Following a lead from South African botanists, scientists at Kings Park and Botanic Garden in Western Australia found that it is not the heat and ash from a fire, but rather the smoke that holds the key to germination for many native Australian plants.
Smoke is now widely used in nursery production, bushland management and mine-site restoration.
What species can be grown?
Over 400 native species of seeds respond to smoke treatment. Smoke can be applied as water (i.e. smoke water) or in the aerosol form to seed trays, bushland soil or directly applied to seeds.
Research has found that smoke responsive native species occur throughout temperate southern and arid Australia. Even species from habitats which are not fire-prone (e.g. the alpine herb fields of Tasmania) germinate well following application of smoke. Note: Tropical species may require research to determine the extent to which smoke may be important for germination.
Benefits of smoke
While heat and ash are of limited value for breaking dormancy in many species, smoke promotes:
- Germination of species which are difficult to germinate by conventional means
- More uniform germination
- Earlier germination
- Seedlings which are more robust.
Which plants respond to smoke?
Trees, shrubs, herbs and annuals respond to smoke-treatment. There is no clear distinction in the relationship between taxonomic groups and the requirement for smoke. Proteaceae, Myrtaceae and most other dominant Australian plant families contain smoke responsive species. Some experimentation is essential to determine if an untested species might be smoke responsive. Do not rely only on evidence from related species to predict if a species is smoke responsive.
The link between soil disturbance and fire
Research at Kings Park and Botanic Garden has found that after a bushfire, smoke is deposited as a residue on soil and is then washed through to the soil seed bank when autumn rains arrive (for southern Australia). It has also been shown that smoke-like chemicals are released from the soil organic layer following physical disturbance, leading to a promotion of germination similar to that following a bushfire.
When to smoke
For regions in southern Australia, smoke is best applied from autumn to early winter. For tropical or arid zone species, some experimentation may be required to determine the best time to apply smoke. As a general rule, sowing and smoking should be done when germination is most likely to occur in nature. Note: Smoke is highly water soluble amd over-watering of seed trays can leach the active agents from the soil before seed dormancy has been broken.
How to make and use smoke
Liquid (smoke water) or aerosol smoke are the two most common methods for applying smoke to soil or seeds.
Seeds can be smoked directly in a smoke tent or alternatively soaked in a dilute solution of smoke water (see below for method) for 6-24 hours. Treated seeds are then dried and sown when required. Alternatively, trays containing sown seeds can be smoked in a smoke tent for 60 minutes and then carefully watered for the first 6-10 days to ensure adequate penetration of the smoke chemicals.
Soil restoration / Bushland management
Broadcast seed which has been smoke treated is an effective way to germinate a wide variety of species. Smoke treated seeds used in broadcasting often germinate better including seeds of species which do not normally require smoke for gemination under nursery conditions (e.g. Eucalypts, Banksias).
If a seed bank is present in soil then good germination is possible following the addition of smoke either using aerosol smoke (area for treatment is limited using this method) or smoke water (using automated sprayers). REGEN 2000 is a synthesised smoke product which is a more concentrated and cost effective means, for broad-acre application of smoke.
Making your own smoke tent and smoke water
See the image slideshow on this page which illustrates a typical smoke tent with smoke generator, cooling pipe and inlet fan. The tent can contain up to three levels of shelving. Leave approximately 30 cm between each shelf to ensure adequate flow of smoke between the shelves.
Smoke water is produced by drawing smoke from the smoke generator through drums (20-30 L) containing water for up to 60 minutes (see illustration). A cheaper and more effective method is to use REGEN 2000. This is a highly concentrated smoke water which is ideal for germinating seeds and for application to bushland soils.
What to burn
Most types of fresh or dried plant of woody materials will produce potentially useful smoke. Good germination results are obtained when a mixture of dry and green foliage and twigs are burnt. Note: Do not combust plants with white sap (e.g. Euphorbiaceae) or Oleander as the smoke produced may be noxious.
For further information on smoke methodology, please contact Dr David Merritt.
Plant genera that are responsive to smoke