A frame piped and heated by steam is shown in Figure 21. Hot water may be used in the same way. When heated by either method, mats may be dispensed with. The principles of steam or hot water heating must be observed to secure satisfactory results. These systems of heating are becoming more popular every year. Drain or sewer tile may also be laid in the soil of the frames, steam being conducted through them under some pressure.
Hotbeds are used for starting early vegetable plants. The seedlings are started in the hotbeds and transplanted into other hotbeds, cold frames or the field. Hotbeds are also used in forcing some crops to maturity. The most popular vegetables for hotbed forcing are radishes and lettuce. Spring and summer crops are frequently matured in hotbeds, being started over the fermenting manure, or over manure whose heating qualities have been (exhausted. Among the crops grown in this manner are cucumber, musk-melon, eggplant, squash, tomato, cauliflower, kale, spinach, radish and lettuce.