Restoration and propagation of the King Fern in Norfolk Island National Park

Challenges and techniques

By Mark Scott and Sara Freeland

The King Fern (Ptisana salicina) is a species native to the subtropical rainforests of the South Pacific, including Norfolk Island. It’s one of the largest ferns in the world, with fronds that can reach up to several metres in length, and thrives in cool, moist environments, typically along streams and in shaded valleys.

Unfortunately, shifting climate patterns and prolonged dry spells are increasingly threatening the King Fern’s natural habitat and ability to reproduce naturally. The species was listed as endangered under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) in 2003. At this time, there were less than 100 mature plants within Norfolk Island National Park.  

To ensure the long-term persistence of the species, the Norfolk Island National Park (NINP) nursery is focused on the propagation and replanting of this species to preserve its future.

Propagating King Ferns

When NINP Nursery Manager, Mark Scott, reached out to experts in New Zealand and Australia about propagating P. salacina from spores, he was surprised to learn that nobody had any germination information for the species, or any pictures of what a gametophyte even looked like.  

“Nurseries (staff) said that propagation was usually done using the nodules off mature plants,” says Mark. “We quickly noticed there were buds on both sides of the nodules, so we cut them in half to produce two plants from each nodule. These were nestled cut side down into 90/10 perlite/fine coir and kept in the hot house. Within four to eight months, most of these had put out roots & new growth.”

However, while this method is achievable if there is sufficient wild stock, due to the very small and specific locations where these ferns occur within NINP, there are only a very limited number of mature King Ferns left in the wild. Mark became increasingly concerned about putting too much strain on these existing plants.

“I didn’t like the idea of having to constantly remove material from the few mature plants, so I knew I needed to try and propagate them in larger numbers from spores instead,” he said.

Trialling different spore collection techniques

Like all ferns, King Ferns reproduce via spores, which develop on the underside of their fronds in small clusters known as sori. While spores are abundant when available, collecting them without contamination of other fern species was challenging.

“It was very hard to get them when the sori are opening, and usually, they’ve got tons of tree fern spores all over them,” Mark recounted.

To address this issue, Mark tried harvesting fronds while the sori were still closed, sterilising them using a 5% bleach solution to kill surface contaminants. After sterilisation, the fronds were placed in a paper envelope and left for about a week. “If they didn’t open naturally, but the sori were mature (brown), I would open them with a scalpel and tap the spores out,” said Mark.

Once the spores are ready, they are tipped into a sterile vial with a loosely stretched piece of stocking over the opening to make a ‘pepper shaker’. Using a careful ‘tap tap tap’ (the technical term according to Mark) and a small torch or phone light to shine over the container, Mark would disperse a thin layer of spores into plastic takeaway containers, filled with 10-15mm of dampened Yates seeds raising mix which had been sterilised in the microwave with the lid on for five minutes on high. “It’s really challenging to minimise overcrowding during the sowing process. Over-sowed trays like this led to stunted growth, and it’ll take much longer for ferns to grow”.

Once the lid goes on, it goes into the hothouse, where the magic happens.

Growth cycle and timeframe

In February 2022, Mark was able to sow a suitable amount of King Fern spores. However, these took many months to germinate, unlike the 20-40 days for most of our other fern species.

“I knew they had to be P. salacina as soon as they became large enough to see easily as they looked like tiny green lumpy slugs,” said Mark

Hundreds emerged, but only the ones with space have grown much larger than 3mm. Due to this, Mark transplanted some gametophytes into separate containers to give them more room. This seemed to work, with several transplanted spores starting to ‘leaf out’. By February 2025 they will already be three years old from sowing and still only about 15mm tall, but will have started to produce a distinctive-looking frond.

In controlled environments like greenhouses or nurseries, where humidity and water levels can be carefully managed, King Ferns should grow relatively quickly once provided with enough space. So far, from spore to a plantable-sized King fern, it’s looking to be around five years. However, if we can continue to refine this process, the National Park Nursery could produce hundreds or even thousands of P. salacina to put back into the areas where they once occurred.

Selecting appropriate planting sites must be carefully considered, as they need to replicate the fern’s natural environment of moist, shaded valleys. However, with appropriate site selection and occasional post-planting maintenance, the NINP is confident we can ensure this species’ long-term survival in the forest.

All images supplied by Norfolk Island Botanic Gardens (unless otherwise stated)