The answer to 'why is my hardwood floor mildew' is that excess moisture is the primary cause of mildew growth on hardwood floors. Mildew thrives in damp, humid environments and can form on wood surfaces that are not properly sealed or dried.
Some common causes of mildew on hardwood floors include poor ventilation, high humidity, water damage, leaks, or excessive cleaning with too much water. Symptoms include a musty odor, visible discoloration or black/green spots, and the floor feeling sticky or slippery. If left untreated, mildew can damage the wood and lead to more serious issues like rot.