasture treatments, followed by sabi grass (Urochloa mosambicensis (Hack.) Dandy) and then red Natal grass (Melinis repens (Willd.) Ziska). These results suggest that grazing strategies that maintain a healthy and competitive pasture layer may contribute to reducing the rate of spread of bellyache bush and complement traditional control techniques such as the use of herbicides.

Bellyache bush (Jatropha gossypiifolia L.) is an invasive weed that has the potential to greatly reduce biodiversity and pasture productivity in northern Australia’s rangelands. This paper reports an approach to develop best practice options for controlling medium to dense infestations of bellyache bush using combinations of control methods. The efficacy of five single treatments including foliar spraying, slashing, stick raking, burning and do nothing (control) were compared against 15 combinations of these treatments over 4 successive years. Treatments were evaluated using several attributes, including plant mortality, changes in population demographics, seedling recruitment, pasture yield and cost of treatment. Foliar spraying once each year for 4 years proved the most cost-effective control strategy, with no bellyache bush plants recorded at the end of the study. Single applications of slashing, stick raking and to a lesser extent burning, when followed up with foliar spraying also led to significantly reduced densities of bellyache bush and changed the population from a growing one to a declining one. Total experimental cost estimates over 4 successive years for treatments where burning, stick raking, foliar spraying, and slashing were followed with foliar spraying were AU$408, AU$584, AU$802 and AU$789 ha–1, respectively. Maximum pasture yield of 5.4 t ha–1 occurred with repeated foliar spraying. This study recommends that treatment combinations using either foliar spraying alone or as a follow up with slashing, stick raking or burning are best practice options following consideration of the level of control, changes in pasture yield and cost effectiveness.

This study compared the efficacy of first and second fires applied during the early (August–September) and late dry season (October–November) on mortality of riparian (climbing) and subriparian (freestanding) infestations of rubber vine (Cryptostegia grandiflora R. Br.). The impact of fire treatments on germinable seed banks of monocotyledonous and dicotyledonous species was also determined. Individually, fire season, habitat type and number of fires significantly affected mortality of rubber vine plants. Late-season fires promoted higher mortality of rubber vine (96%) than early season fires (77%), with rubber vine in subriparian habitats more susceptible (90% mortality) than that growing in riparian areas (68% mortality). On average, fire mortality increased from 32% after the first fire up to 86% following 2 fires. Sensitivity of juvenile, mature, and old rubber vine plants to fire was in the order of mature>juvenile>old. Early fires significantly reduced seed banks of monocotyledonous plants, particularly in riparian habitats. Late fires significantly reduced seed banks of both dicotyledonous and monocotyledonous plants. No rubber vine seeds were detected in the germinable seed bank of either burnt or unburnt plots.

If treatments imposed to control exotic plants also have a deleterious impact on their residual seed bank, the duration and extent of follow-up control may be reduced. Fire is one such technique that has this ability, particularly if seeds are located on or close to the soil surface. Three studies were undertaken in a riparian habitat in the dry tropics of northern Queensland to quantify the effects of spring burning on the seed bank of the exotic weed bellyache bush (Jatropha gossypiifolia). The first determined the distribution of seeds within the vertical profile of the trial site. The other 2 studies were experiments that quantified the effects of fire on germination and viability of both dispersed bellyache bush seeds and seeds held in mature capsules. Dispersed seeds of 2 types (intact and ant-discarded) were placed at the following 6 positions in the vertical profile of bellyache bush infestations: 0.5, 1, 2 and 4 cm depth below ground, on bare ground, and below fuel. Seeds held in capsules were located at random on the crown of bellyache bush plants. For both experiments, comparisons of burnt plots were made with unburnt controls. Fire was imposed in spring (September); the season in which burning for weed control in northern Queensland generally occurs. While maximum fire temperatures averaged 590 ± 46°C, the temperatures that seeds or capsules were exposed to depended on their location within the vertical profile, with temperatures decreasing in the following order: below fuel > crown of bellyache bush > 1 cm > 0.5 cm > bare ground > 2 cm > 4 cm below ground. There were negative correlations between seed germination and peak fire temperature and between viability and peak fire temperature. Seed viability was nil for seed under fuel but >80% for seeds placed on bare ground or ≥2 cm below ground. Fire reduced germination and viability of seeds held in capsules by 31 and 35%, respectively, when compared with unburnt seeds. While ant-discarded seeds generally had a higher germinability than intact seeds, they were more susceptible to fire. This may be attributed to loss of the external protective barrier of the seed coat (exotegmen) caused by the feeding of ants. Bellyache bush seeds were recorded across all soil depths, reaching a peak of 3.8 million seeds per hectare at 1–5 cm soil depth. These results suggest that while bellyache bush seeds are susceptible to fire, many are buried beyond the reach of lethal temperatures. Therefore, viable seeds will be available for post-fire recruitment and other measures, such as chemical control, may need to be employed in conjunction with burning.

