Cardboard Trench Beds

Cardboard Trench Beds - Trench Bed 01

“Got sandy soils? Need an effective garden bed? Improve your soil and save water by making a cardboard trench bed!”

Sandy soils allow water to drain quickly, which washes away nutrients and forces you to water often. Instead of replacing your soil with the fertile soil from elsewhere, take the opportunity to improve the soil you already have! A cardboard trench bed forms an effective semi-permeable barrier below your plants, which traps nutrients and slows drainage. This means less watering, and less fertilising. Organic amendments can freely be introduced into your soil at various depths in the bed to create a deep and thorough profile of great soil.

Benefits:

  • Slows drainage
  • Saves water
  • Saves nutrients
  • Becomes fertiliser

Materials:

  • Collapsed cardboard boxes (plain, not glossy)

Jump to:

Sourcing Cardboard Boxes
Designing
Digging a Trench
Laying Cardboard
Filling the Trench
Planting
Mulching
Watering
FAQ
How deep can my cardboard trench bed be?
Can I plant trees in my cardboard trench bed?
What other materials can I use to line the trench?
How much water will it save?
Related Articles
Cardboard Potholes
In-ground Shared-soil Wicking Beds
Hugelkultur Beds

Sourcing Cardboard Boxes

Cardboard boxes are thrown away by most shops, and you can determine how big the boxes will be by the sizes of the products they sell (e.g. huge boxes can be found anywhere that sells fridges). The greatest variety of boxes can be found at grocery stores. For a cardboard trench bed, ask for cereal boxes, tissue boxes and potato chip boxes. All of these are a good size, and are plain (not glossy)

Designing

Design for my cardboard trench bed, alongside a rebar fence trellis

Design for my cardboard trench bed, alongside a rebar fence trellis

A cardboard trench bed can be adapted to any shape and size, though the depth of the bed should be around 50cm. I chose to design a narrow one along the bottom of my rebar fence trellis, to grow vines, but you can prepare any garden bed as a cardboard trench bed.

Digging a Trench

Dig a trench 50cm deep, keeping the soil nearby for backfilling later

Dig a trench 50cm deep, keeping the soil nearby for backfilling later

Digging is never great for your back. Take your time, take breaks, get friends or neighbours to help. Lift with your legs as much as possible, keeping your back as straight as possible.

  • Dig a hole 50cm deep and wide enough for you to stand in.
  • Stand in the hole and dig the rest of the trench, dumping the excess soil alongside the trench (you’ll need to put it back later).

Laying Cardboard

A cardboard trench bed, with two layers of cardboard

A cardboard trench bed, with two layers of cardboard

The cardboard acts as a semi-permeable barrier for water. This means it will trap water and let it drain through slowly. For this to work, you must lay a blanket layer with no gaps.

  • Start at one end of the trench. Lay a collapsed cardboard box down, so that it sits flat on the bottom of the trench, and rides up the sides a little.
  • Lay a second box so that it overlaps the first. Continue this until you get to the other end.
  • Repeat for a second layer, if you have enough cardboard. Be careful exceeding two layers, as water may not drain fast enough and your plants may drown!

Filling the Trench

Add layers of compost and other amendments as you backfill your trench

Add layers of compost and other amendments as you backfill your trench

You can simply fill in your trench with the same soil you dug out, but now is a great opportunity to introduce soil amendments as you fill, to improve your soil. I filled mine in layers of soil, homemade compost and bentonite clay. You could also add biochar, worm castings & worms, bokashi scraps, mulch (not too much), shredded cardboard, leaves, grass clippings (not too much), manure, even branches and logs (hugelkultur). All of these will contribute to healthy soil and therefore healthy plants.

Planting

Plant into your cardboard trench bed as you normally would. I planted climbing beans

Plant into your cardboard trench bed as you normally would. I planted climbing beans

Fill your new cardboard trench bed with plants as you would a normal garden bed. It is best suited for shallow rooted plants (i.e. most of your annual vegetables), but there’s no limit to what you can plant. I planted vines to train up over my rebar fence trellis, and medicinal plants along my paved path.

Mulching

It’s always a good idea to mulch your garden beds. It prevents evaporation and creates a perfect environment for beneficial bugs. If you’re growing annual vegies, I recommend a light mulch such as hay/straw or grass clippings so it’s easy to move it aside to plant seedlings. If you’re going to be planting less often I recommend coarser mulch such as wood chip mulch, or the mixed mulch you get for free from local tree loppers.

Watering

4 Litres of water from a bucket every 2 days is all it took to grow my climbing beans

4 Litres of water from a bucket every 2 days was all it took to grow my climbing beans

When you water your new cardboard trench bed, the water will soak down through the soil and become trapped by the cardboard basin. The basin will allow the water to drain through slowly, and in the meantime water will also percolate up through the soil back to the plants. The result will be that the soil in your trench bed will remain moist for longer, and you’ll find you won’t have to water as often.

I recommend watering in any new plants, then after a few days scratch the soil near the plants to see if it’s still moist.

Frequently Asked Questions

How deep can my cardboard trench bed be?

You can dig your trench as deep as 50cm. Water will still ‘wick’ to the roots of your plants. Any deeper and you risk having dry soil at the top where the roots of your plants are.
Also, you can dig your trench as shallow as 30cm.

Can I plant trees in my cardboard trench bed?

Yes! It’s a great way to get young trees off to a good start. The cardboard liner in your trench bed will allow roots to grow through it. If you’re only planting trees, consider making cardboard potholes especially for them.

What other materials can I use to line the trench?

Cardboard is chosen for its ability to capture water and let it drain slowly. It’s also biodegradable and freely available. Alternatively, you can line the bottom with grass clippings and compress them into a matting. Make sure there are no grass seeds in the mix! You could also use hessian bags, leaves or weeds.
You can use a plastic liner with holes for drainage. However, for the same effort you could make an in-ground shared-soil wicking bed.

You can fill the bottom of your trench with mulch, branches, or logs to create a ‘hugelkultur’ bed.

How much water will it save?

On the hottest days in summer I find myself watering my cardboard trench bed once per week thoroughly, or once every few days with a 4L bucket. Water thoroughly any time you add new plants, and scratch the surface every second day to see if the soil is still moist. Once it is dry to a depth of 10cm or so, water again.

Related Articles

Cardboard Potholes

“Get young trees off to a good start using cardboard potholes!”

In-ground Shared-soil Wicking Beds

Hugelkultur Beds

 

 

2 comments on “Cardboard Trench Beds

  1. Do cardboard trench beds lend themselves to growing potatoes? Appreciate any pics/details regarding: LOVE to start a thread on this topic! HAPPY GARDENING!

    B. S.
    Zone 7A
    OKC, OK

    • Hi Bill, I can only speculate cause I havent tried it, but I imagine you could plant potatoes toward the bottom of the trench, then gradually fill it up as they grow, same as how we ‘mound’ potatoes above ground. If you get up to ground level with it, you could continue mounding with mulch, and the potatoes that grow there will be totally clean when you harvest. Let me know how it works out!

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