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 this reading yet so here's a mnemonic to help you:
How do you get plants to wither? You hit them with your car (か). It isn't very good for your car either, but the plants will definitely die, so problem solved!
The tree I took great pains to grow withered.