Several years ago in the place where I was living; I had given over the back yard to a flock of chickens which made it awkward to grow vegetables in that area.
Being a back section down a long driveway I decided to construct a raised garden in the end of the turning bay. A good spot facing North-West, so sunny much of the day and also near the house with a wooden fence a metre away making it nice and sheltered.
I had a cocktail kiwi fruit vine growing on the fence and did not realise that would be a problem down the track.
I wanted a raise garden that would be about waist high so easy to work without bending.
Besides a high garden was less likely to have weed seeds blown into it.
I also did not want to spend a lot of money so I decided to build it out of corrugated, galvanised roofing steel.
I purchased 3 new lengths of roofing steel 1.8 metres long and 845mm wide for about $109 at todays prices.
I also purchased two 100 x 75 x1800 fence posts ($30.00) that I cut both in half making them 900mm long.
As the posts are tanalised I did not want the chemicals leaching out into the garden and poisoning the vegetables, so after cutting them I gave them two coats of acrylic paint all over to seal in the chemicals.
This done and dry I laid one sheet of galvanised steel onto the painted posts at each end of the steel.
The steel was flush with the bottom of the posts and thus drilled and then screwed on using roofing screws.
I also did the same with another sheet of steel and the other two cut posts.
I now had two long sides of my new raised garden.
The third sheet of steel I cut in half so that would be the two ends of the raised garden.
Making it 900mm wide a nice width to work on from one side and easy when access to both sides.
I simply drilled and screwed the two ends in place and sat the structure on the ground which was a gravel/stone area.
Now became the task of filling the raised garden up and I did this as I would a compost heap.
Pruning bits and and cut branches at the bottom onto the shingle then lawn clippings, spent potting mix, kitchen scraps, screwed up newspaper, saw dust untreated, weeds not in seed, compost out of compost bin in fact anything organic.
Once the level got up to about 600mm or about two thirds of the way to the top I then placed some sheets of cardboard over the fill and on top of the cardboard placed a layer of chicken manure (could be any animal manure that is available) sheep manure pellets, then a sprinkling of Wallys Ocean Solids, Wallys BioPhos and Wallys Calcium & Health.
Thats the food done for the plants and over this I placed a layer (about 8 cm deep) of my favorite purchased compost, Value Compost which is available from Bunnings and some switched on garden shops.
This left a distance from the top of the growing medium to the top of the sides of the steel of about 20 odd cm.
Now with one long side facing the sunny north, and though the steel nicely warmed up the growing medium and the gap of 20 cm above the growing medium, which means the wind passes over and we have created a micro climate.
This was so good that seedlings of silverbeet I planted took only about 3 to 4 weeks before I had good size leaves to harvest.
In fact every thing grew so quickly and healthy.
The four posts protruding a bit above the roofing steel were great to put a nail partly into each and then place netting over the garden which kept butterflies and birds/cats out of the garden.
My first season was great but the following season when I planted up nothing grew, plants sat there like they were sulking.
It took me a while to sort out the problem which I found that when I dug down into the growing medium I came across the most dense mat of fibrous roots that I had ever seen.
The cocktail Kiwi vine had become massive, spreading out all over the place to the point the neighbour complained about all this vegetation in his area.
The vine had found that there was a massive amount of food nearby and sent its roots across and then upwards to collect all this wonderful food I had placed there.
It completely ruined my raised garden but as every thing was screwed together It was simple to unscrew and remove the posts and roofing steel, which I moved to my next place of residence and assembled again but this time on concrete.
A good lesson learnt if you are going to have a raised garden of any type you must place it on concrete or a solid concrete pad.
It only needs to be 40-50mm thick to prevent invading roots from any plants/shrubs/trees within many metres of the raised garden.
There is no safe distance that I am aware of other than about 10 metres from plants and likely 50 metres or more from established trees.
So put your concrete pad down on the ground and then place the raised garden on top of that.
Maybe a double layer of Black Polythene film which is what I call builders plastic that comes in packs 2m X 5m and 200um thickness might work, as long as it covers the area and has no holes in it to let any roots in.
I prefer the concrete pad. At my current place I have the same original raised garden posts and roofing steel now over 15 years old and placed on an asphalt area.
There is another advantage with this construction which is you can (where there is room) take one end off and with another two new sheets of roofing steel and another post cut in half and painted extend the size on the raised garden to double.
If doing this I would place a brace across between the now two central posts to stop any bowing effect from the fill.
People that do not have the room to construct the perfect raised garden can purchase lower type steel or wooden ones from the likes of Mitre 10, Bunnings or Trade Tested (online).
There are nice small ones 60cm x 60cm and 41cm tall for about $50.00 and larger ones about 12cm x 120cm and 41cm tall for about $90.00
I have several of those and they work a treat for growing any vegetable plants in just be sure they are placed onto concrete to prevent root invasion.
After a crop is harvested you can put over the medium a layer of cardboard smoothing any weeds and then animal manure and other goodies covered with Value Compost ready to replant a new crop of seed or seedlings.
Image credit: PhotoMIX Company
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