The tree I took great pains to grow withered.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means wither, so the verb version is to wither or to die.
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!