Now for evergreen trees the myth of a growing season is even more dangerous as they do not give such clear clues as deciduous trees do when the buds burst in spring or drop their leaves in winter. Also it is possible that they come out of ‘hibernation’ and grow in the depths of winter if the days are warm enough (or if we stick them in a warm greenhouse).
The phenomenon that causes a tree to start growing is known as ‘day degrees’. It is simply the temperature each hour of the day added together to get a total figure for the day. When this figure is high enough the tree will start growing. So a hot day in winter can trigger a higher day degree figure than a longer colder day in spring when you might expect a tree to be growing. Whenever a tree is growing it needs water, so you must always be alert with evergreen trees that the soil remains moist through the winter. I also think it is worth repeating a sentence from the last blog: it is always prudent to ensure the soil is slightly moist as a desiccated soil can draw moisture reserves out of roots and could lead to ‘freeze drying’ (the same way food in the deep freeze can dry out if the packet is open).
Evergreen are also more able to be active at lower temperatures because they quickly incorporate water with their resinous sap, a substance able to withstand freezing at normal temperatures. The plant can pump this sap around the system causing growth to occur even at incredibly low temperatures. For evidence of this look at the growth rings of a felled tree – you will see wide summer rings where growth is at its most vigorous but material is added to stem diameter throughout the winter period.
Therefore it is safe to assume that the Growing Season never really stops regardless of the species and winter watering is needed. Now having said all of that of course trees are clearly more active in the spring and summer and need much greater volumes of water during hotter months.