Europeans have rather a binary view of when plants grow and when they don’t – growth in spring and summer, and none in winter. Deciduous trees give this impression, but as someone who did not grow up in Europe, I never had a sense that there were such specific times of growth – all around me were evergreen trees which seemed to me never to stop growing! So what actually is the truth for roots?

Obviously deciduous trees don’t photosynthesise when they don’t have leaves, but evergreens can and do photosynthesise throughout the year – albeit less intensely during winter. In a study on Picea abies “photochemical activity was high during early fall and then declined from November until April. Photochemical activity was at a minimum in April and then increased quickly to high values in May”^ref. A study in Idaho looked at four evergreen coniferous species and found that their photosynthetic output varied with temperature but did not go to zero^ref (shown in the chart below where the start of the chart is 7th September 2001)

So do roots also show growth activity during winter? Some studies have looked into this question:

• In established Scot’s pines, root growth has been shown to accelerate in early spring, then to back off while there is strong shoot growth, then to pick up again after this has finished in later summer and autumn.^ref Root growth did occur during winter but not at the same levels.
• In 15 & 20 year old Sitka spruce, root growth up to 0.5 m from the stem base had a minor peak of activity preceding and a major peak following shoot elongation in the spring, while further than 0.5 m from the stem, root growth was frequently restricted to the period following shoot extension (effectively – summer and autumn).^ref
• In 3-year-old Sitka spruce and Douglas-fir, height growth ceased in late September, but roots continued to elongate until mid-November, and the peak in root elongation occurred after height growth had stopped in one-year-old Douglas fir.^ref

The data points above suggest that root growth happens when resources are not prioritised for shoot growth. Temperature is another driver that has been identified – in one study covering six species, 85% of all new roots were grown in soil above 9 °C, and 6 °C was identified as the likely soil temperate threshold for root growth – with “a sharp increase of root growth between 6 and 9 °C, followed by a marked attenuation from 9 to 16 °C”^ref (the species in the study were Alnus viridis, Alnus glutinosa, Picea abies, Pinus sylvestris, Pinus cembra, Betula pendula). The root growth in this study was 40% less than the root growth in controls at warmer temperatures (16-23 °C). A similar dropoff temperature for root growth was also found for alpine species in a different study conducted in the Swiss Alps.^ref

What this suggests is that regardless of being deciduous or not, trees are able to use their stored energy to grow roots at temperatures above 6 °C. Looking at ‘Central England’ data from the UK Met Office (“representative of a roughly triangular area of the United Kingdom enclosed by Lancashire, London and Bristol”), during 2021^ref there were 332 days which went above this level at some point (ie. the max temp was greater than 6) and 200 days when it was above 6 degrees for the whole day with nearly 20% of these occurring between December and April. Most months other than January & December had a majority of days exceeding 9 degrees at some point, but the only months for which this was the case for the whole day were June – September. So in Central England there will be some root growth throughout the year, but not as much as during the summer months.

From a bonsai enthusiast’s perspective, this information tells you that roots grow at different times to shoots and often later in the season. Also – if you were wanting to encourage root growth you could consider having your trees in a greenhouse or sitting on a heated horticultural mat, as this would provide the higher temperatures needed.