Note that an excavator is usually the most helpful piece of earthworking equipment for urban permaculture installations - they can be used for digging swales, trenches, French drains and you have a lot of control over where the excavated dirt can be placed (i.e. when making mounds or berms). In my opinion, a dingo is not as suitable for these kinds of tasks - they're more useful for levelling and pushing things around than for digging.
Neither of us had used an excavator before, but you don't need a special license to drive one and you can hire one pretty easily. It did take a bit of getting used to though - and it's a pretty bumpy and uncomfortable ride!
The first job to do with any earth works is to scrape off the topsoil first (about the top 100mm), and put it to one side. This is generally considered best practice - nature puts top soil at the top for a reason - but it is tedious and took about 2 hours before we were ready to start digging.
Once I got the thing working, I'm not sure I'd call it great fun exactly, though it's satisfying when you manage to pick up a big bucketful of dirt. What is noticeable is that it gets quite uncomfortable in the seat, hunching over the controls which seem a little low. There's no back support so my partner and I both had pretty stiff lower backs by the end of the day.
Given the heavy clay content of our soil, water takes some time to drain away, up to a day or two, so I'm thinking they might perform almost like in-ground wicking beds! But just as with any wicking bed, an overflow is essential so they won't fill up with water above a certain level. We will be putting in a couple of overflow channels at the edges of the berms so in a sudden downpour these beds can't fill up too much and get waterlogged - we aren't planning to grow rice in them!
Obviously we can't just plant straight away into the clay - once the bases of the beds are levelled, the next step is to fill the beds with lots of organic matter - compost, mulch, lucerne hay, some top soil, old potting mix from some temporary wicking beds. Then we will plant a green manure to build up the soil ahead of the first summer crops. We need to get a lot of good organic soil built up to act as a sponge on top of the clay base of these sunken beds. The beds won't look anything like as deep once the soil has started to build up.
In case you're wondering about the overall concept, these basins are essentially modified 'rain gardens' (in that they will have overflows to work around the impermeability of clay). My go-to source and inspiration for this approach is Brad Lancaster's series of books on Rainwater Harvesting for Drylands and Beyond. I can't recommend these highly enough for anyone interested in learning more about passive water harvesting. [Note - not an affiliate link. I don't do affiliate links - this is just my personal recommendation of a great set of books].
Our beds are somewhat experimental: we're keen to push the boundaries to see what will work in Canberra's unique conditions of very hot dry summers and cool, usually wet winters. We plan to use them for annual food production because of the high water needs of these plants, especially in summer. Note also that the water running into this system from hard surfaces will not be polluted in any way.
The next step is to level the bases of the beds as much as possible, and stabilise and shape the mounds of earth around them (the berms) in preparation for planting in them - which is a job to do by hand. I'll describe this in the next post.
If you're contemplating doing some earth works in your garden, then here is a summary of key things to consider:
- Make a plan before you start - know what you are doing and why you are doing it.
- Check the locations of underground services on your land (e.g. contact Dial Before You Dig)
- If possible, do earthworks in the cooler months of the year - the soil is more moist and doesn't dry out too fast. In summer it will be very dry and dusty, and digging baked concrete-like clay is no fun. It's also hard to establish new plants in very hot weather.
- If your soil is clay rich (like most of Canberra's is) avoid earthworks in or after heavy rain too, as you'll be working in a mud swamp, and be treading mud into your house for weeks afterwards! In hindsight, we have been very lucky with our timing - with winter being so dry, the soil was only slightly moist, making it a lot less muddy than it could have been.
- Make the smallest change possible for the result you are after. If you don't have to decimate the whole site for your garden plans, then don't. Consider whether you can keep some existing plants in your garden for the ecological services and habitat they already provide.
- Earthworks are immensely destructive to soil life which will take time to recover. The earthworks described above are to my mind reasonably large scale and disruptive (not to mention somewhat experimental), so I would not recommend this for everyone!
- Nature does not like bare soil. Plant it up as soon as you can after doing earthworks - scatter a variety of seeds thickly over the whole site, from green manures to any old seeds you want to use up, just to get a head start on the weeds. Nature will use whatever resources she has to hand to repopulate bare earth - so if you don't want couch, sheep's sorrel, fat hen, rye grass and all manner of other weeds in your garden, get planting! Covering areas you don't want to plant yet with mulch can also help.