A closer look at flowers: thermoregulation in winter

In winter or in cold habitats such as high mountains, an optimal flower temperature is important for successful reproduction. Some plants can actively produce heat in their flowers, such as Helleborus foetidus, using yeast bacteria in the nectar (Herrera and Pozo, 2010). But this is the exception. For most plants in cold regions (or early bloomers), the more heat they can absorb from the environment, or at least not lose, the better. In this story you will learn what influence characteristics such as shape, colour, pubescence or orientation to the sun have on the temperature in the flower.

Bells and discs
Have you ever wondered why many early bloomers have bell-shaped flowers? The answer is that bells collect more heat than disc-shaped flowers. Hanging bells can absorb heat from the ground radiation and so the temperature inside the flower lies 3-11 °C above the ambient temperature (Kevan, 1989). Upright bells, such as gentians, bundle the sun’s rays when they fall at a certain angle. In disc-shaped flowers, the reproductive organs are directly exposed to the environment and are located in the centre, where most of the incoming light is reflected by the flower. But the petals also play a role. In one experiment, the excess temperature in Saxifraga oppositifolia flowers was reduced by 70% compared to the surrounding area after removing the petals (Kevan, 1970).

Another successful strategy is the formation of “micro-greenhouses”. These are, for example, bubble-shaped structures made of translucent bracts, as in the yellow rattle (Rhinanthus minor), or sepals, as in Physalis. These filter light in the UV range, but allow longer wavelengths to pass through, which heats the air inside. Similar to flowers, hollow stems can also have a heating effect if the heat trapped in the stem leads to an increase in internal temperature (Kevan et al. 2018, 2019). This can promote the development of the flower bud immediately above it.

Orientation towards the sun (heliotropism)
During the course of the day, some plants permanently orientate their flowers so that they face the sun. In cold regions in particular, this leads to effective warming of the flower. This can have advantages for the plant, for example through increased temperatures in the reproductive organs, heavier seeds and more visits from pollinators. Many experiments have already tried to demonstrate the added value of heliotropism, but not always successfully. (Van der Kooi, 2019).

Darker colours can absorb more solar radiation. This can be converted into heat, which can increase the temperature of the flower. In a series of experiments with Plantago, a close relationship was found between the colour of the flower-bearing inflorescence and its temperature. Individuals developing at low temperatures produced darker panicles that were 0.2-2.6 °C warmer in direct sunlight than the comparison group (Anderson et al., 2013). Another study found that purple-coloured flowers of Ranunculus glacialis were warmer and produced more seeds than white-coloured flowers of the same species (Ida & Totland, 2014). However, there are also studies in which the colour of the flower has no influence on the flower temperature (Van der Kooi, 2019). Further studies are needed to better understand the relationship between flower colour, temperature and reproductive capacity.

Opening and closing
The opening and closing of flowers through petal movement is widespread throughout the plant world. In particular, cup- or disc-shaped flowering species protect themselves from external factors such as light, moisture or temperature in this way. Opening and closing can take several minutes or hours, depending on the species. It is assumed that closing the flower protects the pollen from precipitation (washing out, damage) or drying out, which increases its viability. There are various experiments that have investigated the influence of flower closure on the temperature inside the flower: For example, if the bracts of Tulipa iliensis close at cool temperatures, a more constant temperature is maintained inside the flower (Abdusalam and Tan, 2014).

The pubescence of flowers is probably important for maintaining flower temperature, but in contrast to leaf pubescence, this has been little studied to date. Plant species that grow in high-altitude, cold regions sometimes develop thick leaf pubescence. This creates an insulating boundary layer to the neighbouring cold air mass, which reduces heat loss (Meinzer and Goldstein, 1985). The pubescence of the flowers can have a similar insulating effect as in the example of willow catkins: In Alaska, it was investigated that it can be 15-25 °C inside willow catkins at an air temperature of 0 °C. If the woolly hairs were removed, the internal temperatures in the catkins fell by around 60% (Krog, 1955).

