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Raised beds can be made from many materials (see
picture gallery
of raised beds) but wood is the commonest. Below we
show you how we built our basic raised bed from wood. All the
materials are commonly available from local stores.
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One word first about wood / lumber / timber.
If wood is to last a long time outdoors it should be
treated. Whilst all wood will eventually rot, treated wood
will last many years longer. There are concerns over the use
of treated wood for raised bed. I suggest that you trawl the
internet and come to your own conclusions. |
For our beds we are using new tanalised wood, which we believe is
safe. The process of tanalising previously used arsenic and chrome
but no longer. These days it's copper based which is much safer.
Indeed, copper based fungicide is often sprayed directly onto plant
leaves.
Our raised bed will be 1.8m (6ft) square. Yours can be longer if you
want but don't make it much wider. 1.8m (6ft) wide allows you to
reach the centre of the bed without too much of a problem.
See our note (construction methods) at the end of this article about
why we recommend part constructing the frame in the raised bed area.
We also strongly suggest that you read this entire article before
starting any construction work.
Wood and tools
needed:
| 1. |
Four 1.8m x 15cm (6ft x 6in)
treated wood. Gravel boards (they form the base of some
fences) will do the job well. See point
1 at the end of this article if you want a higher
raised bed. |
| 2. |
One fence post or similar wood
measuring 1.8m x 10cm x 10cm (6ftx 4in x 4in). See point 1
at the end of this article if you want a higher
raised bed. |
| 3. |
Sixteen exterior nails, about
7cm (3in) long |
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4. |
Four canes and some string to
mark out the bed
Spirit level (not absolutely essential)
Saw and hammer
Spade
Tape measure |
| 5. |
Compost and soil to top up the raised bed. |
How to build the raised bed:
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Mark out the raised bed area using the canes
and string.
Check that the marked out area is perfectly square by
measuring the distance between the two opposite diagonals.
The two distances should be the same.
In the diagram, the line AA should be the same length as
line BB. This applies to rectangular and square shaped beds. |
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Clear
the area of weeds and at the same time dig it to a spade's
depth. This will increase the volume of the soil which
should be piled up in the centre of the area.
Make sure that there is an area at each
corner clear of soil to a depth of 30cm (1ft). This is where
the corner posts will go. |
| If the gravel
boards are not the correct length then saw them to the
correct length. For our 1.8m (6ft) square raised bed, each
gravel board should be cut to 1.8m (6ft) long. Saw the
fence post into four lengths, each 45cm long. |
| You will need some open,
reasonably level grass area or other flat surface to
build the frame. Nail posts at either end of one length
(one of the shorter ones if the raised bed is not square) of
gravel board. Do the same with the another length of gravel
board. |
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| 4 |
Position and complete
building the frame |
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Position both assembled ends of the frame in the raised
bed area. Ensure that the four corners form a square by
measuring the diagonal distances as show in step 1 above.
The "legs" go into the previously dug holes at each of the
four corners. Tap down or add earth to ensure that both ends
are level and that the base of the gravel boards is about
1˝cm (žin) below ground level. |
| Nail the remaining two boards to each end of the frame.
Again, ensure they are level and the base of the gravel
boards is about 1˝cm (žin) below ground level. Fill in the post holes with earth and gently firm down the
surrounding soil with your boots. |
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Add some multipurpose compost to the soil in the
raised bed and mix it in well. Level off the surface
with a rake. We estimate that if the gravel boards are 4cm
(6in) high then you will need around 200 litres of extra
compost. Make a surround of about 45cm (18in) around the
edge of the raised bed using chipped bark or similar. |
This will make walking and kneeling around the bed easier. It will
also suppress weeds.NOTES ON BUILDING RAISED BEDS
1. Height of the Raised Bed Sides
| If you want your raised bed to be higher then simply add
more rows of gravel boards. The picture on the right shows
two rows of gravel boards adding an additional 10cm (4in) to
the height. Remember though, the side posts will need to
be 10cm (4in) longer for each additional row of gravel
boards. |
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2. Method of Construction
| When scanning various books
and internet sites we noticed that some recommend
constructing the entire frame first then lifting it into the
raised bed area. We tried that and our advice is don't do
it!
There is no doubt at all that frame will break if a
single person attempts to lift it into place. It really
requires four people to lift it into place for no clear
benefit over the method we suggest above. |
Construct entire frame first. Not a good idea! |
3. Covering the Top
If you plan to grow low
height plants (e.g. strawberries) or if you want some frost
protection at the start of the season then covering the top
with some form of clear plastic or horticultural fleece is an easy option. You will
need at least two levels of gravel board but only fill with
soil to the first level.
Then cover the top with clear plastic or horticultural
fleece. We used corrugated
plastic. It can easily be attached to the corner posts. |
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PREVENTING CATS ON RAISED BEDS
| Cats can be a real problem
for the raised bed gardener. Not only do they dig large
holes in your freshly dug raised bed but they poo and wee in
them making your produce an unappetising choice!
One common prevention method is to lay
horticultural fleece
over the top of your crops - see "Covering the Top" above. |
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A much cheaper option, free in fact, which works well for us is
to put twigs into the soil around the edge of the raised bed. Click
the above picture to see a larger version of it. It seems that this
is just enough to prevent the cats from getting to the raised bed.
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OUR ANSWER: I have seen suggestions that lining a raised bed with cardboard is a solution, but this is clearly not true.
The only safe solution is to have a raised bed on legs, there are several different types for sale and I'm sure they will give you some ideas. Pallets (often available for free) could also be used as a base to raise a raised bed off the soil.