More than a need for “fresh air” as such, house plants need a daily change of air. In any house, gases, smoke and dust particles will accumulate in the air. Over a period of time they can become quite harmful to any house plant. The big offender in this category is manufactured gas. A pilot light that has blown out, and whose gas exhalation is so slight that normally the odor will go unnoticed until the next time you go to light the stove, is enough to kill almost any plant in a matter of hours. Giving your indoor garden enough ventilation in the warm weather is no problem, just open the doors and let the air circulate. In winter it’s a different story, however. You should try to see that the plants get a daily change of air but that they are never subjected to a direct draft. Allowing plants to stand in a freezing winter draft for even as short a time as fifteen minutes can sometimes kill the hardiest of individuals.
The roots of your plants also need a kind of ventilation. The surface of the soil should be turned over every now and again to the depth of an inch or so to allow air to get in among the roots, and also to allow the plant foods to reach down to where they will do the most good. This is covered in more detail in Chapter 4, About Soil.