When Will Spring Start? Examining Why The Start of Spring Varies Each Year

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Why does knowing how early Spring will start really matter?

Collecting yamadori: Before collecting trees from the wild it is always best to wait until the roots of a deciduous tree are active and the energy of the tree is rising from where it has been stored over the winter. This is indicated in late Winter/early Spring by the 'colouring-up' of the small dark winter buds on a tree, as they take on their Spring bud colour and start to plump up. The collecting season ends when (deciduous) trees have come into leaf.

Repotting: Although I would always advise leaving the repotting of trees until as late as possible (when as much of the energy of the tree has risen into the branches and the tree is about to come into leaf), the appearance of Spring buds is a sign that the roots are starting to become active and can be safely pruned and bonsai repotted.

Knowing when Spring proper, that is leaf-burst, is likely to start and all repotting has to be finished, allows the enthusaist to plan their repotting schedule, important when they have a lot of trees in their collection.

Leaf-Size: The size of leaves of a bonsai can be dramatically altered by when the tree comes out of dormancy. A deciduous tree opening its leaves in early March when light levels are lower and weaker, will have bigger leaves than if it comes into growth 2 months later in May.

The effect of types of (sun)light

Finally, the type of light that hits the dormant buds of a deciduous tree, or the leaves of coniferous species for that matter, makes a huge difference to when the tree starts into growth.

During the winter when the sun is low in the sky, the sun light hitting the atmosphere of the Earth is refracted in such a way that we receive a higher level of UV-A than UV-B.
As the year progresses, the sun sits higher in the sky and the balance of UV-A and UV-B changes as UV-B levels become higher. This increase in the proportion and level of UV-B, triggers leaf-burst and actual extension growth varies for different plant species.

Determining exactly when each species will come into leaf, and attempting to influence this date artificially, is a complex business and can only be dealt with on a species-by-species basis. Some species, particularly shrubby trees such as Acer palmatum 'Kiyohime', native to the woodland floor, have evolved to come into leaf early, before trees in the main canopy open their leaves and soak up the majority of the available light. Other tree species will require a certain number of hours of light before coming into leaf properly; as a for instance, Beech/Fagus sylvatica require a full 13 hours daylight every day before they will open their leaves.

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