Raised Beds

This month’s topic is raised beds and I don’t mean four poster beds in medieval castles or princess-in-the-pea stories of beds fifty feet high! I’m talking about garden beds that have been raised above the surrounding ground level for whatever purpose. Raised beds are often used to improve drainage; they are used to create some height in an otherwise flat landscape; they are used to display plants to better advantage and sometimes they are used to provide easier access for the gardener. Garden beds often become raised all by themselves as annual mulch applications rot down increasing the soil level.
When we first moved into our house, I was concerned that with an anticipated average annual rainfall of 750mm and a flat block of land, all my plants would get waterlogged. I decided to raise all the garden beds to improve their drainage – a plant on a mound is less likely to die of wet feet. To this end I covered up the builder’s rubble with imported soil and hired a bobcat to contour the earth. He created mounded beds (I didn’t use retaining walls) and I ended up with some essential height and pleasing forms with which to start the garden and with drip irrigation it was all okay at first.
Fifteen years later I have changed my mind about the need for raised garden beds here. Not the least because we found out the actual annual rainfall here is 623mm and then you have to factor in climate change which has meant that over the last 11 years it has averaged out at 380mm per year. The raised beds have begun to work against me, in that I can’t get water into the soil – it just runs off. Most of the plants are stressed and some have even died. To see whether removing the soil and going back to the original soil level might help, led to the reconstruction of one entire bed. I removed all the plants last July and got a mini-digger to dig out the soil and builder’s rubble and level the bed at just above the adjacent path height. I replanted the bed and crossed my fingers. The plants settled in well over winter and spring and with the aid of greywater, took off this summer just past. Plants which were struggling before on the raised bed have doubled in size and it is gratifying to gaze at the most crowded and most floriferous bed in my entire garden.
So are raised beds a good idea or not? I think they still have a purpose especially if you live in an area with high enough rainfall that waterlogging may be a problem. If you want raised beds for added height in the landscape then you just have to be aware of the difficulties that can be associated with getting water into the soil and retaining moisture in the soil.
A girlfriend of mine has built an interesting raised bed in her front garden. She surrounded the large oval bed with what we call paddock rocks (small volcanic rocks) and built the bed up about 20-30cm higher than the surrounding ground. However the centre of the bed was slightly hollowed making a kind of swale. Rain falling on the bed was directed into the swale (instead of off the bed) where it soaked into the soil and was available to the plants. Has it worked? Well the trees in the bed have shot up very quickly and the shrubs are doing very well too. Perhaps that’s what I should have done in my garden!
Raised beds that have been created with railway sleepers, rocks, concrete or corrugated iron are a slightly different kettle of fish. Even filled with garden soil, I think these need to be treated like very big pots and a vigilant eye kept on them with regard to watering. Vegetable gardens are often built out of railway sleepers making it easier for the gardener to cultivate rows of crops. Lately on television we have seen the use of corrugated iron rings in which to grow vegetables. We had four of these made for our local primary school and it meant the children were at the height of the vegetables and didn’t have to kneel in the dirt. These would be very useful for a wheelchair bound person or someone with a bad back. They also look very stylish but their drawback is they need a lot of soil to fill them up as we discovered! Raised beds also have some other advantages; the soil warms up more quickly than the ground in spring so plants get a head start. This is great for vegetables in particular.
Another friend who owns and runs a nursery south-west of Melbourne is in the process of creating a display garden for his succulents adjacent to his nursery. When they were reconstructing the Princes Highway outside his front gate, he got the trucks to dump their spoil on the site of his garden-in-the-making. He shaped the gravelly soil into enormous mounds some over 3m in height. The plants have taken to their well drained and sunny aspects extremely well and my friend has a garden with instant impact.
I have come to realise that the dead and dying plants on raised beds in my garden are no longer suitable to the site (or the climate) and am gradually replacing them with plants more suitable. I am looking at choosing from a range of succulents and plants from similar climates like South Africa, the Mediterranean and California! A garden is never static and conditions change – a fact well worth remembering when your favourite plant carks it!