FACTFILE: Birch sap flows during springtime, before the leaves sprout
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Birch sap is a renewable resource. It is a solution of nutrients produced by the tree for its own use. The liquid flows from the roots to the top.
As the birch tree absorbs water and nutrients from the soil, it also converts substances that it has stored into sugar. When the sap collects dissolved chemicals such as sugars, fruit acids, and minerals, the osmotic pressure builds up in the tracheid cells, and the sap flows from the roots all the way to the crown.
The birch sap season in Finland is in April and May, and lasts from three to five weeks.
The sap begins to flow when the snow around a birch tree has melted. The sap season is over when the first leaves sprout.
Sap can be collected from birch trees whose trunks have a diameter of more than 20 centimetres. The permission of the owner of the tree is needed. A tree from which sap is collected cannot be used for the production of plywood.
Sap should not be taken from trees near roads, landfills, or factories and other sources of heavy metal emissions.
There are many different ways to tap a birch tree. Small amounts of sap can be collected by cutting a bunch of tops of branches and pushing them inside a bottle.
Another method is to drill a slanted hole a few centimetres deep into the trunk about half a metre from the ground, pushing a straw or tube inside to collect the liquid.
A birch tree will produce up to ten litres of sap in a 24-hour period.
Birch sap spoils easily, which places high demands on hygiene for the collection process. The hose and covered container should be kept in the shade.
Birch sap should be refrigerated, and consumed within 24 hours.
If it is not consumed immediately, birch sap can be frozen. If this is done, it is best to do so in single servings, because freezing does not kill bacteria, which continue to multiply when the liquid is defrosted.
At the end of the sap season, the hole in the trunk is left open. New holes should not be vertically collinear with those of previous years.
Helsingin Sanomat / First published in print 18.4.2005
More on this subject:
 The birch sap is flowing again!
From Tohmajärvi to Japan, Korea, and Europe
Helsingin Sanomat
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