Sap is the sticky, yellowy liquid you sometimes see oozing from injuries in tree bark, or dribbling out of freshly sawn timber.

This liquid is made up of the water, nutrients and sugars which the plant requires to survive. The plant transports these vital ingredients through a network of tubes. The xylem network pulls water from the roots up to the leaves, the phloem network takes the sugar produced by the leaves (through photosynthesis) and transports it down to feed the rest of the plant. Damage to either or both of these tube systems will result in sap being expelled.

The amount of sap lost will depend heavily on the plant and the time of year. In some plants, such as Vitis species (ornamental and edible vines), when the spring triggers the plant to start growing again the tube networks have to work really quickly to provide water, nutrients and stored sugars to the rest of the plant (known as ’sap rising’). If these plants are cut (eg pruned) in the spring or early summer they can ‘bleed’ so much sap that the whole plant will be weakened and could die. Other plants where this can occur include Acer (maples), Betula (birch), Carpinus (hornbeam), Carya (pecan and hickory), Juglans (walnut), Laburnum, Magnolia (only the deciduous, spring flowering ones), Morus (mulberry), Populus (poplar), Sophora and Tilia (lime).

Maples also have high pressure sap flows when the weather starts to warm up – this is the time of year that maple syrup producers tap the trees for their sap, to boil it down into maple syrup.

Other plants are unlikely to die as a direct result of a cut. However, if the cut isn’t a clean one (ie if it’s left with jagged edges), diseases could enter the plant at this point and move into the xylem or phloem tubes. From there the disease can be transported around the whole plant.

As well as being a natural response to damage, sap oozing from your tree could also be a sign of an infection, particularly if the sap is dark coloured. So it’s a good idea to be vigilant and check to ensure that any sap you see isn’t a symptom of something worse happening underneath the bark.