Climate indicator – Length of the vegetation period

The growing season means the time of year that is warm enough for different plants to grow. The length of the vegetation period affects, for example, agriculture. The trend is that the vegetation period in Sweden is getting longer and longer. This can, for example, lead to larger harvests and the possibility of harvesting more times per year than before.

Comment on the results

The trend shows that the vegetation period starts earlier and earlier in the year and ends later,  meaning the vegetation period will be longer. The extension of the vegetation period is somewhat greater in spring than in autumn. For northern Norrland, it is thus far difficult to see any significant extension of the vegetation period in autumn.

The vegetation period began to start earlier than it did before from around 1990. It is interesting to note that the extent of sea ice and the number of days with snow cover decreased at about the same time.

The growing season in Sweden as a whole today, is approximately three weeks longer than it was at the beginning of the 20th century. In Götaland the extension is approximately five weeks and in northern Norrland about two weeks.

How is the indicator length of vegetation period defined?

As the name suggests, this refers to the period of the year which is favorable for vegetation. There are many factors that affect the vegetation, for example temperature, precipitation, sunlight and disease attacks. For this indicator, the following climatological definition has been used:

  • The start of the vegetation period in a given year has been defined as the first day in a period of six days when the average daily temperature during all six days is at least +5.0°C.
  • The last day of the vegetation period is the day before the first period of six days after 1 July, when all six days have a daily average temperature below +5.0°C. If no such period occurs after 1 July , the vegetation period ends on 31 December.
  • The length of the growing season is the number of days from the first day of the growing season to the last.

Why is this indicator important?

The vegetation period is primarily linked to food production, but in the context of climate change it can also be seen as an indication of how conditions for vegetation have changed since the 20th century.

The start and end of the growing season affects the ecosystems, the water balance and not least the conditions for harvesting. A longer growing season usually means larger yields. A shorter vegetation period, on the other hand, can mean poorer harvests with increased costs for and dependencies on food imports. 

How has the indicator been calculated?

A total of 55 stations have been used. 13 of these stations are located in northern Norrland, 9 in southern Norrland, 10 in Svealand and 23 in Götaland. Data from a station has, if necessary, been connected with a nearby station to get as long a series as possible. No homogenization of the data has been done.

When calculating the vegetation period for Sweden as a whole, the average value of the different parts of the country has been calculated. For the different parts of the country, the average value of the stations in each part of the country has been used in the same way.

Vegetation period in the future

The growing season is expected to be longer. By the end of the century, the growing season may be 30–100 days longer than during the period 1961–1990, depending on the extent of global warming.

The change is estimated to be larger in southern Sweden and smaller in northern Sweden. All climate model simulations provide an extended vegetation period. The growing season is expected to be extended approximately as much in spring as in autumn.