There are other factors that can cause stress to plants and skew the heat-zone rating. Some of them are more controllable than others. Plant cells require oxygen for respiration. Either too much or too little water can cut off the oxygen supply to the roots and lead to a toxic situation. You can control the amount of oxygen your plant roots receive by making sure your plants have good aeration-adequate space between soil particles.
Light affects plants in two ways.
First, it is essential for photosynthesis-providing the energy to split water molecules, take up and fix carbon dioxide, and synthesize the building blocks for growth and development. Light also creates heat. Light from the entire spectrum can enter a living body, but only rays with shorter wavelengths can exit. The energy absorbed affects the temperature of the plant. But you can adjust light by choosing to situate your plant in dappled shade, for instance, if you are in its southernmost recommended heat zone.