by Robert Pavlis
There are some benefits for using raised beds to grow vegetables, but they also offer some disadvantages. Many of the advertised benefits also apply to traditional beds, and some are just not true.
The term raised bed is a bit confusing. If you add a few inches of soil to a garden bed that is at ground level, it is a raised bed. When most people talk about raised beds for vegetable growing, they mean a bed that has been raised with walls surrounding the soil, sometimes called a garden box or framed bed.
I’ll use the tem raised bed to mean a bed that has walls and compare it with a traditional bed that does not. A traditional bed may or may not be raised above grade, but it is usually not raised more than six inches.
Benefits of a raised bed
- The garden looks neater. The walls keep soil in place, and pathways can be kept cleaner.
- If walls are at least two feet high, they require less bending to work on the plants.
- They can be used in areas that have very poor soil, contaminated soil or no soil at all. Containers are small raised beds.
- They warm up quicker in spring, allowing earlier planting.
- They are a barrier to pests such as snails and slugs, and very high walls might deter small animals.
Claimed benefits for raised beds that are not true
- Provides good drainage. This may or may not be true. If the soil in the raised bed is substantially the same as the soil under the bed, then a raised bed will provide better drainage. The problem is that most recommendations for raised beds call for the use of soilless mixes or other special blends. If the soil is very different and you create a perched water table (see Myth #84), then the raised bed may not have better drainage. Some even put a weed barrier under the raised bed, and this will reduce the drainage.
- Need fewer seeds. I don’t understand this claim. Seeds can be planted exactly the same in both systems; this is not a benefit of either system. Since each plant requires a seed, fewer seeds would mean fewer plants – why is this an advantage?
- Fewer weeds. The amount of weeds depends very much on the soil. Traditional soil does contain a lot of weeds, and they show up in vegetable beds. If you use this same soil in raised beds, they will also have weeds. If a soilless mix is used, it will have fewer weeds, provided you don’t add some weedy manure to the mix. Over time, even a soilless mix will start showing weeds that are growing from seed that blew in from neighboring areas. At best a raised bed has fewer weeds in the early years.
- Longer growing season. This is not true. A raised bed will warm up quicker in spring and allow earlier planting, but it also cools down faster in fall, shortening the season.
Claimed benefits that are also true for traditional beds
- Less compaction. The main cause of compaction is people walking on the soil. Traditional beds can be just as narrow as raised beds with specific walking paths between the beds, so the gardener never walks on the planting area.
- Layers of compost maintain soil structure. This is true for both systems.
- Each bed can be customized with a different soil specific for the crops that will be grown there. This is easier in raised beds, but it can be done in traditional beds.
- Produce higher yields. Better soil, intensive planting and more compost will produce higher yields, but all of these techniques can also be done in traditional beds.
- Statements such as “you do not have to space your seeds as far apart like you would when planting directly in the ground” make absolutely no sense. I think someone is comparing raised beds with traditional farming.
- The quality of food is better. This is a common claim with absolutely no data backing it up. It is similar to the misguided idea that organic food tastes better. The quality of food, either the amount of nutrition or flavor, depends largely on the variety being grown, soil type, nutrient conditions and climate. Both systems can produce the same quality of crop when these conditions are the same.
Benefits of growing in a traditional bed
- There is no soil to buy or transport.
- They are less expensive to make.
- Soil does not dry out as fast in summer.
- Requires less watering.
- More sustainable since you don’t need to buy and transport walls and soil.
- No concern of chemical leeching from the material used to build the walls since there are no walls.
Raised beds may in fact reduce yields. The roots of a tomato plant in the ground grow several feet in all directions, but in a raised bed its root system is confined. Raised beds dry out quicker in summer, and water becomes a limiting factor for yield. Given good soil, and the same amount of fertilizer and water, it is hard to see how plants crammed into a raised bed would produce as much food as in the ground.