Many painters use a palette knife with watercolour but I usually substitute parts of cut up credit cards especially for this following technique.
These are two ways of producing a bark effect – especially useful for silver birches.
1. Dip the card or side of the knife into wet colour and drag the paint across the tree horizontally. 2. Place paint onto one of the side edges of the tree and drag it across with the card. The trunk can be dampened first with clean water. Experiment by lifting unwanted paint out and adding more darks but remember to get all the colours mixed and ready first. Smaller branches and twigs are best painted with a rigger. Load the brush with paint and follow the direction of growth. Lift pressure off the brush towards the finer tips of the twigs.
A useful tip to help depict the cylindrical form of a trunk is shown below: Remember sad mouths above eye level and happy mouths below eye level.
Don’t take my word for it- go and look.