Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You have to wash a rock (あら) you found. It looks shiny, but you can't tell until you wash it.
When you return home, please wash your hands and gargle.
Viet is taking the day off today out of shock because his special jeans were washed by his girlfriend.
Oh, we're using pre-washed rice, so you don’t need to wash it.