Q.: We want to put a porous floor in our greenhouse but don't know how this is done. Can you provide information?

A.: A no-fines concrete should be used to produce a floor that is porous enough for water to drain through. Such a concrete is said to be useful to prevent crop damage from water that collects in low areas. The pores in this kind of floor must be made large enough to prevent water from rising by capillarity.

A 3-inch-thick slab is adequate for personnel and light-vehicle traffic. However, if carts or vehicles are to be moved over long distances it is best to use regular concrete in the pathway areas because the porous floor is likely to be too rough for smooth movement. If the porous concrete will be subject to cycles of freezing and thawing it should be placed on a well-drained base.

Rutgers University recommends the following quantities of material for a 1-cubic-yard batch of no-fines concrete:

517 pounds

Coarse aggregate, 3/8 inch


Fine aggregate



3 to 192

The mix is said to be stiff and difficult to handle; addition of an air-entraining admixture might help. After it has been placed the concrete is struck off but not troweled, then cured in a normal manner.

In use the floor accumulates plant debris. It should be cleaned with a stiff brush or vacuum cleaner between crops to promote good sanitation. A floor heating system utilizing plastic pipe can be incorporated in the slab if needed. This can contribute to lower ambient air temperatures and eliminate the need for benches for many crops. A 10-page publication describing such a system, "Floor Heating of Greenhouses," by William J. Roberts and David R. Mears, is available from the Biological and Agricultural Engineering Department, Cook College-Rutgers University, Box 231, New Brunswick, New Jersey 08903.