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How to fill a raised garden bed

Experienced Contributor

Difficulty: Beginner

A raised garden bed can help make growing your own fresh vegetables and herbs a breeze. No deep digging, bending or kneeling is required because they’re above ground and can be built to a height that suits you. They can even be placed on a paved courtyard.

 

Building a raised garden bed is a terrific D.I.Y. project or you can buy flat-pack kits or pre-assembled boxes. The traditional box positioned directly on the ground is the best option but if you don’t have the space, there are orchard crates and boxes with solid bases available.  Just check there are plenty of drainage holes and elevate it on bricks or similar so it will drain freely. Remember that once filled it will be too heavy to move.

 

Once installed, follow our step-by-step guide to how to fill your raised bed.

Steps

Step 1

You might need to do some preparation work before you start filling the raised bed. For example, it’s wise to ensure any grass underneath the bed is dead and removed before you start filling it. And you should also ensure the base is level.

 

Depending on the eventual height and depth of the box, you might find it easier to start filling it before it is fully assembled. Shovelling aggregate and soil over a low wall takes less effort and is safer than filling a high bed.

 

1. Ensure any grass is dead and removed.jpg

Step 2

Add a layer about 80-100mm deep of aggregate / blue metal, crushed brick, broken concrete, polystyrene or similar to the base of the box to help excess water drain away. This is especially important in boxes with solid bases.

 

Cover the drainage layer with a layer of weedmat or geofabric which will allow water through but not soil or potting mix particles. This fabric layer is optional for boxes built on soil where drainage water doesn’t need to be clean because it soaks straight into the ground beneath.

 

2. Add a drainage layer.jpg  2.1 Screenings.jpg  2.2 Weedmat.jpg

Step 3

Deep boxes are perfect for those who can’t garden at ground or low levels, but can be very costly to fill completely with quality potting mix. Use ordinary garden soil to fill the box to about 400mm from the top.

 

If your overall height or depth of the box is less than 400mm, you can proceed directly to Step 4 and use potting mix.

 

3. Use oil to fill about 400mm from the top.jpg

Step 4

Time to add some premium potting mix. You’ll want to a minimum depth of 300mm of potting mix. Most vegies and herbs will grow and produce well if they have around a 300mm depth of potting mix, even root crops like carrots. Allow 50-100mm space on top for mulch.


Use the best available quality potting mix formulated for vegetables and herbs in your vegie box for optimal results, not garden soil which will dry out and compact quickly. Choose from 100% natural, certified organic, premium boosted with organics or premium with controlled release fertiliser.

 

Potting mixes tend to come in 25L and 50L bags. To determine the number of bags needed, calculate the volume required. For example, a bed measuring 1.5m wide x 2m long x 0.3m depth of potting mix will need 0.9 cubic metres, which is 900 litres (1000 litres is 1 cubic metre).

 

4. Use premium potting mix.jpg

Step 5

Don’t press the potting mix down but water it thoroughly so it will settle naturally. Top up again to within 100mm of the top if required.

 

Now add a 100mm layer of sugarcane, pea straw or Lucerne mulch over the surface to keep it moist and cool. The mulch will also help prevent water running off the top of the garden bed.

 

5. Pea straw for mulch.jpg

Step 6

Plant your seeds or seedlings and enjoy your harvests in the coming months.

 

6. Plant your seedlings.jpg

Materials

  • Raised garden bed
  • Aggregate / blue metal screenings, broken bricks / broken concrete (20-30mm in size) or crushed polystyrene boxes
  • Weedmat
  • Soil (for boxes over 400mm high)
  • Premium vegetable and herb potting mix
  • Organic weedicide (for boxes installed on the ground)

Tools

  • Gardening gloves
  • Face mask
  • Wheelbarrow
  • Shovel or spade
  • Hose 

Images

1. Ensure any grass is dead and removed.jpg

2. Add a drainage layer.jpg

2.1 Screenings.jpg

2.2 Weedmat.jpg

3. Use oil to fill about 400mm from the top.jpg

4. Use premium potting mix.jpg

5. Pea straw for mulch.jpg

6. Plant your seedlings.jpg

4 Replies
Junior Contributor

great  tips...nice work. 

Budding Contributor

Hi 

   This is a great idea other than the polystyrene boxes as base filler (unless its the new corn based diodegradable polystyrene). Polystyrene will never break down, is highly hazardous if eaten by animals and contains lots of harmful chemicals that are used to originally make it . Do you really want to put something like that near your edible plants or into your soil to leach chemicals slowly into your groundwater/nearby waterways? 

I great alternative is used cork from bottles or shreded brown cardboard.

https://sciencing.com/styrofoam-biodegradable-22340.html

Experienced Contributor

Hi @Nellstar 

 

Polystyrene has been used in horticulture for many decades in growing and potting media to lighten the mixes and to improve drainage. It is preferred over perlite and vermiculite.  It is also used in commercial horticulture/agriculture for the packing of crops for transport from market gardens and orchards to markets and eventually to greengrocers.  It is quite stable and does not give off "harmful chemicals" when used and handled appropriately. It is safe to use around edibles.

 

In a raised vegie garden, where it is buried beneath the top 45 - 60cm of growing media, polystyrene is used as filler in the base of a deep box to improve drainage and to occupy space, to cut down on the overall volume of growing media required to fill the box.  It sits below the root zone.

 

It is cheap - greengrocers are often more than happy to give boxes to you to cut down the expense of disposal.  It isn't exposed to animals or wildlife so there's no danger of them eating it, and it doesn't break down to leach chemicals in underground water. It will compact over time but that's OK because fresh growing mix should be added to the top of the box every few years to maintain the depth.

 

So overall polystyrene is safe and practical to use in raised vegetable and herb beds and will not leach chemicals into the subsoil or groundwater.

 

Ordinary garden soil as filler compacts and doesn't drain well

Budding Contributor

@Noelle  Thanks for the feedback, ill do some more research into it. Its hard to distinquish between real or fake facts on websites nowadays.

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