Raised beds can be very useful in a vegetable garden. This article will explore the options available, and how to position, build and fill a raised bed.

Why raised beds?

There are two reasons why beds raised above ground level are preferable. The first is strain – no matter how young or old you are, working with soil quite a bit above ground level makes sowing, weeding, planting and picking a breeze compared to doing it on your knees or bending over. The second reason is soil – if the soil in your garden is black-clay or mostly-sand, you are setting yourself up for success by creating raised beds with good quality food-garden soil or compost. Don't expect nursery-bought soil or compost to be great, because it rarely is, but it will be a much better starting point than having to improve really dense or sandy garden soil.

What is the best spot for your new raised bed?

Find a good spot for your new raised bed with the following in mind:

Sun – Give your vegie bed the sunniest spot you have available.

Roots – Don't put it right next to bushes or trees because their roots will invade your garden bed.

Water – Position it not too far away from a tap or hose, so watering is easy.

Wildlife – Ideally put it where the local wildlife (possums, wallabies, birds, rats) is either fenced out, or may not find it.

A flat and level surface – Having a food garden on a slope is fine, as long as your garden beds in them are level. To minimise digging, position your raised garden bed as closely as possible to 90 degrees on the slope and then dig away soil as needed to create a perfectly flat and level base. You don't want your raised bed sloping down in any direction. If a bed is not level, soil at one end will often be too wet and soil at the other end too dry.

What are good dimensions for a raised vegie-bed?

Width – Make sure its width is no more than twice the length of your arms, so you can comfortably reach the middle of the bed from either side without standing on it or having to reach beyond your capacity.

Height – No vegetable root system, including that of carrots and parsnips, will go deeper than 30cm, so your raised bed should be at least 30cm high. A height of about 60cm might make sowing, weeding and picking more comfortable. The perfect height for you will depend on how tall you are, and how much bending you are happy to do. The higher the bed, the more material you will need to fill it.

Length – Some gardeners prefer really long raised beds, instead of several shorter ones in a row. Long beds maximise growing area, but I have seen raised beds that were so long that it was quite a walk to get to the other side of where you are working.

Before you order a nice big pre-fabricated raised bed, consider how the frame will be transported into your garden. Will it fit through the gate?

What ready-made raised beds can I buy?

If a small raised bed is all you need, a kit from a hardware shop or garden centre can be the way to go, provided that the product, once you have it assembled, is sturdy. A kit where wood will touch soil is not going to last, and chemicals may leach out of the wood into your soil.

New pre-assembled garden beds – I see these for sale at road-side stands. They can be fine if they are sturdy and have the right dimensions. I would not trust beds that have legs and are off the ground because soil, especially when it is wet, is heavy.

New steel corrugated garden beds can now be ordered in virtually any size, height and width. They are very sturdy, extremely durable and come in many colours. Professionally made corrugated garden beds will have a top edge that has been treated, so you can't cut yourself on the metal. Using a steel corrugated garden bed is not the cheapest option, but it gives you a sturdy and maintenance-free frame that will last decades. Most manufacturers are happy to transport the raised bed to you.

Re-purposed tip shop items – My favourite shop is the tip shop. Once you open your mind, you will find many items at the tip shop that, with little modification, can be ready-made raised beds. They may just give your vegie garden the quaint look you are looking for. Laundry tubs, old wheelbarrows, bath tubs, old water tanks, apple crates and large containers are all potential raised garden beds. Make sure the item is chemical-free. You may need to make quite a few holes in the item's bottom so excess water always drains away freely.

What do I fill my new raised bed with?

This is not as silly a question as it may sound. In 2012, when I filled my first large raised bed, I bought a trailer load of veggie-garden soil and compost and chucked it in. Not all went to plan. Not only did I need a lot more soil than I thought I would need, but over the years that soil compacted to the point where in recent years I have had to improve the drainage in this bed quite a bit to keep my vegetables happy.

After building five other raised beds over the years, and after having learnt a few lessons, I filled my latest raised bed as follows (from the bottom up, for a 60 centimetre high bed):

• a 10 centimetre layer of not too fine blue metal pebbles
• a layer of geo-textile so the next layer is not going to invade the layer of pebbles
• a 15cm layer of shredded branches and leaves
• a 30cm layer of soil with a fair amount of gypsum added (to avoid compacting)
• 5cm of mulch

Why put good soil or compost in the bottom of a 60cm high raised bed when no vegetable roots are never going to reach it? Instead I put in the bottom of the raised bed four bags of medium-grade blue metal pebbles from Best Mix in Mornington. I spent $18.40. I bought geo-textile at Bunnings. A roll 0.9m wide and 5m long cost me $11.95.

The idea for the shredded branches and leaves comes from a method called Hügelkultur, whereby you create a raised garden bed that is built from the bottom up with logs, sticks and branches, wood chips, grass clippings, manure and leaves. You can find more information about Hügelkultur at richsoil.com/hugelkultur.

Adding this layer meant that once again I did not waste valuable soil or compost. Instead I used a large quantity of garden prunings that otherwise would have little value.

I shredded the branches and leaves in my shredder before putting them in, but that is not essential. You can put uncut branches and leaves in the raised bed and push them down a bit to make sure the layer does not contain a lot of air.

For the top layer I used soil from my compost heap mixed with some gypsum to make sure that the soil does not become too dense in years to come.

The Hügelkultur layer will turn into compost over time. As a result, the soil will sink somewhat. At that stage you could add some soil or compost, but that is not essential. Just organise the soil layer in the same way you do for your ground level beds, eg, dig or no-dig.

Max Bahrfeldt started The Food Garden Group a decade ago after retiring from many years of full-time teaching, managing and designing courses for adult education. The aim of the group is to create an active community of Tasmanian food gardeners, who freely share their knowledge, surplus produce, seeds and plants. Meeting other food gardeners face to face is an important part of the group, and through this many food gardeners have found new friends with like-minded interests. Members include beginners, experienced food gardeners, and some horticultural professionals.

This article first appeared on The Food Garden Group blog (foodgardengroup.blogspot.com).