Most plants in the north temperate zone flower in the spring, summer and autumn, when warmth and the presence of pollinators ensures success. Winter flowering seems like ‘a bad idea’ as the chances of pollination are small. There are just a few plants that usually flower in all twelve months of the year (notable examples are Gorse Ulex europaeus and Annual Meadow Grass Poa annua).
In recent years, it has been observed that some summer- or autumn-flowering species keep their flowers for rather longer than expected. Is it caused by warmer winters? Are the ones we see merely statistical outliers or is there a trend? The Botanical Society of Britain and Ireland (BSBI) organise an annual ‘Plant Hunt’, https://bsbi.org/new-year-plant-hunt and so it will be possible to address such questions when the data base is larger. For now, a few of us recorded the plants we saw in Edinburgh on or around New Year’s Day.
Table 1 Sixty species spotted in flower in Edinburgh, on or around January 1st 2021. The areas of the city were: A, Blackford Hill near the Observatory; B, Midmar Allotments; C, around North Park Terrace; D, Oxgangs pond area and Braid Burn; E, South Morningside; F, Musselburgh..
A few of these plants looked as if they were thriving, but most appeared as if the act of flowering was a ‘last gasp’. One hypothesis to explain anomalous flowering has to do with light pollution. It has been known for some time that trees near street lamps stay green for much longer than others of the same species. The science behind this, is that flowering is often controlled by day-length, or more correctly, by the length of the night. Plants flowering in summer are usually ‘long day plants’ and are stimulated to flower by the short nights. But in winter, if the long winter’s night is broken up by street lights, car-headlights or even the ‘sky-glow’ of towns and cities, these plants may ‘think’ it is summer and duly flower (don’t ask me about moonlight or even starlight, but we do know that some plants are sensitive to very dim light).
Some of the science is described in this open-access review paper by Bennie et al (2016) https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12551
The images we have of the flowers are not up to the standard we usually see on our posts, but a few are presentable.
Fig. 1. Images of some of the species. From top left to bottom right they are: Erigeron karvinskianus in an abandoned church-yard; Phacelia tanacetifolium, often seen at allotments (used to control potato eel-worm); Lamium hybridum (quite common around Morningside and leaves quite distinct from the even more common Lamium purpureum); Senecio vulgaris (on pavement edges everywhere and also a weed); Galinsoga quadriradiata (becoming common – it’s at the clock in South Morningside), Stellaria media (widespread); Poa annua (showing its anthers at freezing temperatures); Hedera helix (flowers and fruits, but most of the flowers are ‘over’).
We might try comparing this list with the BSBI results, https://nyph.bsbi.org/results also looking to see whether there is a North-South difference in a species ‘willingness’ to flower in winter. Maybe next year we can try to compare areas of the city where street lights are nearby with those that are far from artificial illumination.
Oh! Here are a few more pictures that arrived later, on Sunday night, all from Musselburgh, photos by Chris Jeffree.
Fig. 2. Viola arvensis, Sonchus asper, Heracleum mantegazzianum and Cytisus scoparius
Data in this post by Maria and David Chamberlain, Julia and Chris Jeffree, John Grace. Text by John Grace. Photos: John Grace (Fig 1), Chris Jeffree (Fig 2).