Building our wicking beds

Fair warning, this is a long one. It’s also pure garden stuff, so if you’re here for the sewing, try again later!


Well, this post was much delayed! My photos tell me we built the beds in March, and it’s now December. The photo above is from October, showing our bumper sprouting broccoli crop that grew so much it was flowering as fast as we could pick it. But we’ll get to that.

I talked in my last garden post about selecting the beds, and you can see what the garden looked like before they went in. I’m going to write this post a bit out of order because I want to first describe our experience of placing and constructing them, and then afterwards provide a bit more of a step-by-step a single bed so you can see the process. Please don’t use this as a how to, though! There are PLENTY of good resources out there, if you google ‘how to build a wicking bed’ there are tonnes. But one of the things I found useful when researching was specific accounts and examples so that’s what I’m going for here. I also found a real lack of follow ups on how things worked after the beds were built – all well and good to say ‘do it this way’ but does it WORK is the question! So I will try to get to that, as well. If you’re here after the fact, check out the tag ‘wicking beds‘ which is where I intend to put all of that. Hopefully there will be something!

Before we started the beds I had poisoned the grass where they were to go with roundup, and laid black plastic down to solarise it. I would prefer, in general, not to use poisons but I’ve battled kikuyu before and it doesn’t muck around. No point using half measures, it will just laugh in your face. So poison it was, and even then I’ve had to do touch ups around the edge as it tried to rise from the dead. I’ve since piosoned off the rest of the grass in the hopes that the harsh summer sun will keep it dead and I won’t have to re-poison. But that’s a story for another day.

So. We started with the black plastic laid out, and the beds in pieces. It took us a while from getting the beds to building them, because we had to acquire all the other materials. This included:

  • The beds themselves (three 2.4m long x 1.4m wide x 0.6m high beds from Dovetail timbers)
  • Pond liner
  • Wicking fabric/geotextile
  • Sand
  • Gravel
  • Soil
  • PVC pipe for outlets and inlets, as well as pipe and caps for in-bed worm farms
  • Irrigation pipe for the reservoirs.

It’s been a while and I’ve forgotten how much of stuff we got but I know we just bought one big roll of the felt from bunnings – it was with the builder’s plastic which is a bit counter intuitive. I can’t remember volumes for the sand and gravel but it was a couple cubic metres of gravel and one of sand, I think.

Our piles of sand and gravel – this was as close as we could get it delivered so we had to cart it back and forth a fair way. The closest bed is about 5m from there, the furthest one about 12m. Not massively far but it certainly added to the effort this project took.

So once we had everything, we began by laying out the bottom tier in the general area we wanted the beds. Here is the original plan:

Kate's Backyard FUTURE

And here we are laying them out:

The tree in the corner of this photo is the tree in the top left of the plan, if that helps orient people. We weren’t sure that they were where we wanted them, but they were pretty close. And we were sure about the first one, so we decided to build that first and then re-assess, because giving it height would change how it all looked and felt. We did try to simulate this by using the black lining that came with the beds, but it was hard to tell.

Once we had the bed in the right spot, we lined the bottom with sand to even it out and also make a soft surface for the pond liner, to avoid punctures. It’s important for this whole process that everything within the bed is level, otherwise the water won’t wick evenly to the surface, and you’ll get stagnant spots in your reservoir.

Sand lining. We pushed it to the sides to hold up the black liner for the next bit. The liner came with the beds, from Dovetail timbers, and is designed to shield the wood from moisture to extend its life. It also was very handy for us, to shield the pond liner from splinters. I have also found that some of the boards have shrunk over time as they aged – as timber does. This is not a problem at all but it made gaps big enough that, without the liner, the soil would be starting to spill out.

The next step was to even out the beds. The surface we were building them on was pretty uneven. In order to compensate for this, we stacked gravel under the wood and leveled that out.

I say that like it was easy. It was pretty painstaking, gotta say. As you can see, we used the top tier of the bed to hold the gravel in place. We put those logs in place and then S lifted up a corner while I jammed a brick in there.

