Plants are well adapted to life, part 1.
David Beeson, August 2020
Annuals, biennials and perennials
Evolution, through Natural Selection (Survival of the Fittest), is a powerful force. Death does that! If a plant’s strategy is poor, it dies and fails to pass on its genes. On the other hand, if it gets everything perfect, it will have many viable offspring and its genes are not only passed on but multiplied. And, that’s the aim of life.
Annual plants are often agricultural weeds, or plants surviving alongside soil disturbers such as pigs or badgers or rabbits. They germinate in the spring, flower and seed during that year, and die when adverse weather conditions hit them. The parent plant might have died, yet its progeny will have over-winter protection and germinate next spring when adverse conditions are past.
Garden centres are full of annuals in the spring – but put them into your garden too soon and you will lose them all to frost. Poppies are an example of an annual.
Biennials have a different life strategy. Mostly they germinate in the spring, grow a whorl of ground-level leaves and then store their products of photosynthesis in their roots. In the late autumn, they cuddle down for the winter, and remain comatose. Spring temperatures and sunlight levels stimulate growth using up their stored starch or oils and proteins; they grow tall to have their flowers visible to pollinators, set seed and die. In the UK, foxgloves are good examples of biennials.
Perennials can be large organisms. They hold the resources produced each year and do not die back. Redwoods, oaks and eucalyptus can live for many years, and put on tonnes of weight. However, they too have their challenges, especially away from the tropics were frost, snow and freezing temperatures may persist for several months. Of course, other environmental factors can cause dormancy. For example, summer drought or extreme temperatures.
Perennials need well-protected buds, to keep the growth points (meristems) away from weather damage. This often involves having thick, closely overlapping scale buds that hold out moisture. Inside the bud will be low in moisture and may have added anti-freeze to stop needle-like ice crystals forming and destroying the cells.
Scale buds are best seen on horse chestnut buds.
Whatever the life strategy, a plant needs to flower early enough to disperse full-formed seeds. Genetic material that are protected to remain viable until germination time. Flower too early and frost could kill the plant. Too late and the seeds will not have enough development time to be viable.
In my local forest, Harewood, many plants flower in March or April. They store resources in swollen roots or bulbs so they can rush to leaf and flower before the trees shade them out and pollinators vanish. Inside a woodland, temperatures are more moderate than in an open site, so frosts are less likely and so allowing early flowering.
Many plants flower later in the growing season, allowing maximum growing time, photosynthetic resources and, hopefully, lots of seeds. In my garden the flowering peak is May or June for the flower borders and June and July for the wildflower meadows. Some plants only flowering in August, but they are in a minority. I have to selected plants carefully to have flower-power late in the year – Japanese anemones, asters and fuchsias … these being exotics, not native plants.
Our native trees and shrubs flower from February (cherry plum) through to July (privet). Non-natives, such as hibiscus, may flower into September, yet I doubt viable seeds will be produced before the first frosts hit them.
2020 had a wet spring, but lacking cold weather, and seed / fruiting has been exceptionally heavy. Our English oak is currently shedding a huge crop of acorns. The winter birds and active rodents will have plenty of food.
(At this time of the year, domestic pigs are released into the New Forest – called pannage. The hogs are keen acorn eaters, so reducing the chance of forest ponies being harmed by the acorns’ tannins. Pigs are immune to the toxic effects of the tannins.)
(Non-native plants will have evolved under different environmental pressures, so their flowering strategy will be non-adapted to southern UK conditions.)
We will return to flowering shortly. It is a complex topic!