If storing vertically along such a wall, support it between two acid-free boards. Mould or mildew should not develop if the temperature is kept below 75 degrees F. and the relative humidity below 60 percent. Aim to keep the paper free of dust, damp and light wherever you store it. Storing under bed or in a cupboard, or in plastic can cause mildew growth especially in humid climates. Rolling the paper can damage its structure.
Acid is arguably the worst enemy of paper.
Improper storage in acidic materials such as cardboard boxes, ordinary file folders, or surrounded by acidic matboards will rapidly damage your acid-free watercolour paper. Acid in the wood pulp can migrate aggressively into your archival paper, staining and damaging it.