Tree sap is the lifeblood of a tree: a clear, thin, colourless liquid that is sticky to touch. Tree sap is contained within the bark and outer tissues of leaves and stems, but can leak out if the tree is damaged, pruned at the wrong time, or is exuded by sap-sucking insects.
Humans have harvested tree sap to drink and eat, and use for a variety of practical purposes, for thousands of years, and small-scale farmers and foragers still harvest tree sap today. The best-known product made from tree sap is maple syrup.
What is tree sap?
Tree sap is made up of water and dissolved substances taken up from the soil by the roots, along with sugars and plant growth hormones produced by the tree’s leaves. It’s transported around the tree by two different types of vessels known as the xylem and phloem. Xylem transport water and nutrients upwards from the roots, while phloem transports sugars and plant hormones generated in the leaves.
Most types of tree sap are not flammable unless the sap contains aromatic oils. It’s the thicker, sticky substance known as resin, which is produced by coniferous trees such as pines, that is flammable.
Why do trees produce sap?
Trees need sap to live. When sap is exuded by a tree it means the outer protective surface is damaged – just the same as if we cut ourselves and bleed. However, in the case of trees, the sap is more likely to flow, or bleed, at particular times of year. Bleeding sap can weaken a tree or allow disease to enter – again, like human bodies.
Getting the correct timing for carrying out pruning work is important to ensure the tree does not bleed excessively and heals as quickly as possible. For most deciduous trees, major pruning that involves sawing off branches, is best carried out in autumn and winter when the tree is dormant, as most sap is stored in the roots at this time. However, there are exceptions – trees in the genus Pruus (cherry) are best pruned in summer when cuts heal fastest, as they are susceptible to diseases like canker and silver leaf at other times of year.
Insects that feed on tree sap, such as aphids, mealybugs, and scale insects, are known as sap suckers. As they digest sap they exude a sticky, sugar-rich substance known as honeydew. Where honeydew falls onto vehicles, garden furniture and leaves it can attract a powdery black fungus known as sooty mould, so is best cleaned off promptly where possible.
Trees that produce edible sap
The sap of many trees is edible. and some types of tree sap taste sweet and can be good to eat. Sap that is high in sugars has been used as a source of sweetness for thousands of years. Tree sap is also fermented to make alcoholic beers, wines, and liqueurs. The process of harvesting sap is known as ‘tapping’ and can only be done during a short period of the year, when the sap is rising strongly within the trunk.
Maple tree sap is the one that is most commonly harvested and boiled to produce delicious maple syrup. This comes from the sugar maple tree (Acer saccharum). Other species of maple also produce edible sap, as do birches and walnuts.
Commercially produced products containing tree sap are usually processed in some way, often by boiling the sap first to concentrate the levels of sugar. This also kills any harmful bacteria. It is not recommended to consume raw sap directly from a tree.
