Raised vegetable garden beds have long been used in agriculture as a method of enhancing productivity and managing food growing space. This is often done simply by mounding of rows up to a maximum height of 20cm or where extremes of slope demand hard borders to create level growing areas, for example, hillside rice paddies in Asia with clay and gravel borders.
In the smaller scale food garden, raised vegetable garden beds can be as simple as mounding soil in rows. On a gentle slope mounded beds do not need hard borders. However you can choose to border the raised beds with materials such as untreated timber, concrete blocks or rocks.
You may also consider filling a structure made of metal, plastic or wood with soil material.
Situations where it is ideal for raised vegetable garden beds
- Where you have a slope, its important to make your beds as level as possible. By cutting the beds into the slope, making them level and then if necessary retaining the lower side of the bed, you are effectively creating a raised bed on the lower side.
- If you have no slope, you may choose a raised bed as a decorative feature of your garden. The beauty of your growing space has an important benefit on the success of your food gardening, for example, a herb spiral raised up to 1.2m and held in place by rocks can be a wonderful feature.
- Perhaps your back makes it difficult bending down to ground level. In this case, having a raised bed of about 60cm high would be very helpful – making your food garden an easy place to work in is very important.
- Where you have no soil base at all and your garden is on a balcony, concrete podium or rock, having a raised bed gives you a sufficient soil depth to grow food. A depth of 40cm is OK for vegetables.
Advantages of raised vegetable garden bed structures
- In cooler times of the year when it is sunny, say late winter to early spring or mid autumn to early winter, the soil warms up faster with a raised bed giving you potential to extend your growing season because of the warmer soil;
- Drainage is better in raised vegetable garden beds, so long as water can easily seep out of the beds
- Normally soil will not compact as easily, improving air flow and helping plant growth, so long as you do not walk on the beds;
- In cool climates, the potential of frost damage is reduced with raised beds, as your beds are more likely to be above the lowest level where frost sits;
- Having a firm border in your garden bed due to a raised bed structure helps to increase your planting space in the beds as you can plant right up to the border and the hard edge helps to reduce water erosion
- You can use the harder border of your garden bed structure as support for a climbing frame for plants that like to climb such as peas, beans and cucumbers
Materials to use for raised vegetable garden bed structures
- Rocks are durable, solid, look great and provide minerals to the soil as they break down. The only disadvantage is that they can be quite costly.
- Timber is easy to work with, but can be eaten away by ants if its not hard wood. You could treat your wood for this problem but then you would be introducing those poisons into your soil. Using old recycled timber that is not CCA treated is fine. If you can find non-CCA treated pine, then use this. Cedar is also a reasonable wood to use and last longer than pine.
- Metal, such as corrugated iron, is very easy to work with, light and is designed in many configurations. It also comes in different colours
- Plastic planting containers such as pots are a form of raised garden beds and are easy to work with, but generally not used for larger beds.
Some challenges with metal raised bed structures
From a soil fertility point of view, a healthy vegetable garden needs top soil of no more than 30cm deep, so if your metal structure is any higher, you will be increasing soil needs without getting too much benefit. The exception is if you are placing your raised bed on concrete or rock, in which case at least 40cm would be OK.
If your raised bed is on concrete, there is no connection to the flow of life and minerals in the earth. This adds many challenges to maintaining fertile soil. It simply means you need to do more work to maintain healthy soil compared to in-ground beds
The higher your bed, the hotter your soil will become in warmer climates. In summer time this extra heat will cause plants to wilt and go to seed faster. This extra heat will also mean you need to water more and if you have to water a lot then the soil is more likely to be leached of minerals which then means more need for soil fertilisers and this ultimately leads to dead over-fertilised soil
If you live in a hot climate and use a corrugated iron structure, it needs to be a reflective or light colour, to reflect the heat. A non-reflective colour, such as dark green will become very hot to touch in summer. If you live in a cold climate, then the colour should be darker to absorb heat, such as dark green
- The higher the structure, the greater the start up cost and the more fill you need;
- The metal raised bed can become water logged in high rainfall unless there are drainage holes in the sides of the bed and under the bed, particularly if its on concrete or rock. Thus for wet climates pay extra attention to drainage and water logging;
- The materials used to fill a metal raised garden bed should be very carefully considered. I have heard of many metal raised beds of up 1.2m being filled with polystyrene and bricks in the bottom half and then soil in the remaining space. You would be much better off with a 60cm metal raised bed filled completely with soil. This would save money and control temperature better and you will have a healthier soil layer.
Other challenges of raised vegetable bed structures
- If you choose to have a raised bed which is 1m or more, you will need a lot of soil to fill it. Where you already have this soil or its soil cut in from a slope, then using extra soil is not really an issue. If you have to buy in soil or make soil over time just because you have chosen a high border structure, then you are likely to be wasting effort and money.
- Some gardeners get around the lack of soil in a high bed structure by filling the base of their structures with rubbish. This does not help the quality of the soil and should be avoided.
- The fill should not include uncomposted manures, as is very common in “no-dig” raised bed gardening these days. Such manures have hygiene concerns and carry the risk of contamination with herbicide. You would be better served by composting manures before putting them into your food garden or growing cover crops in your raised garden bed until such time as the manures have broken down and then plant your food crops.
As you can see, there are a multitude of factors to consider, which is not unlike most things in successful food gardening and in life, for that matter. We work through these concepts at our regular workshops, webinars and via our one-to-one organic gardening coaching service. At times, we also provide services for food garden build projects.
Author: Peter Kearney – www.myfoodgarden.com.au