How I spent my spring
This is my latest project. Building a series of giant self watering planters.
If these function as planned, plants will draw water up from a reservoir below the growing medium through capillary action. This means that the roots can draw up water as they need it and the plants will have nice, moist soil most of the time. Subsurface irrigation not only requires less water than standard gardening methods, it can also help reduce the risk of fungal disease.
We started by selecting a location with lots of sun, removing the grass and leveling the ground under each tank.
A clean, empty tank was set on each pad.
The bottom of each tank was filled with 8 to 10 inches of 3/4″ washed river rock. We conveniently had a lot of this laying around leftover from another landscape project. This coarse, well-sorted gravel has lots of large pore spaces and creates a reservoir at the base of the planter. A gravel like this can have a porosity of up to 50%!
I installed a section of 1 1/4 inch galvanized pipe at one end of each tank. The pipe has a 8 inch elbow at the bottom and a screw cap on top. I made sure that the gravel at the discharge end of the pipe consisted of pieces that were much bigger than the inside diameter of the pipe to keep it from being blocked. The pipe will deliver water to the gravel reservoir at the bottom of the bed.
The gravel was covered with a layer of landscape fabric. The fabric will allow water to wick up to the soil while keeping the soil out of the gravel below.
I drilled a half dozen drain holes at the top of the gravel layer at the end of each tank opposite the intake pipe. This will encourage the water to flow across and fill the entire reservoir before it hits the overflow.
I filled the section above the fabric with good topsoil. Audie supervised.
I read in several places around the web that “wicking bed wizards all agree that water cannot be wicked further than 300mm (or about 12 inches) in soil”. The wizards didn’t provide any kind of calculations for their magical prognostications and their numbers didn’t make any sense to me. So, since I did rather large amount of hydrogeologic consulting work in my previous career I looked up some general data on capillary rise and then went ahead and put a foot or more of soil over the rock in each of my beds.
For those who may be interested, capillary action pulls water upward in materials against the force of gravity. The empty spaces between soil particles are called soil pores. Below the water table the pores are filled with water and above the water table they’re filled with a variable combination of air and water. Adjacent pores are connected to each other somewhat analogously to pipes in a water system. The sizes of these ‘pipes’ and the degree to which they are connected can vary over several orders of magnitude.
Capillary rise occurs when water migrates upward through soil pore spaces against the pull of gravity. Capillary action involves two types of attractive forces — adhesion and cohesion. Adhesion is the attraction of water to the solid surfaces of the pore walls. Cohesive forces attract water molecules to each other. Adhesion pulls a mass of water upward along the pore walls and cohesion pulls more water upward with that mass.
Capillary rise occurs when the upward pull of adhesion and cohesion is equal to or exceeds the downward pull of gravity.
Anyway, I was comfortable enough with my off-the-top-of-a-former-hydrogeologist’s-head calculations to put 16-18 inches of soil on top of my gravel reservoir. I will update you on how this works.
The beds operate very simply. The male end of the garden hose conveniently fits snugly into the intake pipe, so when I want to fill the reservoir I just pop the hose in, turn the water on and let it run until I see Chucky attack the spray of water discharging from the drain holes.
So far, so good. This is a bed I completed about three weeks ago. Radishes, turnips, beans, kale, chard and a bush-type summer squash (I’ll let it cascade over the side) are already up!
I’ve completed three beds and may put a fourth one in later on. They look pretty good and planting and weeding chores are much easier in these 24 inch tall beds than the ground level beds.