Raised beds and to dig or not to dig, that is the question.

Raised beds have their place, but not necessarily every place (Photo Joe Mabel, Wikimedia Commons)

Raised beds have their place, but not necessarily every place (Photo Joe Mabel, Wikimedia Commons)

Maybe the gardening world divides into diggers and non diggers. The non diggers favour raised vegetable beds and no dig gardening methods. We old diggers, on the other hand, are closer to our peasant ancestry.

Digging is not an activity to be feared. The physical effort is terribly good for you and it is only hard the first time. The more you work the soil, the easier it becomes. One of our vegetable gardens has been tilled for anything up to ten decades in some places and it is like working in potting mix. While Mark prefers to dig with a shovel, I swear by a good, sharp spade. People often neglect sharpening their spade but it makes a huge difference if you keep a good cutting edge on it.

Being of the digging persuasion, we have never felt the need for raised beds. If you put some effort into turning over the soil of your raised bed, it will fly in every direction, spoiling your surrounding paths. Lesson number one, should you decide you want them, is to keep the soil level lower than the surrounds and leave room to build up further as you add compost, green crops and all the additions which keep it fertile and friable over time.

Raised vegetable gardens have their uses in certain situations. Obviously they are of inestimable value for disabled gardeners or those who cannot bend comfortably. They can be useful where you are trying to garden in an area choked with old tree roots or with really bad drainage. They are, my daughter tells me, particularly useful in Canberra where a combination of clay, a new puppy and a heavy population of ground birds named choughs (pronounced chuffs) condemn any vegetables at ground level. But in the main, if you have halfway decent soil, I think it is actually easier to follow the traditional way and garden at ground level. The current vegetable garden craze has seen a building boom for raised beds and too little questioning of the rationale.

For starters, there are issues related to the construction of raised beds. Brick or concrete block is permanent and relatively expensive – devilishly difficult if you change your mind about your garden layout further down the track. Hardwood railway sleepers are expensive and heavy to handle. Most people end up with pine. Herein lies a major problem. If you use untreated pine in a high rainfall climate, it is going to start breaking down within a couple of years. Don’t forget it is permanently wet on the inside of the bed. So what about tanalised pine?

“No,” said my scientist daughter in Canberra. I trust her judgement in these matters and she did spend one summer as a chemistry student looking at sap staining on tanalised timber. She comments that the information on the internet is not particularly up to date, and in the absence of scientific evidence proving safety, she certainly would not want to be growing anything edible near tanalised timber. She points out that the preservative was, and may still be, a mixture of arsenic and chrome and plants are very good at absorbing and holding heavy metals. She wouldn’t risk it. She is constructing her raised beds out of untreated pallet timber which will break down in due course but is at least free, recycled and will last longer in her dry climate than here. She has also coated it with linseed oil.

The conventional approach to vegetable gardens, though one hopes the timber sides are not tanalised

The conventional approach to vegetable gardens, though one hopes the timber sides are not tanalised

Once you have built your raised bed, you have the problem of filling it. Too often, I have seen people on TV wheeling in large, heavy grade plastic bags, each containing 40 litres of soil mix bought from the garden centre. I can’t think of much that is less environmentally friendly. If you are not going to go down the expensive, convenience route, you are going to have to find or create your own soil. It takes a much greater volume than you would ever dream of to fill a raised bed. Then you have to shovel or lift the soil in. Don’t underestimate the size of the task.

At this point, some advocate going the no dig route. I know the theory, but being diggers, we have never done it. It consists of layering a mixture of compostable materials and leaving the worms to do the task of breaking it down. So you layer green material (but avoid weeds, seeding plants and anything diseased such as mildewed tomatoes) alternately with dry matter (fine twigs, newspaper, even old woollen carpet, straw and the likes). Essentially you are building a cold compost heap. It takes time for it all to break down and form a tilth and longer term, it matters a great deal what your proportion of green matter to dried matter is. Kay Baxter of Koanga Institute has written about this and why she chose to abandon the no dig approach after many years to return to old fashioned double digging.

If you want to hasten your no-dig garden, you end up adding large quantities of compost and humus and this is where my vegetable growing husband draws the line. We make large quantities of hot compost here and as far as he is concerned, that compost is destined to be spread annually to a depth of no more than 5cm across as much garden as possible – thereby feeding and conditioning the soil while acting as mulch. There is no way he will allow a single garden bed, whether raised or not, to absorb more than its allotted share just to avoid having to dig the soil.

Raised beds and no dig gardening require higher inputs for the same, or often less output. We are waiting for practitioners with several decades of experience to convince us that these gardening techniques are an improvement, rather than recent converts.

First published in the Waikato Times and reprinted here with their permission.

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