You can see the labeling of the timbers here, to make it easy to match to each other and build. You can also see that the one on the right has split – this was our fault for leaving them out in the weather, lying on the ground, for about a month before building. Whoops.

We repeated this for each corner, and the we shovelled gravel in there, making sure to get it pressed right up against the black internal liner which was now exposed at the bottom. Then we used a spirit level to see which end needed to be adjusted, and either added or subtracted gravel from there until the whole edifice was roughly even. I’d say this process took around about an hour per bed.

Some of the corners needed more support than others – this is in a spot where the ground dips down. We used random rocks from the garden to make sure it didn’t shift.

I was a bit worried this wouldn’t be secure long term. Our plan was/is to get in bark chips to mulch the area, so they’d be the same height as the gravel and make everything a bit more secure and prettier. We haven’t gotten to that yet for a bunch of reasons, and the gravel and beds have stayed put just fine, despite some woolly weather over winter.

Once we had the first bed evened out we built up the tiers, just by stacking them on top of each other. Honestly, these beds were fantastic. The wood is good quality, the craftsmanship is great – the top tier is all beautifully smoothed and each of the joins fits perfectly. Putting them together was a breeze, the people who sold them to me were very helpful, and in terms of comparative products, the price was really good. I would highly recommend these beds. (Just my personal opinion, you understand, but I would 100% buy from them again).

Anywho, once we had the height on that bed, we played with where the other two would go. We ended up moving them a bit from the plan – instead of having them in line with each other we staggered them.

This started as an accident – we’d just put the timbers down in that area to get them out of the way. But we found that the layout was really easy to walk around, and looked better than having them line up. People like to walk in curves and having things staggered generally produces the impression of more space and interest, so it worked well! We built the bottom tier in place and left it like that for a while, to look at it and decide. Once we were happy with it (with some minor moving back and forth) we built the other two up on gravel, and built them up as well.

And then we quit for the day, because we were exhausted.

The next day, we went back to the first bed and began the process of making it a wicking bed rather than just a regular bed.

First we evened out the sand lining the bottom.

Then we checked that they were still even. Some of them had settled a bit overnight and we had to adjust the corners again.

It’s more important that everything within the bed is even rather than the bed itself. So we spent more time getting the sand even than the bed itself. But it does generally help if the bed is on a level footing.

Then we put in the pond liner and the wicking fabric for the bottom layer

You can see that it’s taped in place. This was mildly annoying because the tape came unstuck constantly. But it was important not to fix it in place right away. Once the soil is on top of it, it will sink down lower. I have also read people saying that the pond liner shrinks slightly over time, so we tried to give it some slack so that it wouldn’t pull out from the nails at the top.

This was the easiest way we found to put the plastic and wicking fabric in. Place it over the top and then one person stands on each long side, and you go ‘one… two… three!’ and slowly push it down in. Then you fuss with the edges until it’s mostly in place.

Next we prepared the irrigation pipe. This is to go in the reservoir part of the bed, to provide more room for water. The principle of the bed is that the gravel etc creates an area of water tension between it – it’s important to use smaller gravel and not, say, river stones, because the gravel needs to be small enough that the spaces in between will be small too. This means they are small enough to allow water to pull itself up them by surface tension. I did a LOT of reading before I tracked this nugget down. I found this a really good source of actual information about how it works, rather than just ‘do it this way because’. I made some wicking pots over summer and the ones with bigger stones barely wicked at all, so I already knew size mattered! 😛

The irrigation pipe gets laid within the wicking medium. This creates more room for water. The smaller your material, the better its wicking properties (hypothetically) but the less water can fit in the spaces between it. So the pipe creates more water room while maintaining the general structure of the wicking area. We also laid the geotextile going up the sides, to create even more opportunity for wicking, but this by itself won’t evenly keep the bed wet. The geotextile also helps to protect the liner from punctures – just the thought of getting a puncture in the bottom of the bed is making my eye twitch (it’ll inevitably happen eventually but I’d like it to be a LONG TIME AWAY thanks)

To do the pipe, we connected all the pipe that we had and then cut it into thirds (you can see the blue connector here). Then we capped the ends with geocloth cinched in with a zip tie, to keep the gravel and any dirt etc out of the pipe. The other end gets connected to an elbow bracket which connects to a pvc pipe for intake.

