Number + 棵 + Tree/Plant Noun
棵 (kē) is a measure word (classifier) used for trees, plants, and other vegetation. In Chinese, you must use the correct classifier between a number and a noun. For trees, you say 一棵树 (one tree), not 一个树.
院子里种了三棵苹果树。
Yuànzi lǐ zhòng le sān kē píngguǒ shù.
Three apple trees were planted in the yard.
路边有一棵很高的松树。
Lù biān yǒu yī kē hěn gāo de sōngshù.
There is a very tall pine tree by the roadside.