• Herrera CM, Pozo MI. 2010. Nectar yeasts warm the flowers of a winter-blooming plant. Proceedings of the Royal Society of London B 277: 1827–1834.
• Kevan PG. 1989. Thermoregulation in arctic insects and flowers: adaptation and co-adaptation in behaviour, anatomy, and physiology. Thermal Physiology 1: 747–753.
• Kevan PG, Nunes-Silva P, Sudarsan R. 2018. Short communication: thermal regimes in hollow stems of herbaceous plants—concepts and models. International Journal of Biometeorology 62: 2057–2062.
• Kevan PG, Tikhmenev EA, Nunes-Silva P. 2019. Temperatures within flowers and stems: possible roles in plant reproduction in the north. Bulletin of the NorthEastern Science Centre of the Russian Academy of Sciences, Magadan, Russia 1: 38–47.
• Casper J van der Kooi, Peter G Kevan, Matthew H Koski, The thermal ecology of flowers, Annals of Botany, Volume 124, Issue 3, 16 August 2019, Pages 343–353,
• Anderson ER, Lovin ME, Richter SJ, Lacey EP. 2013. Multiple Plantago species (Plantaginaceae) modify floral reflectance and color in response to thermal change. American Journal of Botany 100: 2485–2493.
• Ida TY, Totland Ø. 2014. Heating effect by perianth retention on developing achenes and implications for seed production in the alpine herb Ranunculus glacialis. Alpine Botany 124: 37–47.
• Abdusalam A, Tan D-Y. 2014. Contribution of temporal floral closure to reproductive success of the spring-flowering Tulipa iliensis. Journal of Systematics and Evolution 52: 186–194.
• Meinzer F, Goldstein G. 1985. Some consequences of leaf pubescence in the Andean giant rosette plant Espeletia timotensis. Ecology 66: 512–520.
• Krog J. 1955. Notes on temperature measurements indicative of special organization in arctic and subarctic plants for utilization of radiated heat from the sun. Physiologia Plantarum 8: 836–839.
• Kevan PG. 1970. High arctic insect-flower relations: the inter-relationships of arthropods and flowers at Lake Hazen, Ellesmere Island, N.W.T., Canada. PhD Thesis, University of Alberta, Canada.

This article was featured as a story in the Flora-Incognita app in winter 2024. In this plant identification app, you can find exciting information about plants, ecology, and species knowledge, as well as tips and tricks for plant identification. Check it out!

Flora-Incognita observations enable phenological monitoring throughout Europe

A new study by our research group shows that plant observations collected with identification apps can provide information about the developmental stages of plants – both on a small scale and across Europe. [Read the paper]

Why is it important to document phenology?

Many plants in temperate climates go through a cycle of flowering, leaf emergence, fruiting, leaf colouration and leaf fall every year. This process is called phenology and is strongly influenced by local climatic conditions (for example, the number of days per year on which a certain minimum temperature required for growth is reached; see “Growing Degree Day” or GDD). It is, therefore, not surprising that climate change has a substantial impact on phenology. For example, spring begins earlier than it did in the 1950s, meaning the growing season starts much earlier than it did back then. Such changes impact agricultural processes and can also lead to ecological imbalances. For example, plants begin to flower even before their pollinators are active. However, not all plants react equally to climatic changes. Species with a broader tolerance range for warm days, or where other factors determine phenology, are virtually unaffected by shifts. To gain a truly accurate understanding of the influence of climate on plant phenology, it is essential to document the phenology of as many different species as possible, in different countries and geographical regions.

How does phenological monitoring work?

Phenology is already being documented using various methods. Satellite images recognize the greening of entire areas, and cameras in treetops produce automated image series on the condition of the vegetation layer below. Such data sets allow statements to be made about large scales, but hardly allow any conclusions to be drawn about the phenology of individual species or even individuals. For this purpose, there are initiatives that are carried out with the help of trained volunteers. However, the number of these citizen scientists is constantly decreasing, and this type of data collection is usually limited to certain plant species (often trees), countries, or even smaller regions.