Studies were undertaken at 2 sites in northern Queensland (Wrotham Park and Ten Mile Creek) to quantify the effects of fire on germination and viability of both loose rubber vine seeds and seeds held within pods. Seed lots were placed at a range of positions within the vertical profile of rubber vine infestations; 0.5 cm below ground (loose seeds only), on bare ground, below fuel, above fuel and on crowns of free-standing rubber vine. The influence of seed age (0 and 1 year old) and seed plumage (with and without plumes) was also examined for loosely held seeds. Late season fires were imposed at both Wrotham Park and Ten Mile Creek, with an additional early season fire undertaken at Ten Mile Creek to enable comparison between seasonal fires. Maximum fire temperatures reached during burning averaged 853, 827 and 871˚C for Wrotham Park, Ten Mile Creek early and Ten Mile Creek late season fires, respectively. The temperatures that seeds were exposed to depended on their location within the vertical profile, with temperatures above fuel > below fuel > on rubber vine crown > bare ground > 0.5 cm below ground. Seed viability after burning ranged from 0 to 100%, with seeds generally most affected if located above or below the fuel, and least affected if buried below ground. The age of seed and the presence of seed plumes interacted with seed placement so that seed germination and viability of loose seeds were affected. Some seeds remained viable, but were inhibited from germinating, particularly if located 0.5 cm below ground. Positive, non-linear relationships occurred between seed mortality and maximum temperature and between seed mortality and the duration of exposure. These results suggest that fire may inhibit germination and kill a substantial proportion of rubber vine seeds.

Phenolic content, germination percentage, and host preference were compared among 11 seed populations of witchweed (Striga hermonthica(Del.) Benth. # STRHE). Significant differences in germination percentage occurred among the seed populations after preconditioning by their exposure to different durations of warm-moist conditions. Greatest percentage of seed germinated after 8 days of preconditioning. Preconditioning for more than 16 days reduced percentage germination. The host specificity range of witchweed seed populations that parasitized pearl millet [Pennisetum americanum(L.) K. Schum] was broader than were those seed populations that attacked sorghum [Sorghum bicolor(L.) Moench.]. Results of thin-layer chromatography indicated large variations in phenolic content among the witchweed seed populations.

The dispersion of ethylene in the soil after appli- cation from a backpack ethylene applicator (BPEA) was determined using a bioassay of witchweed (Striga asiatica (L.) Kuntze #4 STRLU) seed germination. Witchweed seed were buried in packets in ridged seedbeds under field condi- tions. Ethylene dispersion was better when injected into the furrow than into the ridge top. Injections 30 cm deep were more effective in causing seed germination than in- jections 15 cm deep. Seed germination induced by ethylene injection into the furrow 30 cm deep was 13% more than that obtained from the ridge top injection 15 cm deep. Seed germination as high as 90% was obtained at soil depths to 75 cm at a horizontal distance of 100 cm from the point of ethylene injection. Ethylene diffused up to 300 cm hori- zontally and to 75 cm vertically to cause at least 45% seed germination in a field of Lakeland soil (Typic Quartzisamn- ment). The BPEA was found to be an effective application tool in witchweed control operations. Additional index words. Germination stimulants, parasitic weed, Striga asiatica, witchweed, STRLU.

SUMMARYThe parasitic weed striga had significantly greater effects on sorghum yield components than non-parasitic weeds. Grain reduction caused by striga amounted to 65% compared with 32% reduction caused by non-parasitic weeds. Comparable reductions were also manifest in straw yield, grains/head and plant height of sorghum. Early removal of striga (a week after its emergence), and thereafter at regular weekly intervals, may help to improve crop yields.