The directions given for heating greenhouses by hot water or by flues, apply of course only to sections of the country where the temperature during the winter months makes heating a necessity. In many of the southern states there is no need of artificial heat. A greenhouse tightly glazed and placed against a building where it is sheltered from the north and north-west will keep out frost when the temperature does not fall lower than 25 degrees above zero, and if light wooden shutters are used to cover the glass, all those classed as "greenhouse" plants will be safe even at 10 degrees lower, provided, of course, that the conservatory is attached to the dwelling, as shown in Fig. 38. Another cheap and simple method of keeping plants during winter in mild latitudes is by the use of the sunken pit or deep frame, which affords the needed protection even more completely than the elevated greenhouse. This is formed by excavating the soil to the depth of from 18 to 36 inches, according to the size of the plants it is intended to contain. A convenient width is 6 feet, the ordinary length of a hot-bed sash, and of such length as may be desired. Great care must be taken that the ground is such that no water will stand in the pit; if the soil is moist it should be drained. The sides of the pit may be either walled up by a 4 or 8-inch course of brick work, or planked up as may be preferred, but in either case the bach wall should be raised about eighteen inches, and the front about six inches above the surface, in order to give the necessary slope to receive the sun's rays and to shed the water. A section of such a pit is shown in Fig. 46. If a pit of this kind is made in a dry and sheltered position, and the glass covered by light shutters of half-inch boards, it may be used to keep all the hardier class of greenhouse plants, even in localities where the thermometer falls to zero.
Fig. 46. - Sunken Pit.