20.10.25 How to grow orchid seeds with B1 Fungus

Part 2: Pour the medium

This blog series is a detailed description of how to grow orchid seeds with B1 fungus. If you have any questions please feel free to e-mail me. See below.

Now you have all the equipment described in Part 1 it is time to go ahead and make Petri dishes of Basic Oats Medium.

Wear gloves to protect the hands and avoid contamination of the plates: rubber gloves for coarse work, disposable latex gloves for fine handling.

Dilute some Domestos bleach, 1 volume with 9 volumes of water, to make an approx. 10% dilution in an open dish. Soak a J-cloth in this then wring most of it out. Swab the inside surfaces of your work cabinet with the cloth to sterilise them and any objects going in there.

Open a pack of Petri dishes inside the cabinet and stack some in readiness. They are sterile coming out of the pack. If using cleaned, re-cycled Petris give all internal and external surfaces a thorough wipe with the bleach cloth, then stack them.

Allow 30mL of medium for each Petri dish when calculating the volume to make up. For example, 200mL is enough for 6 Petri dishes.

Put 200mL tap water in a Pyrex glass jug. For 3.5g/L oats final concentration add 0.2 x 3.5 = 0.7g of oat flour or porridge oat powder. For 7.5g/L agar final concentration add 0.2 x 7.5 = 1.5g of agar powder. If you don’t have scales use teaspoon measures. The internet says a teaspoon is 5mL but mine seem to be more like 3mL! Anyway, try a level teaspoon of oats and a heaped teaspoon of agar powder. These can be tweaked in future with growing experience.

Marmite is added at nominally 0.1g/L, a tiny amount, and it doesn’t need to be accurately weighed. Use a blob about the size of a peanut but don't put it straight in! Stir to fully dissolve in a separate 100mL of water. Then add just 1mL for each 100mL of final medium. So in this case add 2mL or about a level teaspoon. Discard the surplus or freeze it as ice cubes for future medium prep.

Stir the oats, agar and marmite thoroughly into the 200ml water. They don’t fully dissolve at this stage, but form a suspension. Have 4 Duran bottles or jam jars clean, dry and ready. Put the appropriate amount of water in your pressure cooker and have it warming gently on the hob.

Heat the medium in a microwave for a few minutes with occasional stirring to just below boiling so that the agar dissolves. Pour equal amounts into 3 of the Durans or jars. Don’t fill more than two thirds full. A good amount is 65mL in each of 3 bottles. Later each can be used to pour 2 Petri dishes. Fill the 4^th bottle with tap water. This provides sterile water for later, when the seeds are being prepared.

Screw lids onto all the bottles or jars but do not fully tighten. The looseness avoids pressure accidents inside the cooker but still prevents medium from boiling out. Load bottles in the cooker and cook for at least 20 minutes on maximum pressure. The heat may need to be turned down a bit once max. pressure is reached – see your cooker instructions for safe working.

If you don’t have a pressure cooker you could try boiling the bottles in a pan of water - it might work?

Remove bottles from the cooker taking care not to burn your fingers. They need about 15 to 20 minutes to cool at room temperature before it is comfortable to pour the medium. Don’t wait too long though or agar will eventually set in the bottle.

At the back of your cabinet lift the Petri lid and pour medium in so that the base is about half full – a bit below the horizontal line that can be seen in the wall.

The poured Petris can be left in a stack to cool and set. Put two empty Petris on top to insulate and slow the cooling so that internal condensation is reduced.

Two hours is enough for the agar to set. Condensation forms on the underside of the lids. This is annoying as it stops you seeing inside and excess water encourages contaminating microbes. Carefully replace the lid with a clean, dry, sterilised one.

The Petris are now ready for orchid seeds and fungus to be added!

However if you want to do this later the Petris will be OK for 24 hrs or so. For longer storage seal them in a sterile kitchen bag and store in the fridge. Even so it is best to use them within a few days because they invariably develop some microbial contamination with time.

Next time     Part 3: Add orchid seeds and fungus

Anthony.heys@sky.com