A Festive Bouquet – why do plants flower in winter?
What do botanists do in winter? Do we require a period of dormancy (the plant equivalent of hibernation) like many of the plants we study? I decided otherwise and chose to remain active, making a list of all the plants I found in flower over the festive period. I soon found this had the added advantage of encouraging me to put on my walking boots and get out into the local countryside. It is to be recommended.
I excluded any plants growing in gardens, or deliberately planted, which included all those Polyanthus and winter-flowering Pansies loved by local councils. Altogether I found 26 plants in flower, admittedly not all prime specimens, detailed in the appendix below. Despite visiting several local nature reserves, most of the flowering plants I found were in the urban environment in local towns and would probably be regarded by many as weeds. This may be because of the ‘heat island effect’ of towns or alternatively light pollution. Timing of flowering in many plants depends upon the length of darkness, not daylight which would seem more logical. Fewer nighttime hours due to street lighting might trick the plant into thinking it was either no longer winter or winter had yet to arrive.
The BSBI (Botanical Society of Britain and Ireland) has been running an annual New Year plant hunt for 15 years. As you might expect, more species were in flower, the further south or west you went, and at lower altitudes. Their top four has always remained the same: Daisy, Groundsel, Dandelion and Annual-Meadow grass .
Looking closer at those plants in flower, we can recognise four categories:
Autumn stragglers – summer plants flowering late
All year-round flowerers
Winter specialists – mid to late winter flowering plants
Early spring flowers – flowering early
Autumn stragglers account for most, amounting to 50% of species recorded. This is even more marked when November and December are warm, with temperatures not dipping below 4 °C. Such conditions allow plants to continue flowering well into winter, presumably because of the absence of frosts which would normally kill any late-flowering shoots.
All year-round specialists made up a further 20%, including Gorse, Hairy Bittercress, Annual Meadowgrass, Petty Spurge, Chickweed, Shepherds Purse, and Procumbent Pearlwort. The latter six having a typical weed-like habit. These ‘weeds’ can all successfully self-pollinate, useful in winter with the absence of pollinators. Hairy Bittercress and Procumbent Pearlwort go one stage further showing cleistogamous behaviour, with the flower forming but the petals never opening, but with pollination and fertilisation still occurring.
Examples of mid and late-winter specialists include Oregon Grape (Mahonia), Snowdrop and Winter Aconite, accounting for 5%. Despite keeping my eyes open, I surprisingly never found a Snowdrop or Aconite in flower; they tend to flower in late January here in Wharfedale.
Lastly spring flowers flowering early make up the remaining 25%. Given global warming and climate change you might have expected more spring plants like Primrose, Cowslip and Lesser Celandine. However, the evidence from the BSBI and my list suggests that more species are flowering late than early. This is because very large advances in flowering would be needed for plants normally flowering in March or April to flower at Christmas. In addition, many spring-flowering (vernal) species require periods of freezing temperatures in January and February to break dormancy and begin growth.
Coming across a plant in bloom in winter always brings a smile to my face. Stopping for a period of reflection and amazement that they are in flower (the equivalent of their summer clothes), meanwhile I am wrapped in multiple layers of Rohan clothing.
My festive flower list (plants in flower between Christmas and New Years Day found in Wharfedale. In the order they were found):
• Groundsel, Senicio vulgaris
• Thale Cress, Arabidopsis thaliana
• Shepherds Purse, Capsella bursa-pastoris
• Petty Spurge, Euphorbia peplus
• Daisy, Bellis perennis
• Creeping Bellflower, Campanula rapuncloides
• Gorse, Ulex europaeus
• Oregon Grape, Mahonia aquifolium
• Creeping buttercup, Rununculus repens
• Dandelion, Taraxacum officinale
• Ivy-leaved Toadflax, Cymbalaria muralis
• Annual Meadow grass, Poa annua
• Narrow-leaved Ragwort, Senecio inaequidens
• Creeping Compfrey, Symphytum grandiflorum
• Aubretia, Aubrieta deltoidea
• Primrose, Primula vulgaris
• Hairy Bittercress, Cardamine hirsuta
• Smooth Sowthistle, Sonchus oleraceus
• Yarrow, Achillea millefolium
• White Deadnettle, Lamium album
• Pineappleweed, Matricaria discoidea
• Mexican Fleabane, Erigeron karvinskianus
• Wood Aven, Geum urbanum
• Red Deadnettle, Lamium purpureum
• Chickweed, Stellaria media
• Procumbent Pearlwort, Sagina procumbens