I’ve decided to put in a high raised bed on one side of the back garden. We are still working on the drainage problem and I will write a lengthy article about this once it’s all sorted. Part of my waterlogged clay drainage plan is to build a high raised bed (30cm tall by 2m wide by 1m deep) which can accommodate a whole load of the displaced soil from other areas of the garden (it should take about 0.6 metric tons of soil, if you put those measurements into a calculator).
This came about because we have about 3 tons of soil that’s been displaced from digging 50 metres of drainage trenches around the garden. The soil type we have is heavy clay soil, and although we have a south-facing garden, the fence at the bottom is 4 metres high so that part of the garden is in shade for most of the morning, and even in this heat (I measured a 45 degree ground temperature two days ago) the clay soil just can’t dry out because behind the 4 metre high fence is a huge garden whose ground level is 2m above our ground level.
This means all the water from their garden comes to ours. Their garden is the lowest on their street (the street goes uphill from there) so we’re getting water from about 20 houses percolating into our back lawn.
Even with the drainage ditches (which have created a beautiful stream water feature in our garden), it’s too waterlogged to grow anything useful or interesting. And I can’t have a pond and a water garden because I have a toddler (and will soon have 2 under 2) who can drown in an inch of water.
So instead I’m building up. The positive about clay is, it’s fantastic for holding nutrients in the soil for plants. It’s just the drainage that’s an issue. So I’m making adjustments to the soil (more on that later) to make it drain better.
The plan is to grow onions over winter in this raised bed, or to plant green manure (to fertilise it naturally), and dig that in, ready for squash or pumpkin planting next year.
Being quite pregnant, a very raised bed is great because it means less bending over to work with my plants. I used offcuts of recycled wood and built each side separately (long sides are 2 metres by 30cm, short sides are 1m by 30cm). Once each side has been made, you can nail them together using square chunks of wood in each corner to give them stability and strength.
First of all, I prepared the site by laying down some thick (but not waterproof) weed control fabric (aka weed proof membrane) to kill the grass and weeds that were already here. The membrane stops the sunlight getting to the grass and it dies because it can’t photosynthesize.
There was a lot of moss in this area and weed control fabric isn’t much use for that. However, iron sulphate works well for moss. Usually, iron sulphate is used to acidify soil (to make so-called ericaceous soil, suitable for erica, heather, rhododendrons, hydrangeas, citrus plants and other acid-needing plants). Since moss thrives in waterlogged alkaline conditions (especially in shady areas), acidifying the soil can help with this.
My problem with this method was, when the soil is this waterlogged, all that will happen is the water will dilute the iron sulphate until it’s not very effective (any pH of acid or alkaline can be neutralized when diluted with enough water). We would need to drain the area before using that remedy on the moss. Instead, I dug over the whole area after removing the weed control fabric so the moss was dug into the soil. It needs light (even a little bit) and oxygen to survive so this was another effective way of getting rid of it.
The wood needs to be thick enough to hold a ton of soil and remember, that soil will be wet, which will shorten the life of the wood if it’s untreated. However, untreated wood is better for your plants because the chemicals from treatments can leach into the soil. With a large bed this is less of an issue.
The structure was easy to make and here’s instructions, if you want to make one, too.
Instructions for making a high raised bed:
You will need:
6 pieces of wood (length 2m, width 2cm, height 10cm)
6 pieces of wood (length 1m, width 2cm, height 10cm)
4 square corner posts (length/width 5-ish cm, height 30cm)
12 screws or nails
Step 1: Attach 3 pieces of 2 metre wood to 2 of the square corner posts, using one corner post at each end of the wood. Repeat this step with the other 3 pieces of 2 metre wood and the other two corner posts. These are the two long sides of the bed.
Step 2: Attach 3 pieces of 1 metre wood to one square post at the end of each long side. These will make the short sides of the bed. Repeat with the final 3 pieces of 1 metre wood. See diagram above (the green lines are the square posts and the grey dots are nails or screws).
Step 3: Put your bed where you want it. That’s it!
Once the whole structure was complete, I sited it in the ground. I left a 6-inch (15cm) gap between the end of the bed and the fence, to protect the fence and to ensure better drainage from the higher garden behind ours (I don’t want their garden draining into my onion bed haha). In the images below, the bed isn’t in its final resting place, yet.
Because I used recycled wood, one of the long sides has an extra piece of wood nailed on the inside that doesn’t appear on the plans.
Next, I filled it with soil. This soil was all extra stuff from digging drainage trenches all around the garden, so if we didn’t find a use for it, we would have to figure out how to dispose of it, which seemed weird, because it’s soil.
The soil needed to be adjusted to it useful for growing plants, which involved adding sand and manure. The sand will improve drainage and the manure will increase the nutrients available for plants. You can’t use just any old sand, however, and there are several ways of adjusting soil.
You also need to take care not to use the soil too soon after you’ve made adjustments to it, because the repaired soil needs time for its structure to change after you’ve worked on it. I’ll go into detail on how to adjust your soil in another article.
I removed the turf all around the bed, too, and replaced this with gravel for better drainage and access to the bed. Clay soil suffers badly from compaction when it’s waterlogged, and walking on it will literally damage the peds (the individual cells of soil) by making them platy, so they can’t absorb water, which makes the waterlogging worse.
Compacted soil also makes it hard to grow anything. You can see this out and about if you’ve ever walked past a farm gate where cows have stood around, compacting the soil with their hooves. In summer, when that soil dries out, there will usually be a bare patch around the gate where things don’t grow so well. Compacted soil can produce dangerous conditions around farm animals (especially cows) as their feet can get stuck in it and then they might get injured.
In a garden, the main issue with compaction around walkways (where you’re not growing your flowers or vegetable crops anyway) is that it looks really unsightly. It also usually produces a very sticky mud that attaches itself to your shoes and refuses to let go, resulting in lots of scrubbing to get them clean!
Overall, then, soil compaction is not great for a number of reasons, and it’s best to avoid it wherever possible.
Switching the turf for gravel around this bed should also reduce the chance of excessive grass/weed growth around the edge of the bed, which has been a problem with my much smaller (120cm by 120cm by 15cm high) bed which is currently housing most of my crop for this year.
Also, the gravel looks pretty and will mean I can work on the bed without having to wear wellies to keep my feet dry!
So there you have it, that’s how we changed this dark, squelchy corner of the garden into a large high raised vegetable bed.