With trees in young leaf and their flower pollen dusting the air, I re-visited Russell Square’s Black Poplar to determine its sex and true identity. It was heartening to see the naked, lanky specimen of my winter visit fully clothed in semi-lustrous foliage and looking more like the noble tree of our lowland river valleys.
Examination of the blunt-toothed leaf with its twisted petioloes suggests that this is no cross-species but true Populus Nigra. These contortions are caused by the spiral leaf gall aphid, Pemphigus spyrothecae, which apparently do not occur on any of the Cotton Wood or Balsam Poplar hybrids.
The pointed, triangular leaf shape and furrowed, heavily bossed bark also make an identity match and only the lack of pubescence on the foliage raises some doubt as to this being from native wild Black Poplar stock.1 It could be a sub-species hybrid but without DNA testing at my disposal I cannot discern a betulifolia from sbsp nigra.
Although not a racing certainty, I had assumed that the Russell Square tree would be male. The prejudice against planting female Poplars especially in urban settings arose from the annoyance of having fluffy seeds wafting around. Populus nigra subsp.betulifolia are currently more endangered due to this sex selection; of the estimated 7000 in the UK, only 600 are female.
Flowers appear before the leaf and so with the season prematurely advanced, I had expected to see red ‘devils fingers’ of spent male catkins lying on the ground. A few stray fallen seeds confirmed instead that ITS A GIRL.
High up in the canopy the yellow-green catkins were already fertilised and fruiting as small grape-green clusters. The towering height provided no opportunity for a closer look but true to the saying: “it’s an ill wind that blows nobody any good”, the day’s strong Easterly had broken off a 6 foot length. Naturally I brought the branch home for further study although carrying it required a certain nonchalance and dexterity.
Propped against the wall and looking extremely top heavy in a large flower vase, life was nevertheless temporarily resumed.
In close-up the textured berry-shaped fruit bears the residual two-tipped stigma, which received the wind-blown pollen. Although there is no male tree intimately close by, the estimated radius for pollen travel is about about 9 miles 2 putting other London Black Poplars within pollinating distance, including Green Park, Islington and even possibly those on the Hackney marshes.
Interestingly scientific studies confirm that where there are both male Black Poplars and poplar hybrids in equal proximity, P. Nigra preferably accepts pollen of its own species. Otherwise cross-pollination will occur. 3 Given the proximity of the Black Italian Poplar in Hyde Park for example, it is just as likely that the Russell Square poplar has been fertilised by a hybrid interloper.
Progressing rapidly on, the seedpods burst open within 24 hours, revealing the tell-tale powder puffs of white down, after which its close relatives, Cottonwood trees, are so-named. With an uncanny resemblance to new born stars in a cosmic Magellanic Cloud, the white seeds can be clearly discerned within. Unfurling on tangled, silky filaments, these are eventually frothed out, descending in cumulus clumps. The tree’s resin helps the seeds to stick where they fall.
Germination of the short-lived wind-blown seeds requires a wet, bare, alluvial ground but much of the flood-plain conditions on which they rely have disappeared, due to changes in practice of managing our waterways.
Evidently the tended, crowded bedding of Russell Square affords no chance of propagation for this Black Poplar. It is also possible that the fruit will be sterile if it has hybridised. Nevertheless I shall attempt to bag at least one Black Poplar from this seed collection even though emulating the natural conditions of germination is going to be a major challenge.
p.s. I’d be interested to know from any others who have grown a tree from seed – not counting the fecund sycamores, tree of heaven or Horse chestnut which propagate against my will, in the garden.
p.p.s. this post is inspired by b-a-g, an engineer who likes to weigh, measure and generally Experiment with Plants
1. Black Poplar also known as Downy Poplar, Water Poplar, Willow Poplar and Cotton Tree
2. Black Poplar Biology
3. Journal of Forest Science 2002 ‘Introgression in black poplar (Populus nigra L. ssp. nigra) and its transmission
The Forestry Authority: Black Poplar: the Most Endangered Native Timber Tree in Britian
©Copyright 2011 Laura Thomas.
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