This process is the firming of the tissues in order that the plants will be able to endure the hardships of transplanting and of open-ground conditions such as freezing, hard drying winds or hot sunshine, any of which may damage or destroy soft, tender plants. Figure 42 shows a frame of well-hardened cabbage plants which when photographed were of reddish-blue color, short and stocky. Such plants will stand a temperature of 12 or 15 degrees above zero.
Plants are hardened by watering sparingly, subjecting them to low temperatures and by providing free ventilation. These operations are equally valuable. When hardening is begun, no more water should be used than is necessary to prevent serious wilting. Air is admitted more freely from day to day. At the end of three or four days the sash may be removed entirely during the day, and the frames closed late in the evening and opened earlier than usual the next morning. Matting is not practiced after a few days more of such treatment and, finally, no protection of any kind is given day or night. This general plan of hardening is primarily for the more hardy plants, as cabbage and lettuce. Tomatoes, peppers and eggplants must be handled more cautiously, although hardening is just as necessary for them. Millions of plants are lost annually because they have not been properly hardened.