Bleeding is when

Sap is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

You may notice sap oozing from the wound directly after pruning, or you may simply notice staining some time later where the sap has flowed down and wetted the bark.

Bleeding occurs due to the pressure of sap within the tissues that conduct water and sugars around the plant.

Normally, the sap of deciduous plants falls in autumn to early winter, as the plants lose their leaves and become dormant. Many deciduous trees and shrubs are best pruned while dormant and bleeding is minimised at this time.

In late winter to early spring, the sap starts to rise again, delivering water and sugar to the new leaf buds as they swell in readiness for spring. The sap of some trees and shrubs, such as grape vines, birch and walnut trees, begins to rise very early. These plants are particularly vulnerable to bleeding if pruned in mid- to late winter or early spring, as the rising sap spills out from the pruning cut.

There is no practical method for stopping the flow of sap from a bleeding wound, and in most cases, this bleeding is completely harmless. Binding and wrapping the cut is not recommended, as it is better to allow air to reach the wound and let it heal naturally. Wrapping or binding can cause dampness and lack of ventilation, encouraging the development of fungal diseases. We no longer recommend painting pruning cuts with wound paints (such as Arbrex), as this too may trap moisture and promote fungal infection.

Instead, gardeners are advised to check the recommended times for pruning particular trees, shrubs and climbers. This should minimise bleeding.

The following plants are particularly prone to bleeding, so pruning at the correct time will help prevent it.

Acer (maple): Prune Japanese maples (A. palmatum and A. japonicum) after leaf fall but before January; prune snakebark maples (e.g. A. davidii, A. pensylvanicum, A. rufinerve) in late summer.
Betula (birch): Avoid any pruning if possible; otherwise prune late summer to mid-winter.
Carpinus (hornbeam): Prune late summer to mid-winter.
Carya (pecan and hickory): Prune in autumn or early winter. 
Juglans (walnut): Prune mid-summer, before mid-winter. 
Laburnum: Prune late summer, before mid-winter.
Magnolia (deciduous, spring-flowering types only): Prune early to mid-summer. 
Morus (mulberry): Prune from autumn to early winter. 
Populus (poplar): Prune late summer or early autumn, except for P. x candicans and its cultivars, which can be pruned in late winter and do not usually bleed. 
Sophora: Prune in summer. 
Tilia (lime): Prune mid-summer to mid-winter. 
Vitis (grape vine – edible and ornamental types): Prune after leaf fall but before Christmas to be safe.

Check individual plants in the book, RHS Pruning & Training, by Christopher Brickell and David Joyce. Available from the RHS Book Shop online.