Is it possible to document phenology with Flora Incognita?

Data collected via plant identification apps such as Flora Incognita can be a solution. The scientists in our project already proved this in 2023: Plants are primarily noticed and photographed when they are conspicuous and also flower, bear colorful fruit, or autumn leaves. This results in observation patterns that indicate phenological events. These patterns often coincide with those published by the German Weather Service (DWD) concerning the onset of flowering species in Germany. Assuming that the DWD registers an earlier start of flowering of the elderberry in one year than in the previous year, this shift is also reflected in the identification requests from Flora Incognita.
Details of this study can be found in this article: Phenology monitoring with Flora Incognita plant observations.

A new study shows phenologies and bioclimatic correlations across Europe

Our new publication now shows that smartphone observations even reflect known supra-regional phenological patterns, such as

  • the later flowering of many species in Northern and Eastern Europe or
  • the later flowering of many species at higher altitudes, but also
  • a Europe-wide shift in the start of flowering between years, as has already been proven for Germany.

This proves that the data generated by plant identification apps is a reliable source for the occurrence of plants at a specific time and place and is well suited for answering further research questions – even on a larger scale.

The results of the study at a glance

Plants are more likely to flower when there are more warm days
We compared the Europe-wide observation data (sources: Flora Incognita and reporting platforms such as iNaturalist) from 2020 and 2021 for 20 different plant species. We found that spring-flowering plants in particular, such as the gamander speedwell Veronica chamaedrys, flowered earlier in 2020 – up to two weeks earlier than in 2021.

An analysis of the temperature at each location showed that there were significantly more days in spring 2020 on which an average of 5°C or more was reached, meaning that the plants could absorb more heat in a shorter period of time. The effect was less pronounced for species that flower later in the year, such as the tansy Tanacetum vulgare or the common viper’s bugloss Echium vulgare.

Plants flower later if they grow at higher altitudes, further east or north.
However, patterns could be recognized not only between the years but also between different regions. It is known that the same species flowers at different times depending on the location. (-> Hopkins’ bioclimatic law) For example, if the same plant species occurs in Sweden and Spain, the Spanish plant will flower a few days or even weeks earlier than the one in the north. The exact days until flowering naturally vary depending on the plant species. This regularity can also be illustrated with Flora Incognita data:

This figure shows the median of the observation data for the three plant species already presented. Pink and orange indicate that the species flowered early in the year at the respective location, while the same species flowered later at another location (coded dark green and blue). The longitudes and latitudes are clearly differentiated, as well as the low and high mountain ranges. Details of the data and methods used can be found in the publication linked at the end of the article.

All 20 plant species analyzed could be classified into one of three main patterns, illustrated here as examples. Veronica chamaedrys shows more reddish colours in 2020 than in 2021; as already mentioned, this is due to the warmer temperatures in spring 2020. Echium vulgare shows only minor responses to different climatic conditions over the years. For Tanacetum vulgare, we found that phenology shows an inverse pattern compared to the other species: Tansy is more likely to flower in eastern, northern, and high altitudes than its siblings in western, southern, and lower-lying parts of Europe. This phenomenon has also been described in the scientific literature. Species that need many warm days to flower have adapted to cold locations by shortening their vegetation period and flowering earlier.

For the first time, the new publication shows temporal and spatial shifts in plant phenology on a Europe-wide scale using data not collected explicitly for this purpose. For the users of Flora Incognita, this means that each individual plant identification satisfies more than just their curiosity. By documenting plant occurrences at a specific time in a particular place, they create a growing and robust data source on phenology that knows no national borders, includes new species, and can answer numerous further research questions.

Thank you for your curiosity.

The new publication is now freely available:
Rzanny, M., Mäder, P., Wittich, H.C. et al. Opportunistic plant observations reveal spatial and temporal gradients in phenology. npj biodivers 3, 5 (2024). https://doi.org/10.1038/s44185-024-00037-7

Veronica chamaedrys in the title image was captured by Ilse Schönfelder.