From my Besom Making class:

The besom has long been a tool long used by both magickal and mundane people to cleanse the home and superstitiously to ward off “evil”.  As you may know, in the times when pagan beliefs had to be concealed, symbolism was found in mundane objects, such as the broom.  Many other ritual tools would be made out of clay, wood or some other easily destructible material so that when the witch hunters came knocking they could be destroyed or easily hidden.  An obviously phallic handle of Ash, buried in a knurl of Birch twigs and bound together by Willow withe allowed for maximum symbolism in a very mundane, abundant and necessary household item.  

 Ash is a masculine wood and is often the chosen variety of wood for the handle because it represents strength and flight.  Ash is also one of the few woods that will burn whether still green or cured, indicating constant enduring strength that is always at the ready.  Ash was also the favored wood for arrows and spears due to it=s natural strength and straightness.

 Birch is a feminine wood representing protection, fertility, purification, blessings and beginnings.  Birch is found in cleansing rituals and has been used extensively for that purpose, which is probably why Birch is traditionally used for the brush, or knurl of the besom.  (Traditionally, when people would “beat the bounds” in the springtime to chase out the accumulation of negative energy, it was done with a bundle of Birch.)

 Willow is the binding, and it represents exactly that, the binding force.  Willow is a feminine wood and is generally held sacred to the Goddess.  It has an affinity for water and exudes emotion.  Willow is the tree of the Lady and is suitable to balance the force of the Lord.

 The besom was also involved in different marriage lore.  It is customary for a bride and groom to jump over a broom after they have been ritually united as it is a symbol of fertility and male joining with female.

A besom laid over the threshold or stood in the corner with the brush up was said to keep unwanted negative energy out of the home.