Overhead view of a sub-irrigated planter (SIP) made from a green plastic tote box. Water/Oxygen reservoir(s) system made from recycled 1/2 gallon milk containers. An overflow drain tube connects from the top height of one of the milk containers (4") to the outside. NOTE: There are holes (large slots) in the bottoms of the milk containers. Water flows along the bottom of the tote box and rises to fill each of the reservoirs limited by the height of the overflow drain tube (4"). The water level can rise no higher than the overflow tube. The container bottom acts as a "connecting pipe". There is no need for connecting tubes or pipes between the milk containers. The spaces between the container reservoirs forms the soil wicking system. Planter mix should be firmly packed down in these spaces to create a well functioning wick system. Water will rise up into the root zone by capillary action. Note that for illustration purposes the planter is only partially filled with planter mix. Fill it all the way to the top rim before seeding or installing starter plants. See this prior post for other views and information. See this Flickr photo set for other planter bed & box SIPs. Photos of SIPs for growing plants indoors. See all of Greenscaper's Flickr photo sets.
The function of the overflow drain hole in sub-irrigated planters continues to prompt questions. It may be that some think of it the same context as the drain hole in a traditional plant pot. They serve very different purposes.
There are 3 functions of the overflow drain hole.
1. It is analogous to an LED red light warning when filling the reservoir. The first sign of water dripping from the overflow drain hole tube serves as an alert that the water/oxygen reservoir is full and to stop adding water.
2. It determines the height of the water level in the reservoir. No matter the physical height of the reservoir, water can go no higher than the height of the overflow drain hole.
This appears to be the point of most confusion. The water/oxygen reservoirs in the green tote box SIP (top photo) are recycled ½ gallon milk containers. They are 4” square; therefore, the physical height of the reservoir is 4”.
The usual location of the overflow drain hole would be at the same height. The water level when full is then 4” or the same height as the physical reservoir in this case.
Note that the water level will not remain full for very long. Within a matter of minutes, water will start rising by capillary action up into the soil and root zone. Photosynthesis will add to the lowering of the water. There will quickly be an air space at the top of the reservoir chamber(s).
This oxygen supply is a poorly understood benefit of sub-irrigated planters (SIPs). It is arguably as important as the water supply. Lack of oxygen is a leading cause of poorly developed and dying plants, including those grown in the ground in poorly aerated soil. This is of course a common problem in the city where most of the soil is of poor quality (and often contaminated with toxic metals such as lead).
If you position the overflow drain hole at 3”, you will have a 3” water level when full and a 1” layer of air inside each of the reservoir chambers. I would not do this but mention it only for illustration purposes.
If you live in a very hot climate, you might want to experiment and expand the water holding capacity of the SIP by positioning the overflow drain hole at 5”. This could extend the period between refills.
You should definitely product test this before converting multiple planters. You want to be sure that the extra high overflow is not causing over watering.
When you fill the planter, you will have a 5” high water level, 4” of it in the reservoir and 1” of wet soil above. In a hot climate, this is probably of little consequence since the rate of photosynthesis and transpiration is very high. The 1” of very wet soil will dry down in a short time and there will be no root rot.
3. It serves as a conventional drain hole in the event of rain. The foliage canopy of the plants and plastic sheet mulch typically prevent much water from entering the planter but the overflow drain hole provides a system to drain excess water.
You can convert traditional drain hole pots to SIPs by using this simple method created by Johanne Daoust in Toronto. There is no need to plug the bottom drain hole or drill a hole for an overflow drain tube. It is a clever idea. Note that she used flexible corrugated drain pipe for the water/oxygen reservoir. It would work just as well using a reservoir made from a recycled plastic food container(s).