Raised beds are a highly functional and productive way of organizing a vegetable garden. Plants are grown in boxes or beds with raised sides that can usually be approached from one or more directions. Raised beds allow us to mix the right kind of garden soil for our food plants as we build up the soil inside them and this is especially relevant in gardens where soil is very poor – either heavy, sticky clay or fine sand – or if there is no soil at all, say on a roof or in a courtyard. Soil in raised beds drains well with rainwater easily running away rather than sitting on the surface or in the main body of the soil for too long. Good drainage is essential for a wide variety of vegetables, herbs, fruit and flowers that can be grown in them. The height of raised beds can vary greatly but they generally reduce the amount of bending over when it comes to weeding and planting.
Width - Ideally you should be able to reach into the middle of a raised bed from either side without walking on the soil contained in it. A good way of working out what dimension might be right for you is to lie on the ground and stretch out your arms. Notice where your finger tips are and use the distance between them as the width of your bed. In my case its 2 metres (two good strides). As for length – beds can be as long as you like - often they are around twice their width.
Height – The height of your bed might depend on the materials you use. Macrocarpa sleepers and boards often come in standard sizes and so the height of a bed can depend on how many boards you build up with. Recycled roofing iron can be cut down but otherwise it creates a fairly tall bed. When contemplating bed height think about what you hope to grow, the minimum soil depth for lettuces, silverbeet, onions, radishes and spinach is about 20cm (a good hand’s length) whereas the likes of beans, cucumbers, potatoes, tomatoes and carrots need 45cm (from finger tip to elbow). A good standard bed height and soil depth is about 45cm, this will be adequate for just about all general vegetable plants as well as many fruit plants. Beds set at about 45cm tall are average seat height and this makes life more comfortable as you can perch your bum on them whilst you are weeding, watering and feeding.
Materials: Best to avoid, if you can, using treated, or tanalised, timber that leaches chromium and arsenic into the soil - although there are those who do use this stuff and line it with heavy duty polythene to keep chemicals away from vegetable roots. Far better to use macrocarpa that has natural oils which preserve it in and on the ground without any adverse effects on your veggies. Loads of folks use macrocarpa sleepers which are usually 20cm x 15cm x 2.1m but they can be a bit pricey. In our garden we use ‘landscape grade’ macrocarpa boards 20cm x 5cm bolted to 15cm x 10cm corner posts. Popular alternatives to macrocarpa are sheets of roofing iron laid on edge and tied to waratahs, old native timber weather boards (with the painted side facing out), old bricks, ponga stems, plant pots filled with soil, logs, even tea tree branches cut to short lengths, part-buried and tied together – you can even use wine bottles filled with sand and buried neck-down to put some curves onto your veggie patch. More expensive and permanent are beds made from concrete blocks or bricks – you’ll probably need the help of a builder with these.
Weed prevention: When you decide where your bed is going to go, mark out the area and, using a spade, cut or scrape off any grass and dig up any weeds that are in the way before you build your bed. If there are persistent weeds in the ground like kikuyu or couch grass and bindweed, before adding any soil it might be worth laying some weed-proof membrane (buy it by the metre or in rolls from garden centres) or old wool carpet in the bottom of the raised bed. This should stop weeds coming up through the soil in your raised beds over time.
Soil in raised beds:
Raised beds can contain a mixture of normal garden soil and what is called ‘garden mix’. ‘Garden mix’ is a growing medium made from organic material, fine drainage particles and soil – often with a few extra nutrients added in the form of slow release granules or compost. It is sold loose from Landscape supply outlets or in bags from garden centres. Filling raised beds with ‘garden mix’ can be costly but once you have it in your beds all you need to do is to give it life by careful and regular composting and mulching to increase soil microbiology that will help make essential nutrients in the soil available to your plants.