Ferns don’t have seeds, instead they reproduce from spores. The spores occur on the underside of fronds on established plants usually around late summer. The age that plants reach maturity and start producing spores varies but they are usually only found on plants which have started to form a trunk.
Each spore is capable of producing a new plant, but of a different form. Once released, the spore grows into a small, thread-like or heart-shaped structure that grows close to the ground. This structure is the sexual generation called the ‘gametophyte’ as it possesses the egg and sperm (or gametes). The gametophyte releases sperm cells that need to land in water in order to survive and travel to the female egg cells. When a sperm has fused with an egg cell, the fertilised egg produces a new plant – this time the sporophyte generation. Thus ferns are said to have ‘alternating generations’ because each new generation of ferns that is produced will be a different form from the previous generation.
Dicksonia antarctica is one of the easiest species to try propagating. I always think the easiest way to understand propagating from spore is to try and imagine the environment that the fern comes from. By replicating these growing conditions you are more likely to have success. I would try and imagine the damp cool forests of Tasmania and Victoria in Australia as these are the conditions the species thrives in. The species thrives in the dense understory of the forest where only dappled sunlight will find the forest floor. I would recommend trying to propagate the tree fern on a cool window sill protected from strong sunlight.
The process of spore cultivation may sound very scientific but it is fairly simple but slow process. It can take up to 2-3 years for small plantlets to be produced. Propagating tree ferns from spore is the most sustainable way to grow tree ferns as it does not require plants to be removed from their native habitats.
Step 1. First you need to obtain your spores; this is best done from mid-late summer. The spores can be found on the underside of mature fertile fronds, the spores will be held in pin head size yellow brown capsules. When these capsules have a dust around them they have usually opened so try and select a part of the frond where the spores have not been released. If you are unsure try a few different fronds from around the plant. It doesn’t hurt established ferns to remove a section of a frond although it may ruin the appearance of the fern so ask before removing the material.
Once the spores have been collect we put them in clearly labelled envelopes and place them in the airing cupboard for 3-5 days to allow the spores to shed. Alternatively contact your local fern society to see if they have a spore bank or look on eBay.
The pictures below show spores growing on the underside of fronds. The image on the right shows spores which are ready to be harvested. When spores look like this take a small section and place this in an envelope. The image to the left is a close up of spores which are still a few weeks away from being ripe.
Step 2. Next you need to prepare your container and soil. Do this at least a day in advance of sowing the spore. It is best to select a container that can be sealed airtight to retain the humidity and to keep this sterile. Once you have selected your container make sure it is clean as dirt may carry other plant seeds or spores. Fill the container with your soil. I have found coarse ericaceous compost works is a good soil to try. Once you have filled your container with the soil this needs to be sterilized. I normally pour boiling water over the entire surface of the compost ensuring this has been scalded. I normally use a 9cm pot and place the sterilised pots into zip seal freezer bags to make things quick and easy.
Step 3. You should now have fresh spores to sow and a sterile container to sow them. The spores are best sown in a still environment to reduce unwanted cross contamination occurring. The spores now need to be cleaned before sowing. One of the advantages of using an envelope to dry the spores, is it’s useful for cleaning the spores. Tap the contents of the envelope into one corner and tear the corner off to create a 5-10cm equilateral triangle. The torn off triangle will create a funnel to carefully separate the remaining spores and debris on. Angle the funnel at 45° and very gently tap the bottom, you will see the larger debris start to fall off, leaving behind a very fine layer of brown dust, these are the spores!
Step 4. You should now have cleaned spores ready to sow. It is very important when sowing the spores to do this thinly. The more spores you sow the more competition there is, and the plants will crowd themselves out. I would recommend no more than a small pinch of spores in a 9cm pot. Once you have sown the spores label them and find a cool window with indirect sunlight.
Keep an eye on your sowings for the next 6-9 months as the surface of the soil starts to turn green. If after 9-12 months you see no activity you can spray with a small amount of fresh rain water to act as a catalyst.
The image below shows the early stages of growth from spore which all ferns, mosses and liverworts go through. This is the gametophyte stage, which happens after the surface has gone green. From the gametophyte stage you will start to see the first fronds developing.
Step 5. Once small plantlets have developed on the surface with several true fronds you can divide them into small clumps in separate pots.
At this stage the small plantlets are very sensitive to changes in humidity so it is important to keep them covered. These small plantlets are usually not ready to be pricked off into individual pots until they are around 5-10cm in height. When the individual plants have established on there own and have grown to around 15cm you can start hardening them off.
When hardening off young plants the easiest way is to slowly open the freezer bag or remove the covering over several weeks to avoid sudden humidity changes.
Ensure that the plants are well watered with rain water during this process.
If you live in an area where tree ferns occur naturally collect spores and follow the steps but place the pots under mature specimens and keep an eye on them. You should get good results from this.