Various types of raised vegetable garden can be used in different ways

Using a raised vegetable garden design may seem like a quick way to start, but it has a few pitfalls you should try to avoid. These mainly relate to the soil you use, since (in most cases) you will need to buy top soil rather than use what you already have.

Even though you will be buying soil, you should not ignore the soil you have. The objective for a raised vegetable garden is to make more efficient use of water by increasing the available root zone.

If you just put new soil on top of old, the effect will be like putting oil on top of water. You will be restricting your root systems to the top layer only. For this reason, it is important to start with improving the soil that will eventually lie beneath the raised garden bed. You can read about doing this in our soil improvement section.

The only common exception to this is if you are building a raised vegetable garden that will be more than about 2 feet above the surrounding ground level. In this case, you will have little or no need to worry about the soil beneath - so long as you remember to install drain holes at regular intervals around the base of the retaining wall.

There is one other option though. Called the Garden Rack, it looks something like a cross between container growing and hydroponics.

However, if you are building a raised vegetable garden that is not so high (as a means of increasing the available depth of soil), you will need to begin well before the new top soil arrives.

You will need a good soil test - Not just a quick result from a home soil test kit, but a proper Laboratory test that gives good advice on improving structure and friability. The only one I ever use or recommend to others is the test from SWEP Analytical Laboratories.

The same testing and soil improvement process should then be applied to the imported soil. In this way, you will ensure that both layers have the same conditions of friability and moisture infiltration, etc. The result will be a raised vegetable garden with a genuinely increased root zone, rather than just the dream of one.

The other trick is not to overfill the new bed with soil. When you first put the new soil in, of course, you should fill the bed right up, because the soil will settle and the level will go down a bit, but don't rush to add more. We will talk about using this space a bit later.

Building your raised vegetable garden

Naturally, you will need to put up the sides before bringing in the new soil and there are many ways to do this. You can get ambitious and build brick or stone walls if you like, and I have even seen corrugated steel used quite well, but this option can become rather unsightly over time.

The most common option is timber and the process is no more difficult that building a child's sand pit - just a bit taller. The same broadened steel stakes used for the corners of a sand pit can be used here. So you just butt the timbers together and bang in the corner stakes to hold the whole thing together.

OK, now you are ready for the soil, but again, you will need to pay attention to improving it. The reason is that the mix you get from the garden center only needs to look good in the heap. How it behaves in your garden could be quite a shock.

When I did my raised vegetable garden I found the soil set hard at the first sign of moisture - not even the weeds would grow. A SWEP soil test revealed the problem - too much Magnesium! Several bags of gypsum and some heavy rain showers to leach the offending Magnesium out and "all is well in the garden".

The next step in developing a raised vegetable garden is to start the crop rotation. You may be tempted to start planting your raised vegetable garden right away, but try to restrict this enthusiasm to one-third or less of the area. Your soil improvement efforts need time to work and you still need to build up the nutrient fertility and biological activity. Any initial plantings you make at this stage should also be kept to the hardier herbs like rosemary or thyme and vegetables that like less fertile conditions such as onions. Another alternative could be legumes (like peas or beans) that can fix nitrogen from the air.

For the rest of the area, work in as much horse manure and garden compost as you can - even if the soil level initially goes above the sides of the bed. If you can, a thick mulching layer of straw would also be a good idea. Give this a couple of weeks to start breaking down and then plant a green manure crop. The best one would be a leafy legume like broad beans. However, don't try to keep this until harvesting stage (although you could be forgiven for keeping a few plants). All green manure crops should be turned in at the start of flowering.

Another useful choice of green manure crop would be a brassica like Mustard. When this is turned in and starts to break down it releases a bio-fumigant into the soil that will selectively control any potential pathogens and nematodes.

Lastly, because a good result relies on such extensive preparation, the best time to start developing a raised vegetable garden would be late summer or Autumn, while the soil is still warm.

Oh, and one final enhancement you may find useful to keep pets and birds away from your seedlings. You can use the side walls to anchor some steel hoops (like the ones used on igloo-style greenhouses) and cover the whole thing with a light shade cloth. Like a mosquito net, this will also help keep out many insect pests.

Once you have finished building your raised vegetable garden, you can start planting according to your crop rotation. Why not take some time to start selecting the vegetables to grow.

After all the digging you have done to this point, you probably won't want to read any more about it, but as each crop finishes, you will need to cultivate and prepare the area for the next one. Here is some essential information on various digging methods and when each one should be applied.

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Further reading and useful links

Crop rotations
This is not a rigid sequence of crops but a flexible set of stages, with a few guidelines to follow.

Green manure and cover crops
Green manure crops help improve nutrient availability in the soil and stimulate biological activity. Some can even help control soil-borne pathogens or weeds.

Garden mulching
Mulches are used to control pests and weeds, as well as for conservation of soil moisture.