At the JBG they have a great collection of Australian and South African plants: the displays occupy two of the gardens regional sections. Most plants in the garden come from seed due to import restrictions upon live plant material. This means we sow lots of seeds for these areas in the nursery.
Seeds from Western Australia and parts of South Africa often require fire to germinate. To simulate the necessary conditions, plant material can be burnt and mixed with water for a 24 hour seed soak. Interestingly, the seeds don’t actually require heat to germinate, only the chemicals generated by the fire, which are contained in ash. This method is useful for small amounts of seed, but is very time consuming.
For a more efficient seed treatment, you can buy vermiculite infused with smoke that releases the chemicals when watered. By topping seed trays with a thin layer of this smoked vermiculite, simulated conditions can be achieved. The big draw back of this is that smoked vermiculite is very expensive and only seems to be available from Australian vendors; with delivery it’s about a dollar a gram. Due to the cost and the fact that the garden needs about 1kg of it, I decided to try and make our own.
To smoke the vermiculite, I simply used a BBQ with leaves and bark smouldering in the base and a wire box of vermiculite on top. The material was collected from the respective sections, one batch for South Africa and one for Australia. The vermiculite must be wet to help with absorption and prevent it from combusting. Every thirty minutes, fresh leaves are added and the vermiculite is damped again. The process takes an hour and a half (three changes of foliage). Even with this improvised set up, around 500 grams an be produced at a time. After smoking, the vermiculite is air dried, and can be bagged and stored after three days.
When seeds require smoke treatment, they are sown in a tray and covered with a thin layer of the smoked vermiculite. Upon watering, the volatile oils held by the vermiculite are released slowly onto the seeds, thus replicating sitting in ash as they do in nature.