Cross-pollination experiments with one of New Zealand’s rarest trees

Cross-pollination experiments with one of New Zealand’s rarest trees

When an incredibly rare native tree – the Bartlett’s rātā – flowered for the first time in a quarter-century, Botany curator Carlos Lehnebach was ready with his tweezers.

Bartlett’s rātā is one of the most threatened trees in New Zealand. It’s also one of our rarest species, with only 13 trees left in the wild – mostly on private land. There are many in cultivation, but they’re mostly clones from a single tree.

The tree is found naturally only in the far-north, near Piwhane/Spirits Bay on the Aupouri Peninsula. Only three tiny populations exist in the wild there.

The main threat to the tree is possum browsing and habitat destruction/loss. It is also, along with other species of the myrtle family, under immediate threat by myrtle rust. This fungal disease has reached New Zealand shores and it attacks new leaves, flowers, and fruits. Repeated infection can kill the plant.

Barlett's rātā
Barlett’s rātā at Otari Wilton’s Bush, 2017. Photograph by Daniel Crichton-Rouse. Te Papa

Surprise flowering

A cutting from one of these trees from Spirits Bay is in Otari Wilton’s Bush, and this year against expectation it has flowered. The tree has been at Otari since 1992 and in the past 25 years no one has ever seen it flowering.

One hypothesis as to why it is only flowering now is because the tree is simply fussy and takes time to reach full maturity and the right conditions for flowering to occur. Normally, a plant originating from a cutting would flower faster than that – generally within two years.

Bartlett's rātā
Bartlett’s rātā in flower, 2017. Photograph by Carlos Lehnebach. Te Papa

This is not an isolated flowering either: trees in the Auckland Botanical Gardens (a cutting from the same tree that resulted in the one at Otari Wilton’s Bush) and Auckland University (a genetically different tree) are also in bloom.

Using the opportunity that these flowerings provides, Carlos Lehnebach, Curator Botany is conducting a pollination study at Otari Wilton’s Bush in collaboration with Karin van der Walt, Conservation & Science Advisor at Otari.

The study hopes to uncover whether the tree needs pollen from a different individual to form viable seed, or whether it is happy using its own.

Batlett's rātā
The team in front of the Bartlett’s rātā at Otari Wilton’s Bush, 2017. From left: Karin van der Walt, Conservation and Science Advisor at Wellington Botanic Garden and Otari Native Botanic Garden; Carlos Lehnebach, Botany Curator at Te Papa; Rewi Elliot, Manager of Otari Native Botanic Garden and Wilton’s Bush Reserve. Photograph by Daniel Crichton-Rouse. Te Papa

The pair are using pollen from the tree at Auckland University for the cross-pollination experiments in the hope of getting genetically variable seed, which will produce fitter, robust seedlings.

“Usually flowers would fall off if they are not fertilised,” says Carlos. “The ones we pollinated are still in place and looking healthy, so fingers crossed.”

Aiding reproduction with peroxide and emasculation

“The time frame of when the stigma [the female reproductive part of a flower] is receptive for pollen is an important consideration for effective hand pollination,” says Karin of the work they’re doing. “To determine this in Bartlett’s rātā, stigmas were submersed in 3% hydrogen peroxide. The presence of bubbles on the stigma coupled with a blue/purple discoloration indicate that the stigma is receptive”.

“The testing was conducted every 24 hours from when flowers began to open (Stage 2), through to when they were wide open (Stage 6). Based on this information we can now ensure that hand pollination is done during the floral stages when stigmas are most receptive.”

flowering stages2

In addition to using chemicals, emasculation is used to aid cross-pollination.

“We use tweezers to remove the male parts of the flowers,” says Carlos. “This is to prevent interference of the tree’s own pollen, so if we have seeds in the end we are confident it is from the pollen we have placed on the stigma, and sourced from a distinct individual – in our case the tree from Auckland.”

Bartlett's rātā
Carlos with pollen collected from flowers from a tree in Auckland, 2017. Photograph by Daniel Crichton-Rouse. Te Papa
Bartlett's rātā
Stamen and pollen, 2017. Photograph by Daniel Crichton-Rouse. Te Papa
Bartlett's rātā
Carlos using a brush to pollinate the rātā flowers, 2017. Photograph by Daniel Crichton-Rouse. Te Papa

If pollination experiments are successful, the tree should bear fruit in about four months, then seed germination will take a month or so.

The study will take over a year to complete.

A late developer

“The description of Bartlett’s rātā as a new species was put on hold for more than 10 years after its discovery in 1975,” says Carlos. “Flowers, which are important to confirm the distinctiveness of this species, were not seen until 1984.

“In the herbarium collection at Te Papa we have some of the very first collections of Bartlett’s rātā. Some of these specimens were collected by John Bartlett himself and used in its description. Only four specimens were collected in flower, from 1984 and a later collection in 1991.”

Bartlett's rātā
Specimen collected by John Bartlett, dated 12/07/1978. Te Papa (SP093676)
Bartlett's rātā
Flowering specimen, dated 21/11/1984. Te Papa (SP093673)

Update 15 Dec 2017:

“We just removed the pollination bags today, a month after hand-pollinating over 400 flowers,” says Carlos. “Many flowers have fallen off but those we cross-pollinated (with pollen from the Auckland tree) are still in place and have nice swollen ovaries, which means seeds are developing. Now we need to wait until the fruits mature.”

About Bartlett’s rātā

  • Only found in New Zealand
  • Current conservation status is Threatened – Nationally Critical
  • Grows to a height of 30m, with a trunk diameter of up to 1.5m
  • Discovered in 1975 by Auckland school teacher John Bartlett
  • Numbers peaked in 1992 with 34 trees but have fallen due to possums
  • one of the few species with white flowers in an otherwise red-flowered genus (Metrosideros, the same genus as the pōhutukawa).

Further reading


  1. Hi there, where is the tree at Auckland University, for inspection. Thanks

  2. Hi Karin and Carlos.
    Great to see your efforts documented here. Quite sad to hear we have lost some of the trees. Were they in the Unuwhao or Kohuronaki population? I can report from January 2017 the two trees at Te Paki were looking good but sadly no signs of young plants anywhere in their vicinity.

    1. Oh dear, I just read Peter’s post on the project crimson site, that is super bad news. I will keep in touch and do let me know if there is anything else I could do to help.

  3. One at Percy Scenic Reserve has flowered too but Rewi does know about this one.I think they have visited to collect pollen

  4. Hi
    This is exciting, thanks for posting. I wonder if Paul’s tree mentioned above is genetically different from the 2 ‘lines’ mentioned in the post?

    1. Thanks Barbara. Karin from Otari is looking into this. It would be great if the trees in Whanganui originated from a genetically distinct tree.

  5. We have a Metrosideros bartletti in our Whanganui garden.
    Acquired c 25 – 28 years ago from Graeme Platt’s Albany nursery.
    First flowering last year [minimal] and a better showing this year.
    Specimen at Virginia Lake & Bason Botanical Gardens [Whanganui] also flowered well this year.

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