Mildew -- a type of mold -- comes from fungi and can grow on fabric in dark, damp places. If you've had your curtains in storage and they got wet, you might find small black dots all over them. This is mildew, and it is most common on natural-fiber curtains such as those made of cotton, linen, rayon or silk. But the good news is that a little mildew doesn't mean your curtains are ruined. Silk drapes might require dry-cleaning, but plant-fiber curtains are usually machine-washable.
Add water to baking soda in a small bowl, stirring constantly until it is a thick, sludgy paste. How much baking soda you need depends on how much mildew you have. Make enough paste to scrub all the mildew spots with the paste.
Dip a toothbrush into the baking soda paste and scrub all the mildew spots on the curtains. Allow the baking soda to sit for 30 minutes on the spots.
Set your washing machine to the gentle cycle. Adjust the water temperature to cold to prevent plant fibers from shrinking.
Add laundry detergent and bleach or a bleach alternative. If your curtains are white, add 1 cup of chlorine bleach. If they are colored, use a scoop of oxygen bleach instead, or add 1 cup of white vinegar, hydrogen peroxide or baking soda to the wash.
Remove the curtains from the washing machine after the laundry cycle finishes. Hang them in the sun to dry.
Add 20 drops of tea tree, eucalyptus or lavender oil to 2 cups of water in a spray bottle. Shake up the bottle and spray the curtains after hanging them in the windows to inhibit future mildew growth. If you live in a damp climate and mildew tends to grow on the curtains while they're hanging, spritz them weekly.