How High are the Pipes From the Soil Surface?

Q.  I have a question concerning the height of the pipes in grow boxes.  The book says to lay 2″ X 4″ X 6″ blocks down across the bed width, but shows two pictures – one with the block laying flat, which would put the pipe 1.5 inches off the ground. The othe picture shows the block on its side, which would put the pipe 3.5 inches off the ground. What is the recommended height or what works best?

A.  The PVC pipes must provide water to the roots of your plants.  In order to do this the height of the pipes from the ground may need to be different, depending on circumstances.  In an 18″-wide Grow-Box, we recommend the pipes be at least 3 1/2″ from the soil surface.  The reason is that the water will not travel very far horizontally, but goes mostly DOWN when it hits the soil.  Therefore, it is important to get the water spray close to, but not hitting the plant stems.  Otherwise you will waste water, and your plants may not get enough.  On the other hand, if the water hits the plant stems – particularly when they are tiny, it may wash them right out of the soil and kill the plant. 

If plant stems are 2″ from the sides of an 18″ box, pipe sitting 3 1/2″ from the soil surface on a 2″ X 4″ block will be 6″ MOL from the plant stems.  With holes drilled at 45 degree angles, the water will hit the soil 2 to 2 1/2″ from the stems.  Once the plant is established, or if you’re using transplants, the root systems will go deep enough that the water will reach them.  But with plants from seed, the water may not be close enough during the first 2-3 weeks.  If this is the case, you may need to raise your pipes another inch or so. 
In Soil-Beds, unless you are growing in very sandy soil, the water will generally fill the entire bed from side to side and end to end, and so the height of the pipes from the dirt is not as important, so long as you do not wash your plants out of the ground by having the water hit the plant stems.  It is still most efficient to place the water close to the plant stems, and so a 2″ X 4″ block works well.
Some pictures you see from the Mittleider books may be of 12″-wide Grow-Boxes, in which case the pipes would not need to be as high off the soil surface in order to have the water reach the plants’ root systems.