The pros and cons of raised garden beds

    It might seem like a strange idea. Most people will have a yard that has at least some form of soil. You might even have weeds, grass, flowers, vegetables, and even trees growing out of that soil. So why do people go the effort and expense of building raised garden beds?

    There are several reasons people build garden beds, but before we proceed, let's look at the structure of one.

    My latest garden bed

    A raised garden bed can be as simple as a pile of dirt that is higher than the surrounding ground. However, over time, the soil will level out if it is not supported. So some sort of low retaining wall, frame, or other structure is needed for it to last a long time. The choice of material you use is up to you. Some people make them out of old railway sleepers, others will use brick walls, or concrete for a long lasting, termite resistant structure. Some will use treated timber, but that's going out of fashion because of the risks of chemicals leaching into your soils. Some people even use corrugated iron, or plastic.

    Regardless of the material used, garden beds have several advantages over the typical ground-based gardens.

    Pros of a raised garden bed:

    • Easier access: Younger gardeners won't have to worry about this as much, but by raising the garden between 30-60cm makes a world of difference to those of an older vintage. You don't have to bend over as far, people in wheel chairs can lean over and do their gardening. Planting, weeding, and harvesting all becomes much easier. When gardening is easier, you want to do it more. This increases involvement and the likelihood of gardening success.
    • You can customize your soil: It doesn't really matter if your existing soil is sandy, rocky, or a hard clay pan. You can fill it with the soil that best suits your intended plants. As the plants grow, you can ensure that the plants have the best start possible, but roots can continue down into the existing soil, instead of becoming root-bound like they would in a pot.
    • Amazing drainage: Air and drainage are as important to soil's microbial ecosystem as nutrients and water. Because the soil is above ground, there's usually a lot of opportunity for water to drain away when needed.
    • Soil stays light and fluffy: As I said in the previous point, air and water are important to soil. By putting your garden into raised beds, you're far less likely to step/walk on your soil. This avoids soil compaction, which makes it easier for roots to grow, increases the availability of nutrients, assists in the decomposition of plant matter, encourages more worms and other beneficial soil-based organisms.
    • Easy to separate and organize crops: With garden beds as permanent and obvious fixtures, you can plan out your garden, and easily perform crop rotations, simply by changing the plants you put in each garden bed. Alternatively, if you put your garden beds on a trolley, as a mobile entity, you can move your crops to suitable areas. This is a popular approach for people who have flat surfaces, (roof, courtyard, larger verandahs, balconies, etc).
    • Problems become easier to manage: If your garden bed has a highly acidic soil, you can test the bed easily, and even check the surrounding beds, and get a very accurate idea of where the problem is located, and treat accordingly. The separation also makes cross contamination of biological issues like pests less likely.
    • Higher elevations in soil gain extra sunlight for plants: For people like me who grow gardens in narrow yards between two tall townhouses, adequate sunlight can be a precious commodity. By raising the garden bed, the angles that sunlight can reach a bed widen, which is beneficial in shaded areas.
    • Invasive plant species become easier to manage: If you have a love/hate relationship with plants that like to take over such as mint, having it in a raised garden bed can limit the growth more than if it's directly in the ground. Many plants are more likely to spread across, rather than down, so putting your mint in a raised garden bed can keep it contained more easily.
    • It actually improves the soil under the raised bed: The way to improve both sandy and clay-ridden soil is the same. Add organic matter. By putting rich soil (rich in organic matter) on top and watering your plants, the nutrients will slowly leach into the underlying topsoil. This makes deep-rooted plants more likely to burrow into the existing soil, which can break up your harder clay like soils, or slow the excessive drainage of sandy soils.
    • Digging is so much easier: Without the rocks, hard clay, compaction, and easier access, and a convenient raised garden edge you probably can lean on, you can be reasonably sure that soil will be much easier to work. I typically try to reduce my digging/tilling/etc activities because this can be detrimental to the soil's ecosystem, but it's nice to know I can without too much effort when I need to.
    • Frost protection: Everyone knows that hot air rises, but few people think about cooler air. The coldest air will of course sink and cause frosts during winter. This is why frost and the resulting condensation (fog) sinks into hollows and frosty air flows downhill. By raising your plants even 20-40cm (or more if you want) and allowing frost to flow away from or around the bed, you'll find the garden suffers less from frost. Interestingly, our property is lowest at the front. By opening our gate to the side of the house, the back yard actually gains a few degrees at the surface because the cold air is allowed to flow down onto the street. If you have solid colourbond, brick, or paling fences, consider putting in a grill or opening on the lower side to stop the cooler air from pooling in your yard.
    • Earlier start to spring: Soil in the ground takes a long time to warm up to seed-germinating temperatures.  Conversely, soil in a raised garden bed takes comparatively less time. In the tropics, this might not mean anything to you, but in Canberra, the difference a week or two makes can be significant to the entire spring crop. Getting your plants established before all the pests emerge gives them a better start overall, as well as more time to grow the crops you might want to harvest.

