Cycads

Dioon spinulosum

This very large cycad (up to 16 m, with 2 m long leaves) is found only in eastern Mexico, where it is threatened with extinction due to its restricted distribution and the clearance of its native rainforest in many areas.

Its scientific name describes the plant's appearance; spinulosum refers to its spiny leaflets (most other species in this genus have entire leaflets). The female cone of D. spinulosum is the largest cone of any gymnosperm, reaching 50-80 cm in length and 20-30 cm in width and weighing up to 15 kg. Each scale of the female cone has two seeds, hence its genus name Dioon meaning 'two eggs'.

The related plant D. edule has seeds with starchy endosperm, which are ground to produce an edible flour or consumed after boiling or roasting. Many cycads have recently been found to contain carcinogens and neurotoxins with long-term harmful effects and the seeds of all cycads are more or less toxic.

Dioon spinulosum at Kew

We have two plants in the Palm House, one male and one female, which were donated to Kew in 1888 from Washington Park Arboretum. The last time the female specimen produced cones was seven years ago. Pollination was attempted but not successful.

Pollination attempt, July 2003

In July 2003, we observed that both the male and female plants were producing cones, providing a fantastic opportunity to use fresh pollen to fertilise the female. After researching how to tell when the ovules become receptive, we pollinated the female cone in August 2003. We knew we were successful because the female cone soon started to swell up as the seeds were developing inside. It takes up to 18 months for the seeds to ripen, so we can expect to harvest our seeds, maybe up to 300 in total, some time in early 2005. When the cone is mature it will burst with a loud noise, scattering the seeds. We will then collect them and test their viability. Some seeds will be sent to Kew's Millennium Seed Bank, some will be germinated to provide new plants, whilst others may be distributed to other botanic gardens, depending on the numbers available.

Update November 2004: mature seeds harvested

After pollination fifteen months earlier, in mid-November 2004 we noticed the cone had started elongating, a process caused by the the scales separating. In just one week the cone became 10cm longer. It then quickly started to break up from the apex of the cone up towards the base. Seeds have been collected, the fleshy seed coat removed, and a crude test for viability undertaken by dropping them into a bucket of water - those that sink may be considered as potentially viable, those that float are probably not viable (not a 100% secure method, but generally quite accurate).

Depending on seed numbers collected, we would like to germinate a proportion ourselves. If the seeds are suitable for storing then some could go to Kew's Millennium Seed Bank, and the rest distributed to other botanic gardens at home and abroad (subject to clearance under the Convention In Trade of Endangered Species regulations - CITES).

The literature concerning this plant suggested it would release its seeds with a sudden 'bang', but unfortunately we did not observe this as it has been more of a gradual thing.

This is the first time this plant has been propagated at Kew in its long history, so our ability to grow future generations to eventually replace our original, older specimens is greatly anticipated.

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