I gave the cactus too much water and I killed it.
You already learned the word 枯れる which means "to wither," but this is when you are the one making a plant wither. It means to let dry or to kill your plants. They didn't just die on their own, you did it.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You already learned this reading when you learned 枯れる, so you should be able to read this word as well.