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. What are my thoughts on gravel? Click Here.
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" (including the reservoir depth). That means 3 rows of 6" boards (5.5" actual width). Don't deviate too much from this. The more shallow the planter, the more overly saturated the soil will become. This could lead to root rot. Plus your plants' roots will be robbed of space for growth. Anything less than 12" is asking for trouble.
On the other hand, going deeper might start to reduce the ability of the soil to wick moisture all of the way to the top. I would suggest 24" as an absolute maximum. If the soil gets too dry at the top, you might need to irrigate it at the soil surface from time to time. To compensate, you might try to improve wicking by adding some coir or vermiculite to the mix.
A depth of 16.5" is an excellent reference point. That said, there are lots of variables. The physical properties of your potting mix will be a factor. How absorbent or porous is the mix? Climate is also worth considering. Cool, rainy climates will hinder the performance of shallow SIPs. And plant type also comes into play. Some plants HATE wet feet. Others do fine. Ultimately, going too deep is better than going too shallow when it comes to plant health.
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? Please do not use a 6 mil plastic lining or a tarp. The reservoir itself will hold up much better if you select a sturdier material. A fish-safe pond liner is the way to go. Although 20 mil might suffice, I advise trying 45 mil, especially if your climate includes freeze cycles. PVC should work ok, but I've been quite pleased with the EPDM liners. 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.
WHAT ABOUT THAT SPECIAL "WICKING" LAYER? I typically mix up a special layer of wicking mix that I use for the very bottom section of my SIPs. This is the mix that's stuffed in between the black corrugated perforated drain pipes. So what's the magical formula? Is this super critical? Rather than making a strict, hard fast rule, the idea is to offer a general guide.
If you're making a very small system and materials are hard to come by, you could probably just use the same potting mix that is applied to the top section and it should work ok. If you want to mix up a special wicking base, you can use peat moss / vermiculite / perlite at a ratio of 3:1:1. This is not set in stone. The ratio could be 1:1:1 and it would work well. Anything like that is fine. You could replace the peat moss with coir (coco). I like using this special mix, because it is easier to stuff in between the pipes, since the particles are small & uniform. Also, I'm not applying a nutrient rich potting mix in the very bottom of the box. Instead, the fancy potting mix is reserved for the upper layer.
There's some flexibility here. But here's what I do NOT recommend for a wicking base. Do not use gravel or sand. Gravel does not wick well. Sand is a little better at wicking, but not as good as vermiculite, peat moss or coir. Both sand and gravel are MUCH heavier. It's not worth it. Do not use wood or bark products as these will not wick well and they may cause air gaps as you try to stuff around the pipes. Also, although peat moss is good at wicking, it does have a limitation. If peat moss becomes bone dry it becomes hydrophobic and actually REPELS water. So I never use pure peat moss in the bottom section since it could create a failure point if it ever happens to dry out.
DO YOU NEED A PLASTIC COVER OR MULCH? 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.
There are other mulch options though. You might try applying a layer of straw. Then at the end of the grow season, simply pull off the straw and compost it. The same would apply to newspaper with grass clippings on top. Or instead, a thin layer of pine bark fines would work. In the Spring the old bark can be turned into the potting mix to refresh it before planting.
SHOULD THE PIPES BE WRAPPED IN CLOTH? After YEARS of use, none of my SIP reservoir pipes have EVER filled up with mix! Even plant 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 top soil where tiny particles are constantly washing through. These are under POTTING MIX not silt-laden soil.