| Norfolk tree fern | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Division: | Polypodiophyta |
| Class: | Polypodiopsida |
| Order: | Cyatheales |
| Family: | Cyatheaceae |
| Genus: | Sphaeropteris |
| Species: | S. excelsa |
| Binomial name | |
| Sphaeropteris excelsa (R.Br. ex Endl.) R.M.Tryon[1] | |
| Synonyms[2] | |
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Sphaeropteris excelsa,synonymCyathea brownii,[2] commonly known as theNorfolk tree fern orsmooth tree fern, is probably the largestfern species in the world. It isendemic toNorfolk Island, in thePacific Ocean nearAustralia andNew Zealand. It is named after the botanistRobert Brown (1773-1858).
In its natural habitat,Sphaeropteris excelsa is reported to reach 20 metres (66 ft) or more in height.[3] The broad, lance shaped, bipinnate-pinnatifid to tripinnatefronds can reach 5 metres (16 ft)) in length. Stipe is long and has a line of white, stitch like dashes along its length. Rachis and stipe are covered in white-brown and darker orange-brown scales. The trunk can become smooth with age and may display oval scars left from fallen fronds.[4]
Sphaeropteris excelsa occurs naturally insubtropicalrainforests on Norfolk Island. Average daytime temperatures reach around 23 °C (73 °F) during the summer months falling to around 17 °C (63 °F) during the winter. The highest temperatures likely to be experienced in this environment are around 29 °C (84 °F), the lowest around 6 °C. Relative humidity is fairly consistent at levels between 70 and 80% throughout the year. Yearly rainfall is approximately 1,200 millimetres (47 in).
The once extensive forests ofNorfolk Island are now reduced to a single small forested area which has been designated part of anational park.Sphaeropteris excelsa is protected within this park. It is available in cultivation in many countries and though rare is not considered endangered.[5]
Sphaeropteris excelsa is cultivated as anornamental tree. It requires a neutral to slightly acidic soil[6] rich in organic matter. The soil must be moisture retentive but well drained.[5] The tree fern will suffer if left to stand in waterlogged soil.Sphaeropteris excelsa requires high humidity and either high rainfall or frequent watering. During the summer months even a very young pot grown plant can consume 0.5 litres (0.13 US gal) of water per day.
Sphaeropteris excelsa requires good light but does best when shaded during the hottest parts of the day. It responds well to fertilization but can be intolerant of some commercial fertilizers producing malformed fronds and a larger but weaker plant. It is able to withstand only light frosts. Larger plants may survive overnight temperatures as low as minus three degrees Celsius.
In countries such asBritain it is best grown in a conservatory. Its large size however often makes this impractical. A beautiful and rewarding plant but in colder regions it is strictly for the more dedicated of enthusiasts.
It is easily grown from spore and can be very fast growing when young.