HPS Lamps Can Cause Plant Elongation

If you are considering using an HPS lamp exclusively in an enclosed space that does not receive any natural light, your plants may tend to elongate with thinner stems in a shade-avoidance response.

The bean plants on the right was grown from seed under only HPS lighting; note its very elongated internodes. The bean plants on the left was grown under a CMH lamp.

Gardening Under Lights Book