Building SIP Raised Beds:
Self-Watering, Wicking Bed Design Principles
I can't keep silent any longer! After years of testing, I'm ready to share the most important elements that every self-watering wicking bed needs to have in it. I've grown tired of incomplete or even faulty tutorials. So let's set things straight and get you on your way to building your own custom SIP raised bed!
NOTE: This material applies to the type of wicking beds that use some sort of aeration screen, such as drain pipes. But it does not apply to raised beds that use an entirely gravel sub-base or beds that are constructed with AquaBlox & sand.
You have a lot of flexibility in the scale of your design. But there are certain key factors to consider as well.
LENGTH: Your bed can be as long as you want. Anything from 2 feet to 12 feet is pretty realistic. Just make sure your boards AND lining will be big enough.
WIDTH: You won't ever step in this bed. So you need to be able to reach all the way in. Maximum width should be 4 feet if you can access BOTH sides. Don't go any wider than 2 feet if you can only reach in from one side.
DEPTH: This is critical. I advise a depth of 16.5". That means 3 rows of 6" boards (5.5" actual width). Don't deviate too much from this. Going too shallow will result in overly saturated soil. Plus your plants' roots will be robbed of space for growth. Going deeper could start to reduce the ability of the soil to wick moisture all of the way to the top of the bed. The physical properties of your potting mix will be a factor, but 16.5" (including reservoir) is the ideal fit.
OVERFLOW TUBE: Drill the hole(s) for your tube(s) so that the center is 3.25" up from the bottom.
As a side note, I don't seal the lining to the tube in my video. But if you want to be extra thorough, you could apply a bead of caulk in the inside of your box so no water seeps out around the slit that is cut for the drain tube.
4" PIPE SPACING: Select an appropriate number of 4" corrugated, perforated drain pipes. The spacing between the outer edges of each pipe should be a gap of 0.25" minimum up to 0.75" maximum. The pipes should not touch, but they should not be spaced loosely either.
Video: Self-watering SIP Sub-irrigated Raised Bed Construction (How to Build)
WHAT TYPE OF WOOD? Selecting the right wood for your wicking bed is matter of personal preference. But one thing to keep in mind: Depending on where you live, using treated lumber might not be a concern for you. In most places, chromated copper arsenate treatment (cca) is no longer sold to consumers. This applies to the US, Canada and EU. So if you live in these areas, you don't need to worry about arsenic exposure when using brand new treated wood. If you live in Australia, New Zealand or any other nation, then I suggest asking for a CCA alternative.
Of course, lining your bed with plastic creates a nice barrier between your lumber and the potting mix. But what if you just don't want treated, whether it contains arsenic or not? You could always opt for cypress, cedar or any naturally durable wood of your choice. Then there are also composite boards, cinder blocks or even corrugated metal panels. You have lots of options.
WHAT TYPE OF LINING? You could try to make due with a 6 mil plastic lining. But the reservoir itself will hold up better if you select a sturdier material. A pond liner is a great option. Anything around 20 to 45 mil should suffice. If your climate includes freeze cycles, 45 mil would work best. It could be PVC or EPDM. Look for it to say "Fish Safe". I bought mine off of Amazon.com.
WHAT TYPE OF DIRT? It's very crucial that you do not treat this wicking bed like an in-ground plot. Do NOT use top soil or bagged compost / manure. Treat this like a large container. Fill it with a nice quality potting mix. You could buy a bagged retail mix in bulk. Or you could also buy raw components and make your own custom mix. Use things like peat moss, coir (coco), vermiculite, perlite or even a little pine bark mulch. Worm castings make a nice addition. The bulk of the mix should be peat moss / coir. You want something with strong wicking capability while also allowing for healthy aeration.
"Yeah, but. Yeah but!" I've gotten reports of people who have used 1/3 manure, 1/3 peat moss & 1/3 vermiculite (Mel's Mix) with great success in SIP containers. This might work for some people. This is your bed and you can try what you want. But manures and bagged composts may have very small particles which could fill up your pore spaces. They also have a very broad range of NPK values and using them may be hard to predict. So I don't recommend it to others. Instead, I stick with the guidelines that EarthBOX recommends.
DO YOU NEED A PLASTIC COVER? No. This wicking garden system does not require a cover. Including one is purely optional. Plastic covers block the exchange of gases between soil and atmosphere. They prevent you from harnessing natural rain water. They also make it hard for you to apply liquid root drench fertilizers. But there may be reasons why you want to use a cover. Perhaps, you are suffering from severe drought and there's no rain to harvest anyway. If you decide to try a plastic mulch / cover, then more power to you.
SHOULD THE PIPES BE WRAPPED IN CLOTH? After YEARS of use, none of my SIP reservoir pipes have EVER filled up with mix! Even roots have trouble getting into the tiny slits on those pipes. Meanwhile, roots may pass right through any cloth or fabric lining you try to employ. So with this type of wicking bed, you don't need to waste your time or money. Remember, we aren't burying these in soil where tiny particles are constantly washing through. These are under POTTING MIX not silt-laden soil.