The plural of पेड़ is पेड़ itself. However, in the oblique case ( i.e. when it is followed by a postposition), the plural पेड़ becomes पेड़ों while the singular पेड़ remains the same.
Let's take an example sentence and elaborate:
मैंने पेड़ लगाये (I planted trees.)
You can write the sentence in two ways:
मैंने पेड़ लगाये। - where 'पेड़' is in the direct case and the form of the verb लगाये indicates that it (पेड़) is a plural word.
मैंने पेड़ों को लगाया- where पेड़ों is in the oblique case due to the postposition को