Growing in raised vegetable beds in Tasmania is easy and convenient but there are some things to consider when designing and setting up your plot.
Our horticulture students are exposed to a wide range of growing methodologies and systems. Raised bed systems are always something they want to know more about and many of them end up creating them in their own gardens.
The advantages of growing plants in raised beds in Tasmania are well known. The ease of access and relief on gardener’s backs and knees are both good reasons for choosing these beds over growing in the ground. Raised beds are also good for those of us with limited growing space, nutrient poor, tough or contaminated soils and lets face it, they can look pretty good too!
Some research also says that growing in raised beds can speed up plant development and seed germination because the soil is on average, warmer sooner than the surrounding soil at ground level.
So once you have decided to grow your veggies in raised beds, there are a few things to consider.
Raised beds vs no-dig gardens?
No dig gardens are a methodology that involves layering of growing media, compost, decomposing straw or hay and other nutrient sources like rock dust. There is no reason why the no-dig methodology can’t be used in a raised situation as well. In fact, ABCTV legend Peter Cundall used raised no-dig beds very successfully at the Royal Tasmanian Botanical Gardens for many years.
How high to make your raised bed?
Opinions vary on how high the sides of your raised bed should be. The beds at the community food garden at the Royal Tasmanian Botanical Gardens vary in height. Some are as little as one or two sleepers (20-40cm) high and others are up to 75cm or the height of an apple bin. There are various commercially available options as well with a range of heights. Plan or select a bed that allows you ease of access above all else. If you find it hard to maintain, you will let it go, but if it’s easy to get to, you are more likely to use and maintain your bed.
Dimensions for a raised bed?
As with bed height, select dimensions that are right for you and will allow you to access all parts of the bed with minimal effort. We recommend a bed width of no more than 1 – 1.5 meters. This will allow you to reach the middle of the bed and plants on both sides. Anything larger than this means you will have to physically get up onto the bed to weed and plant. This is a big no-no! Its unsafe and more importantly compacts your soil! Never put heavy weights onto your soil. Light friable soil texture is what you area after in the growing media, not heavy compacted areas that plant roots and water cant penetrate.
As for bed length? It’s a personal choice based on the area you have available to you but remember, build only what you can maintain easily. It’s easy to get carried away at the building stage, but remember things like how much soil you will need to fill the bed, distance from taps and hoses and bed position in relation to sunlight in your garden.
What materials to use for a raised bed?
The short answer to this is “just about anything”. We have seen galvanised roofing iron, bricks, blocks, sleepers, crates, pallets, even logs used as walls for raised beds. Choose something that will be helpful to you when you are maintaining the bed. If you are planning to sit on the side of the bed while you weed, choose a wide material like sleepers or brick over roofing materials and tin. If you want to do most of your gardening standing up, an apple bin might be better for you.
Contamination issues associated with raised beds?
This is an important consideration when making your raised garden beds. Treated timbers, used roofing iron, railway sleepers and just about any second hand material can be a source of contamination in your soils and into your produce. Treated pine sleepers can contain harmful chemicals like arsenic that can leach in to the soil. Used roofing iron can have lead paint flaking off and even hardwood railway sleepers may have been treated with oils like creosote or be saturated with years of railway carriage debris.
Choose a material that’s history is known to you. New hardwood sleepers are a good option. If you are planning to use another material, simply line the inside of the bed and any surface that will come into contact with the soil with black plastic before filling it. Don’t forget to leave the bottom of the bed open for drainage, or your bed will be a pond!
What soil to use in a raised bed?
A good loam or sand/loam mix is a good start, but remember your drainage! Filling the entire depth with soil is a common mistake. Remember, all of your plant growth takes place in the top 30-40cm of the bed. Placing soil deeper than that will risk the bottom levels becoming too compacted to allow air to penetrate. This will create “anaerobic conditions” in your beds. You will know if this has occurred when you dig into those layers and they smell sour and acrid. This is the reason many raised beds don’t produce in their second year.
Good drainage will solve this problem. First put down a layer of weed mat or several layers of wet cardboard to stop existing weeds breaking through the depth o the bed from the ground up. Then use a coarse material with lots of gaps to fill the bottom of the bed. Broken bricks or blocks or a coarse gravel are good options. On top of that, a layer of finer gravel and then a cover of shade cloth or geo fabric that will hold in your soil but allow water and nutrients to pass through to the drainage levels. Then put your soil in on top of the fabric to a depth of 30-40cm depending on what you intend to grow.
Watering a raised bed.
There are many options for watering a raised bed. Hand watering or irrigation systems are fine. We would recommend a drip irrigation system if you are putting in fixed watering. This cuts down on fungal problems and diseases and takes the water right to the roots of the plant.
Another option is a wicking system. This involve placing a “reservoir” of water (commonly a pvc pipe with small holes drilled in it) deep under the soil and filling it periodically from a branch of pipe leading to the top of the soil.
A good video of wicking bed methodology can be seen on the Gardening Australia website here: http://www.abc.net.au/gardening/stories/s4010599.htm
As we have said, the variations on the raised bed system are limitless. Take a picture of yours and share your tips on our facebook page. Gardening is all about leaning and sharing knowledge. Good luck!