In all cultural notes there is more or less occasion to refer to bottom heat. There was a time when few cuttings were thought to root well without the aid of bottom heat, unless it were the cuttings of the ericas and conifers. Practice has taught us that to have the heat of the sand or propagating material greater than the temperature of the house is with many cuttings entirely unnecessary. Ten or 15 degrees warmer will certainly hasten the rooting of most of our soft-wooded plants, and with some it is a decided advantage, while with others, carnations and geraniums, for instance, it is not desirable.
Where bottom heat is essential there is no way so inexpensive or durable as having the hot-water or steam pipes under the benches and inclosed so that the heat will remain under them. The hotbed (primitive greenhouse) is ideal as a means of affording bottom heat, but it is of short duration, being available only during the spring and summer months, and is always liable to neglect. Years ago in growing plants requiring bottom heat many a day was laboriously spent in carting into the houses tan-bark, leaves and other fermenting material to afford heat to plants. That, however, is past and only the hot-water and steam pipes are now used.
You yet see, in many extensive places, the propagating bench made of boards and cypress boards would last many years. If you wish to build more substantially, then the walls could be cement, or rather what should be called concrete, six inches thick. If the walls are of concrete then a frame should be built into the wall every three or four feet to afford a sliding shutter to let heat escape from beneath the bed in extreme cold weather. These openings may be one foot to two feet long. If your walls are wood then one of the 12-inch boards the whole length of the bed can be hinged and can be closed or open, just as desired. It may be well to say just here that in small establishments the propagating bed is more often a bench against a side wall. In larger establishments, where cuttings are rooted by the hundreds of thousands, the propagating bed is removed from the wall, is perhaps six feet wide, with a path on each. In this case openings should be made on both sides of the bed. The floor or bottom of the bed should be of slate or slate and brick. A 1-inch board would be a poor conductor of heat; in fact, none at all.
Across the walls should run pieces of tee iron (not at all expensive) every twelve inches. These will bear the slates or tiles. Large slabs of slate are expensive and we have used for years a common roofing slate 12x24 inches. The trouble with these thin slates is that they heat rather violently and cool very quickly. On these slates we have seen bricks laid, and then the four inches of sand. This is an excellent bed, keeping a moderate, uniform heat. There is also a clay tile made some 12x18 inches and one and a half inches thick. These make an excellent bottom for the propagating bed, much preferable to the slate.
Whether the heat is supplied with hot-water pipes or steam, you should have control of them; that is, you should have valves to turn the heat on or off as desired. The heating pipes should be at least eighteen inches below the floor of the bed, so that the heat will be well diffused and not strike the tile or slate too violently on one spot. We have had three 4-inch hot-water pipes in a bed three feet six inches wide. It was too much. Three 2-inch pipes would be plenty, or two 1 1/4-inch steam pipes. For a bed six feet across three 1 1/4-inch steam pipes would be ample, with valves on every one. For the construction and shape of the propagating house see Greenhouse Building.