Smoke treatment and seed germination

This counts as this week’s geeky plant post. My mental health is good but I am unbelievably tired this week.

As many of you know, I have a great interest in plants adapted to a summer-dry Mediterranean climate. This includes the Fynbos habitat of South Africa which is what ecologists call a ‘fire climax community’. This basically means that the style of vegetation is maintained by periodic bushfires which kill off older, woodier plants and leave room for the cycle to start again from buried seeds, bulbs, tubers etc. Eventually, over several years the amount of flammable woody material increases until a fire resets the habitat again.

Fynbos, with a large Restio in the centre and the tubular red and yellow flowers of an Erica species in foreground

The plants of the fynbos have many adaptations to this cycle. For instance some bulbs (which stay safe underground during the fire so do not burn) will only flower after a fire. Fire not only releases nutrients from ash to feed the bulbs but also removes herbivores that might eat the resulting flowers or seed. Some extreme examples do not even produce leaves in the years between fires – sometimes remaining dormant below the soil for a decade or more until a fire triggers them into growth.

Protea (Silverhill seeds, sown 2002)

A Cape Erica, also from Silverhill seeds

Protea nana from Silverhill seeds

In a lot of Fynbos plants (including the famous Proteas) the germination of seed is stimulated by chemicals in smoke, enabling them to respond quickly to the fresh habitat after a fire and grow in the absence of herbivores and competition from other plants. It also ensures that seeds in the soil can restore a species after burning.

This phenomenon has been known to gardeners for decades and treatment with smoke was often done by burning dry vegetation over seed pots or inside a greenhouse or even using a beekeepers smoke making contraption.

Advances in technology have allowed the identification of the triggering chemicals in the smoke and the production of less hazardous methods. Kirstenbosch botanical gardens in Cape Town devised a method which involved impregnating filter paper discs with the relevant compounds and then these could be dried and then used later by adding a disk to water in which seeds were soaked. These discs are readily available by post now.

More recently some gardeners have experimented successfully with using Colgin Liquid Smoke designed for culinary purposes! This stuff is very strong and diluting it 100 fold is recommended or germination can actually be inhibited.

In my experience some Protea will germinate without smoke treatment, as will some Erica (Cape heathers). The Cape reeds (Restios) seem to be much more dependent on smoke treatment.

(Incidentally, I once visited a native plant nursery in South Africa and the owner did nothing more fancy than pass a blowtorch flame over the top of his seed pots of Erica species.)

The process as I carried it out in Feb/March this year. Fynbos plants usually grow through the cooler months in the wild as water is scarce in summer. They therefore germinate better with cool nights and warmer days – keeping the seeds in a constant -temperature propagator will not work. I find the two best times for sowing are autumn or spring. In the UK I feel that a spring sowing works best as the resultant tiny seedlings do not then have to face a winter when still tiny.

• Fungal pathogens are deadly to young Fynbos seedlings, with Protea being especially vulnerable. I therefore sterilise the outside of the seed before soaking in the smoke solution.
• The instructions for the smoke solution say to just drop the seeds in it. Fine if you want to just sow one or two species but otherwise wasteful and a risk of mixing them up.
• So I decant the seeds onto a filter paper (coffee filters are good). These are then folded round the seed and secured with a paperclip and an identifier or name written on with pencil.
• Soak the parcels in a bowl of 1% Hydrogen peroxide for ten minutes. This sterilises the seed coats without harming the seed.

• Transfer the packages to a bowl of smoke solution and leave for 24 hours.

Packs of seeds in (pink) smoke solution. The big brown seeds are Grielum and were too big to wrap! (Apologies for the shaky photo taken in very poor light)

• Prepare a seed compost which must be acidic, well draining and sterile. Use something very low in nutrients as too much phosphorus is lethal to fynbos plants. If you are happy to use peat then a mix of peat and sand or chopped sphagnum moss and sand is a good mix. This should be sterilised by either microwaving or pouring boiling water through it once it is in the pots.
• Sow the seeds, they can be washed off the surface of the filter paper with a few drops of tap water. Erica seed must be on the surface of the mix as it requires light. Protea and Restios can be covered with grit or more mix.
• Water in with tap water or water that has been boiled and cooled. Stored rainwater is full of plant pathogens.
• Place in a spot that is protected from rain but which is warm in daytime but much cooler (but frost-free) at night – an unheated/cool greenhouse or polytunnel is ideal.
• Keep the seed pots moist by misting daily with tap water. Keeping them in an unheated propagator or even a plastic bag is very useful in keeping humidity up.
• Germination usually takes around a month.

Protea seedlings April 2018

• Erica seedlings are TINY and cannot be moved into another pot until big enough to handle safely. Protea can be potted into a bigger pot fairly soon after germination as they are big seedlings.

Tiny seedlings of  Erica copiosa. April 2018

 

Many Australian seeds also benefit from smoke treatment as habitats similar to Fynbos occur in Western and Southern Australia.