We did this by just shoving it in as hard as we could. Then we packed in scraps of geotextile to exclude dirt and keep them jammed together.

Then we placed the intake into a corner of the bed, and laid out the pipe in the middle, and shoveled gravel around it.

The pipe is going to move about a bit as you shovel so you’ll have to adjust it as you go. We found it was best to shovel directly on to the pipe, because otherwise the gravel found its way under it and pushed it up. You want the pipe to be completely covered, so that the top layer is all gravel.

Also, think carefully about how you will move around the beds before you chose where to put the inlet pipe. I wish I’d put the inlet for the first bed we made at the corner closest to the furthermost bed. It would have meant an easy triangle when topping them up. As it is I end up having to drag the hose around another corner and it’s a pain. I did it that way because I based the position on the outlet, wanting it where I could easily see it while doing other garden tasks. But I find the inlet is more annoying than the outlet, so were I to do it again I’d think more carefully about that.

So it should look like this.

General wisdom is your reservoir should be 20-30cm high. We went for 30ish, leaving about 30cm depth for the growing medium. You want the growing part to be as deep as or deeper then the reservoir, or else the plants can’t draw up the water from the very bottom, and you’ll end up with a stagnant part in the bottom (ugh). Any more than 30cm in the reservoir and it’s supposed to not wick as well, although I’ve read varying things about that.

Once that was done we had to make sure the gravel was even. We did this by filling the bed with water until it touched the top of the gravel. The water finds the level, and then we could rake out the gravel until it was at the same level as the water. Props to the ancient Egyptians for this technique, we weren’t building a pyramid but the principle for getting a sound footing is the same.

Once this was done, we folded the extra geotextile edges over the gravel. Then we laid down another layer of the geotextile over the top of the gravel and up the edges. This stops the soil from getting into the reservoir medium and clogging it up. It also provides some of that extra wicking up the sides.

Next step is the overflow! I read a bunch of different opinions about the level this should be at but most places said just above the reservoir. You want to make it, basically, so that the reservoir can be totally full but that the soil above it will never be more than moist. The other benefit of this is that, even if your hole seal leaks, it’s not too big of a problem. One of my beds isn’t sealing well now, and I suspect all of them eventually will have leaks around the outlet because it’s just plain hard to seal an outlet like that when there’s soil involved. But since it’s only wet enough to leak out when the bed is first topped up, I’m not worried. It’s not ideal but it’s not like it’s constantly wet and leaking, it just weeps a bit when it’s filled.

We (that is to say, S) drilled a hole with a wide drill bit and cut through all the layers of plastic and geotextile and poked a piece of pipe through. The wood smelled so good when it was drilled! Like honey. Then we used silicon sealer around it to make it (mostly) watertight.

It didn’t stay this long, we sawed it shorter. I wanted it to be sticking out a bit though, to not get the base of the beds wet. We made an error with the first one and didn’t angle the hole down. We also didn’t leave the silicon long enough between doing it and adding the soil. This spring we had to re-do it and plug the old hole, because water was collecting between the wood and the pipe and slowly leaking out, and it would have meant the wood degraded a lot quicker. It was really annoying to have to dig back part of the bed and deconstruct the innards. So, this is worth taking a bit of time over.

Because we didn’t angle the pipe down, the water can’t run down it. Instead it leaks through the inside and has discoloured the wood. If it kept doing this it would rot pretty quickly.

I know some people advocate having a lower outlet which you can use to regularly drain the beds. I would very much have liked to have done this but thought, given that we have a wood bed and it’d be hard to seal, and water leaks would degrade it, that it was better not to risk it. I definitely think this was the right decision. In winter the beds get enough rain water that they flush themselves, I think I’ll just have to flush them out fully a couple of times in summer. I topped it up the other day after a 40 degree day and it did smell a bit. Plus, salts etc will build up and a flush is a good idea, every now and then.

