Changing seasons

The world of horticulture has always been dictated by the seasons.

Seasons happen due to the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis. In temperate (such as the UK) and polar regions, the seasons are marked by changes in the amount of available sunlight which may cause animals to go into hibernation or to migrate, and plants to be dormant.

Although the astronomical dates and meteorological dates for the start of each season don't change (see table below), the behaviour of plants and wildlife and weather patterns have begun to change. Many now see this as outcome of climate change.

Season start dates

Season
(Northern Hemisphere)
Astronomical Meteorological
Spring 21 March 1 March
Summer 21 June 1 June
Autumn 23 September 1 September
Winter 21 December 1 December

Signs of change

The growing season — the period of time each year during which plants can grow — is an important indicator of what is happening to the world of horticulture under climate change.

The length of the thermal growing season is defined as beginning when the temperature on five consecutive days exceeds 5 °C, ending when the temperature on five consecutive days is below that threshold.

Research, looking back 233 years, has found that since 1980 the length of the growing season has been significantly changing.

  • The earliest start of the thermal growing season was in 2002 when it began on 13 January
  • The longest growing season in the 233-year series was 330 days, in 2000
  • The shortest growing season was 181 days in 1782
  • The 2006 growing season was 269 days, well above the 1961-1991 average of 252 days.

According to research, Gardening in the Global Greenhouse, spring has advanced by two to six days per decade and autumn has been delayed by two days per decade.

So far researchers have found that:

  • The flowering and leafing of plants are happening 10 days earlier and particular species seeing even earlier flowerings.
    • Lesser celandines, commonly thought of as the first spring plant, are flowering three weeks earlier than in the 1950s.
    • Wild cherries are now blossoming two weeks earlier than in the 1970s.
    • Oak leaves in southern England are sprouting 26 days before they did in 1950.Poppies are flowering a fortnight ahead of when they used to be.
    • Stinging nettles are 10 days ahead.
    • Rowan, box and cow parsley are all flowering nine to 15 days earlier than they did 20 years ago.
    • Sycamore is responding fastest to climate change compared with other large trees. Hawthorn and hornbeam are also coming into leaf earlier.
    • Snowdrops have been flowering in November (traditionally a February flower).
  • Summer drought, as seen in 2006, results in premature leaf aging
  • Higher autumn temperatures result in delayed leaf fall
  • On average, lawns are having to be cut almost two weeks earlier than seven years ago. In some areas they are having to be cut throughout the year.

By 2080 it is likely that the growing season will have increased by around 40 days.