    Cons of a raised garden bed:

    • Cost: To make a raised garden bed is considerably more expensive than simply sticking plants into the ground. Whether you use sleepers, sheet metal, treated timbers, bricks, stone walls, etc, the simple fact that these materials cost money, sometimes considerable sums (like $30-$100 per sleeper) add up extremely quickly.
    • Time & effort to make, modify, and install everything: It can be surprising just how long it can take to make some simple garden beds. A typical break down of my sleeper-based beds includes:
      • Clearing area to build garden bed. I've done this in as little as 5 minutes, or in more extreme cases 4 days.
      • Shopping & ordering materials: 2-3 hours.
      • Moving delivered materials: 2 hours.
      • Cutting sleepers to size: 2 hours.
      • Preparing sleepers with UV, natural pest resistant oil: 1 day.
      • Moving sleepers into position, leveling the ground, etc: 2 hours.
      • Drilling holes, and assembling sleepers into beds in situ: 3 hours.
      • Ripping up old pipes and running irrigation to new beds, running two separate lines along the length of the property around and through many obstacles: 7 hours.
        • 81 metres of pipe used. 126 L bends used. 14 straight joiners, 18 T-joiners, 335 hose clamps used. 40 Hose saddles used (so far) and I still haven't finished the main line run for the front hose yet. Nor have I added any actual drippers, or sprays into the new beds yet.
      • Filling beds with compost, straw, manure, and soil: 4 hours.
      • Preparing trellises where necessary: 2 hours.
      • Total time: Basically a week.
    • Tools, consumables, abilities and skills needed:
      • Rudimentary woodworking skills, including planning/design, cut list, and shopping list.
      • Strength to handle (and carry) some seriously heavy wood (and/or a friend to distribute the load)
      • 4L can of organic decking oil
      • Paint brush
      • Long 8mm drill bit (or two)
      • Drill (or two, with spare batteries)
      • Impact driver (failing that, a ratcheting socket set)
      • Long 125mm screws
      • Hex head bits to drive screws
      • Saw to cut wood to length
      • Spirit level
      • Irrigation system tools:
        • Pipe cutter
        • Screwdriver (for releasing hose clamps)
        • Hole punch (for adding 4mm off-take lines to drippers and things on the main 13mm pipes)
        • Pliers (for crimping hose clamps)
        • Hammer (for nailing hose saddles)
        • Smart phone (for testing zones attached to the wi-fi irrigation controller, and calibrating each zone for soil type, slope, sun exposure, intended plants, etc)

    My old garden bed (left) and new (right)

    Above: You can see the difference between the old bed (top left) and new bed (right). The old bed has been there for roughly 4 years, and I estimate that that I'll get another 5-10 years of use out of it. These are all Ironbark sleepers. Yes, my strawberries from the old bed have escaped and taken over. However, I don't mind having a free punnet of strawberries every day or two over December, with lower yields trailing off in January and Feb. I sometimes get a crop in the mid year as well.

    \Two garden beds used an entire tin of oil

    Yep, I used the whole can.... but I also did about 4 coats to really soak in and protect the sleepers. A can like this costs somewhere around $80-$100. I used an organic decking oil made by a company called "Organoil", an Australian company that I love to support, (no I don't get ad fees or "kickbacks") and while the oil can cause skin irritations if it drips on you (I find it makes me a little itchy) it certainly helps to protect the wood in a way far safer than the treated timbers, and most paints.

    Garden beds need maintenance over time...

    I've discussed the initial setup, but I haven't really covered what to expect "down the track". Ok, so imagine you've built a beautiful bed, you know what you're going to plant in it, so you fill it up with appropriate soil (notice the order there), you add some extra compost, some mulch, and you plant your plants.

