Then, when the rains come, the vines will drop roots and extract additional nutrients from the soil. The common white styrofoam is an open cell foam. It will absorb water over time and will sink if left in water. The blue styrofoam, is a closed cell foam. It resists water and holds up much better over time.
Light is precious, and excess heat is hard to remove.
The white styrofoam reflects light back onto the plants better, and keeps the plant shoots cooler. A simple and reusable, plastic nursery grow cup, for larger plants. Your medium should wick water, and breathe well. In the photo below, taken from the moon by Apollo 16, we see the effects that light has on our planet's atmosphere. Photons from the sun, knock electrons out of their orbitals, around atmospheric gases.