Here are some signs to watch out for that may indicate mildew growth on a waterproof outdoor throw blanket:

Musty odor

One of the first signs is a musty, earthy smell coming from the blanket. Mildew has a very distinct odor.

Visible gray fuzz or powder

Look closely at the fabric for grayish fuzzy patches or a chalky powder on the surface. This is mildew growing.

Discolored spots

You may see lighter or darker dots, circles, or discolored areas where mildew colonies have taken hold.

Damp feel

If the blanket feels cool or damp to the touch, that moisture could be feeding mildew.

Loss of water resistance

If the blanket is no longer beading up water and soaking through, mildew may be breaking down coatings.

Structural damage

Over time, mildew can deteriorate fibers and weaken blanket materials leading to holes or tears.

Bunched filling

If it’s a quilted blanket, look for sections where the filling is starting to clump together from moisture.

Musty smell when stored

A blanket that smells musty when removed from storage likely has mildew present.

Catching mildew early is key to preserving outdoor blankets. At the first signs, clean thoroughly and allow to fully dry before reusing. For stubborn mildew, washing with bleach may be required.