@villasun: it can be pretty cold in Peru at night even in summer, especially if you plan to wonder in the mountains collecting plants.
You'll want to pack a lot of layers for night time and carry plenty of sun screen and short layers for sweaty hot days.
I'd imagine you'd be hitting the rainforest area, of course, to collect plants. Be aware that it is much warmer and wetter than say
Cuzco, Lima or other high altitude cities, so be prepared for all of it.
Also, if you're going to spend time in the rainforest, be sure to get your shots and immunizations vaccines, and bring water purification tabs and your own toilet paper. Have fun!