Nature’s Calendar: The Dutch phenological network

Nature’s Calendar (De Natuurkalender) is a national citizen science project that aims to monitor, analyse, forecast and communicate the timing of yearly recurring life cycle events. Nature’s Calendar concludes that due to the increase in temperature the length of the growing season is almost one month longer in the period 2001 to 2010 compared to the situation fifty years ago. Nature’s Calendar is coordinated by the Environmental Systems Analysis Group of Wageningen University and the Foundation for Sustainable Development and involves over 30 organisations, 8,000 volunteers and hundreds of school children.

Aim of the project

Nature’s Calendar aims to:

  1. Monitor, analyse, forecast and communicate the timing of yearly recurring life cycle events (phenological events).
  2. Determine the impact of changes in the timing on society.
  3. Develop and implement tools and methodologies that allow society to adapt to changes in the timing.
  4. Increase the awareness on changes in climate and changes in biodiversity.


Nature’s Calendar is a citizen science project which means that the general public is asked to participate in the research by monitoring phenological events like the start of flowering, leaf unfolding, leaf colouring, leaf fall and the first appearance of migratory birds, butterflies or dragonflies. Over 8,000 volunteers are registered as observer. In addition, hundreds of school children participate in the research in the context of the GLOBE program. The observers report their phenological observations via or via paper forms.  The observations are visualized via the website.

Climate change impacts

The Nature’s Calendar project started on 4 February 2001 and is based on historic phenological monitoring programs in The Netherlands. Historic observations are available since 1868. The historic observations form a nice reference to the current situation. Analysis of the historic and recent observations clearly shows that the variation in the timing of life cycle events is largely explained by variation in weather variables. Especially temperature seems to play an important role. Based on the observations we conclude that the length of the growing season in the period 2001 tot 2010 was on average almost one month longer than in the period 1940 to 1968. This difference is mainly explained by the increases in temperature that we have observed.

Applications of phenology

Knowing when life cycle events will take place is important for many sectors in society including health, agriculture, fisheries, tourism, nature management, gardening and forestry. The timing of flowering of pollen producing plants determines the start and duration of the hay fever season. Flowering of fruit trees determines the risk of frost damage and pest control. The timing of fruit ripening determines the timing of personnel availability for fruit harvest. Nature’s Calendar has mainly focussed on health related topics and developed several tools to help society to adapt to changes in timing. Examples include (Allergy Radar), (Tick Radar) and the Oak processionary caterpillar Expert Center (

Based on the relation between weather variables and the timing of life cycle events we, together with our partners, have developed various models with which we can forecast the timing of life cycle events. Examples include the flowering and leaf unfolding of plants, the flight period of butterflies and dragonflies, the start and duration of the pollen season, the migration of toads, the first eggs laid by birds and the activity level of ticks.


Nature’s Calendar is actively communicating the results of monitoring, analysis and forecasting to society via its website, twitter, presentations, markets, educational program and via media. Since the start we actively cooperate with the national radio program Vroege Vogels (Early birds) of the VARA. The active communication to media resulted in hundreds of newspaper articles and numerous radio and television programs. The active outreach is crucial to keep the citizen science network alive. In 2008 the co-ordinators of Nature’s Calendar launched the website Together with over 15 nature organisations we publish at least two nature reports on current developments in nature in The Netherlands.