    <cue sense of justifiable accomplishment here>

    The plants do well, you tend to them. You might go away for a much needed break, and you come home to find that the crop has exploded, and you have a lot of stuff waiting for you to pick. Great! The crops are so overgrown with a verdant green you're literally playing a natural equivalent of an "Easter egg hunt" with whatever bounty you've been growing. As all things do, the season ends, the plants die off, and you realise just how much the plants have taken out of your garden bed.

    Where did all my soil go?

    Let's say that like me, and you have stacked two, 200mm sleepers on top of one another to form each side of your garden bed. Approximately 400mm of high-grade soil can grow a lot of produce. However, with my first crop of leafy greens in the newest garden bed, in just 5 months, more than half my soil is gone and the lower sleeper is exposed. Even accounting for compaction of the straw and manure I initially added to the garden bed, this is more than I anticipated. However, large leafy greens rapidly absorb nitrogen (one of the big three nutrients), and so this brings me to my point...

    Some crops will rapidly lower the soil levels, while others don't take much from the soil, but trap airborne dust, leaves, and other biodegradable stuff, and actually build soil. However, if you're eating the produce from the plants, you are probably draining the nutrients from the soil at a faster rate than it is being replenished.

    In short, get used to topping those garden beds up. I highly recommend compost, regularly sprinkled on the beds to stop them receding so far.

    This also leads to an important consideration for the plants you put into a garden bed. Certain plants will do much better in a garden bed than others. Deep rooted plants will generally do very well. They have the best soil when they're small, and can go into the ground when they're large. Similarly, small, shallow rooted plants will also do well, running entirely off the soil put in the raised bed.  However, things become a bit unstuck when it comes to shallow rooted trees.

    Shallow rooted trees tend to send their roots outwards rather than down. When the roots inevitably hit the edge, they can become stunted. Similarly, because the roots are so shallow, they deprive nutrients from smaller plants around them. Also, as the soil level drops, the roots (and the entire tree) will drop. This can lead to problems with trees being blown over in high winds as there isn't enough soil to anchor it. This becomes particularly problematic for heavily laden fruit trees. Also, simply topping the soil up to the normal level is problematic because this might invite disease for the tree. After a while, simply "re-potting" a two metre (6' 7") high tree in a garden bed isn't really a realistic option.... so it's a pretty strained situation all around. So pick your plants carefully!

    Regular sprinklings of soil, compost, and mulch will help a lot. But now let's look at the bed itself over time...

    Obviously, no matter what you make your raised bed out of, it will need maintenance, or it will start to break down. If you have plastic, the UV rays will start making the bed very brittle.  If you have metal, it will start to rust. If you have wood, then obviously it will rot. If you have a brick wall, the mortar may need patching up, and if concrete... well.. ok.. that probably doesn't need much maintenance itself, but the metal corners, and other accessories you used to build your bed will start to rust eventually too. So when the soil is low, and the plants are done for another season, perhaps this is a good opportunity to dig out the remaining soil (put it in the compost for recharging) and have a look at the condition your bed is in. Let it dry out for a few days, brush the dirt off, then give that bed a thorough inspection, and repair where needed. This might be fixing a missing screw, or give the wood another coat of preserving oil, the plastic might be cracked... patch it up as best you can. Then you can refill and put it back to work.

    Summary:

    Garden beds bring a lot of advantages, but can also bring significant costs as well. If you're handy, and have tools on hand, you may even be able to handle cutting and moving concrete sleepers, build brick or stone walls, and use these or other long lasting materials. Conversely, I've seen people use hay bales as their raised garden bed walls, and when they break down, they've simply got extra soil to work with. This is a much cheaper and simpler solution, but also doesn't last as long.

    The main reasons I make raised beds, is to offer better soil for my plants, get easier access and long term management of the garden, and to add value to my home. I'm often time poor in the busier growing periods in the garden, and unwilling to do much gardening during the bitter winters here in Canberra. So garden beds really help me during the spring and summer growth periods, and autumnal harvesting time too.

    Perhaps your rationale will be similar, or even completely different to my own. Or perhaps you'll decide it's all "too much". In any case, I hope this helps you to make informed decisions on your garden design, and whether raised beds might be something worth looking into for you.

    As always, stay safe, be well, and happy gardening!

    Ham.


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