On the other end of the pipe I laid a piece of flyscreen over a piece of geotextile, and attached it with a rubber band to the pipe.

The weight of the soil will help hold it on.

Here’s the first bed, all ready for soil. The bucket is covering the outlet pipe so we could add soil while it was still drying (a mistake, as I mentioned, but we had a limited time to get them done)

Then we added the soil!

I ordered a mix of soil and compost from SA composters.  I was a bit disappointed at first because it was very dry and sandy, and a bit hydrophobic. But as soon as it got moist it revealed itself as a lovely mix. I would definitely buy from them again.

You can see the tarp out the front, near the mailbox. That’s the soil. You can also see that we were running low on gravel! By the time we got to the third bed, we had almost none. But we still had a bunch of sand. So we used a mix of both for the reservoir.

I had read some places that sand is even better than gravel. Six months in, I’d say I can’t really tell much difference. It;s a bit hard because the intense wind and sun, plus different plantings, mean that each bed actually has pretty different conditions. I’d say they both wick as well but the sand stops wicking before the reservoir is empty. So I often will refill them when the top 3cm of soil are dry. The sand bed will take five minutes to fill and the gravel will take 10-15. They took about the same time to fill initially. So I’d say from that that the sand bed doesn’t wick as high. I’d love to hear if anyone else has experience with this. If I were pushed for height in my beds I’d do a smaller sand layer, say 15cm. I’m guessing on depths here. Although now I’m looking at this again I wonder if it isn’t just that the gravel on top of the sand was wicking and the sand wasn’t wicking much at all. I wish I could remember how much gravel went into the mix here. The downside of delayed blogging!

Here’s a bed all full up with soil. You can see how we dealt with the intake pipe by wrapping the geotextile around it to prevent dirt etc falling between the sides.

Here’s that in progress – after the first bed we nailed around the inlet before adding the soil, otherwise we kept getting stuff down there as we shoveled.

Once all the soil was in we nailed the geotextile and liner up

Behind the textile we tried to leave some slack in the liner, to allow for shrinkage. It’s less terrible if the geotextile shrinks and tears.

Then we emptied a couple of bags of sheep manure onto each bed and left it for a couple of weeks before planting.

And that’s it! That’s how we built our wicking beds.

I do intend to stain the beds, although given that it’s summer and that hasn’t happened yet I’m not sure it will happen. I’m trying to find a nice stain that isn’t totally toxic. I’m now thinking perhaps I’ll just use linseed oil, although that will need reapplying – I just think it would make them last a bit longer and also look nicer, as they’ve started to grey.

If I did it again, I’d plan better and 1) store them undercover before building (we didn’t have space but I should have MADE space because some of them did crack while lying on the wet ground, and that was a shame and totally avoidable – none of the others have cracked since it was just from being in contact with the ground and then moved) and 2) stain them before building them. But really, if I don’t get around to staining them then it’s not a big deal. They are hardwood and appear to be lasting very well although of course it hasn’t been that long yet.

Big thanks to both S and G who were champions in this process. I skipped a lot of resting and swearing and shovelling and measuring in writing out the process. It was HARD WORK and I would have gotten very overwhelmed and exhausted doing it by myself. So they each get a medal, especially as they couldn’t care less about the garden (although are both happy to eat out of it). It has saved a lot of work in the long term, though, and I’m really pleased with the beds. They save a LOT of water – I watered them maybe once over winter and in spring and autumn only needed to top them up occasionally and then re-wet the top bit which dries from the wind.

I am planning to do a follow up, what I planted and how the beds have worked and behaved since. Hopefully soon but I make no promises! Please let me know if you have any questions, I’m far from being an expert but I’m happy to share my experience! If anyone wants a closer look at the photos, they’re all on my flickr, here.


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