This report is about improving the reliability of direct seeding of native tree and shrub species, through matching field sowing times to germination requirements of species. Two pilot experiments were carried out in low rainfall areas of South Australia and Victoria to determine the effects of direct seeding at different times of the year. Sowings occurred between September 2004 and September 2005 and monitoring occurred until August 2006. Rainfall during this period was below the long term average. The differences in germination and survival among sowings carried out in spring, summer, autumn and winter are reported for the eight native tree and shrub species used. The report recommends greater monitoring of germination and survival by direct seeding practitioners, to determine optimum sowing times for common revegetation species.

 Who is the report targeted at?

This report is targeted at direct seeding practitioners, both new and experienced, working in temperate Australia. It is also relevant to people and organisations who plan, prioritise and fund direct seeding work in these areas, such as landholders, catchment management organisations and community groups. The research recommendations are relevant to research organisations such as universities, state government departments and CSIRO.

 Background

Under the Australian Government’s National Action Plan for Salinity and Water Quality, and the Natural Heritage Trust, regional organisations have set ambitious targets to revegetate large areas of land to improve natural resource management. The cost of meeting these targets through tubestock (seedling) planting alone is prohibitive. If these targets are to be achieved, costs must be reduced and some commercial return from the plantings will be necessary. Direct seeding costs less than planting with tubestock seedlings, but can be less reliable. Improving the reliability of direct seeding will reduce establishment costs for both environmental and commercial revegetation, and reduce the quantities of seed required. Understanding which species are favoured by sowing at particular times of the year will enable practitioners to make better use of scarce seed resources. Direct seeding of commercial species in a wide range of environments will also be necessary to reduce costs and increase returns from revegetation.

In 2003-04 a review was carried out to determine whether there were any advances that could be made to the technique of direct seeding, to increase its reliability to the point where it could be used in commercial revegetation, such as oil mallee plantations. This review included four consultation workshops with leading direct seeding practitioners in temperate Australia, and a literature review.

One of the recommendations from this review was to conduct a small research project to confirm the effects of sowing at different times of the year. To use less seed in direct seeding, we need to ensure that seed is sown when it has the highest chance of germinating, in order to avoid losses from predation, desiccation and disease.

 Aims/Objectives

The trial aimed to determine whether:

1. Some species are favoured by sowing at different times of the year, and whether the differences in survival between species are great enough to recommend multiple sowings.

The trial was not intended to make recommendations for all species used in direct seeding; rather to see if there was evidence for an effect in the field, which could be elaborated further with local research using species mixes commonly used by practitioners.

 2. One particular time of the year was better for direct seeding at these sites than another. Note that this project only sowed trials within one 12 month period.

 3. The use of a commercially available Bradyrhizobium inoculant affected the survival of the acacias in the trial and whether there was a differential effect over the four sowing times.

 Methods used

Trial sites were established in South Australia (near Palmer), northern Victoria (near Pyramid Hill) and Western Australia (near Northam), although the latter site was abandoned early due to drought and grasshopper predation and is not reported here. The trials used a mix of eight acacia and eucalypt species native to the study areas.

At each site, four sowings of the mix of eight local native species were carried out, with one sowing occurring in each of spring, summer, autumn and winter. Sowings occurred between September 2004 and September 2005, and these were monitored sporadically until August 2006 at Palmer and May 2006 at Pyramid Hill. The trial used a randomised split plot design, with time of sowing as the main effect and presence or absence of inoculant assigned to subplots. The number of germinants per species was monitored from a sample of the trial plots for each sowing. Both trials were sown using modified Burford seeders, which are commonly used in these areas.

 Results/Key findings

The climate during this trial (September 2004 to August 2006) was different from average long term trends, with a late arrival of the ‘autumn break’, hotter than average summer temperatures in 2005 and 2006, and rainfall at Palmer was considerably below average from October 2005 to August 2006.

Although limited in scope, the trials found that there were differences in germination and survival among the four different sowing times. As expected, the summer sowing had the poorest survival.

Autumn and spring sowings gave the best survival at both sites, with winter sowings also giving good results at Palmer. This confirms current practice in these areas.

Significantly, there were differences in survival for individual species across the four sowing times.

For example, at Pyramid Hill, Acacia pycnantha and A. acinacea were strongly favoured by sowing in autumn while Eucalyptus microcarpa was strongly favoured by an early spring sowing.

There were some significant differences in survival of the Acacia species between those inoculated with Bradyrhizobium and those not, but only at the Pyramid Hill site. At Pyramid Hill, the better survival in inoculated acacias was only for the autumn and winter sowings. At Palmer, the germination of the two acacia species was low and the differences between inoculation treatments were not significant. However uninoculated Acacia had slightly higher germination than inoculated in spring and winter.

It appears that the use of the inoculant ‘Wattle Grow’ improved the survival of the two Acacia species at Pyramid Hill compared to the uninoculated plants. This supports other studies that show that the inoculant helps survival by increasing the effectiveness of the root system in gaining access to soil moisture. Further trials are needed.

 Recommendations

The implication of these results for direct seeding practitioners is that despite the best efforts at site preparation and planning, if the whole suite of species is sown together at a particular site in a particular season it is unlikely all species will germinate and survive. Sowing times and seed mixes may need to be altered to reflect species’ preferred sowing seasons.

Until we can more precisely predict the range of environmental conditions that favour germination and early survival for all of our common species, practitioners must rely on observations they make in the field. This presents a strong case for regular monitoring of sites for several years after sowing.

Simple methods such as counting the number of germinants over a sample length of seeding line for each species will give an indication of success. These records can then be compared with site specific weather records to give an indication of how the conditions affected the germination and survival of individual species used on the site. Practitioners will then be able to build up a picture over time of which species are favoured by certain conditions.

The ideal moisture, light and temperature regimes for individual species should also be verified experimentally. This could then be matched to long-term climate averages for particular regions to predict the optimum sowing windows for individual species. The effect of inoculants such as ‘Wattle Grow” should also be further tested for a range of Acacia species sown in different seasons and sites.

Direct seeding practitioners cannot be expected to bear the burden of this monitoring alone.

Catchment authorities and other regional organisations should assist or carry out the monitoring. Funding bodies, principally the Australian Government through the Natural Heritage Trust, should make monitoring and reporting a condition of funding. To effectively achieve a national picture of species response to sowing times and climatic conditions, a national monitoring and reporting program should be established. Such a program would provide a simple method for field officers to use and a web-based database for reporting the results. Sufficient funding would also need to be provided to enable field collection of data, data entry and analysis to be undertaken. A critical component of such a system will be its ability to provide meaningful recommendations back to direct seeding practitioners. Regional universities and state research agencies could also assist, in order to build up a picture of the conditions favoured by common revegetation species in direct seeding. Practitioners could then add to their existing experiential knowledge to determine if particular sites and species warrant separate sowings at different times of the year. Such research should initially target species of high commercial or conservation value.