How to: Raingardens, and vegetables under the downpipe

A raingarden is a (usually small) planter or mini-garden watered only by rain when it falls. They’re a lovely way of creating a garden feature, should you have the right kind of climate for them. The right kind of climate is one that has rain often enough to keep your chosen plants alive -grin-.

I first came across them in a Star Wars novel, and kept the idea in the back of my mind for a time when it would be useful. Eventually I did come across them in the real world – in Melbourne. Which is an ideal kind of climate. There is a reasonable amount of rain, but more crucially it’s spread across the whole year with no really large breaks. They were being promoted by Melbourne Water as a way of managing stormwater runoff. With some success, as they now have more than 10,000 raingardens around the city. This relates back to something I’ve said before about half of Melbourne being on clay soil. When it rains, the water can’t absorb fast. With housing density increasing, hard-surface runoff’s becoming a bigger problem – there’s more water to absorb and much less soil for it to go into. A raingarden is a way of catching the water from (usually) one drainpipe and slowing it, filtering it, sending it off into the stormwater drains both clean and slow. Slow means much less erosion, less flooding, less need for bigger drains and infrastructure management, a whole host of fewer problems. So raingardens have a lot of benefits as well as looking cool, fitting in small spaces and being a relatively low-maintenance garden feature.

It amused me at the time to see that the designs Melbourne Water were promoting mostly used Western Australian wildflowers. That’s because they are already highly adapted to sandy conditions – the top layer of a raingarden is sand – and they are also well adapted to random long bursts of dryness across summer, so any variability from year to year won’t impact them much. Apparently the local Melbourne wildflowers were a little more sensitive, or at least needed better soil than the crap Perth offers (which is pretty close to pure sand). But at any rate, using the W.A. wildflowers meant not really having to worry about the plants too much. I did wonder though whether it had to be just wildflowers or if you could do productive raingardens. Wildflowers are easy – they don’t need a lot of perfection to be pretty. But when you’re growing vegies and herbs you want a bit more reliable growth.

Turns out, you can. At least, Melbourne Water think so – they’ve released a design that should allow you to grow vegies under the downpipe. I imagine this will be somewhat seasonal – choice of plants for the right growing conditions will be crucial. Overwintering plants put in in autumn, such as broccoli, cabbages, kale and similar will no doubt do well. Perhaps some root vegies as well. Summer vegies might need an occasional top-up – but in theory that shouldn’t be too often as you know the beds are getting the benefit of any rain hitting your roof. And raingardens typically aren’t large so I’m guessing it’s watering-can-sized work to give your tomatoes a summer refresh. Avoiding water-hungry summer plants might be the go though. If anyone tries this in Melbourne, let me know how you go, I’ll be interested to hear what works for you.

Applying this tech in Perth is a bit different. To start with, our soil already *is* a raingarden. We *are* those layers of sand with an aquifer built in, across most of the city. So our downpipes aren’t designed to channel water to stormwater drains and from there to the bay. They’re designed to absorb all the rainfall on site, using settling tanks if need be but in general just by sending the water into the ground anywhere as long as it’s the minimum regulation distance from the house footings. Consequently, you’ll never see a major Cave Clan gathering here – we don’t have a huge underground drain system for the curious to explore. We have occasional sumps and some surface channels leading to them, and it doesn’t get too much more infrastructure-intensive than that. You could still build a wildflower raingarden under your downpipes if you wanted to, but it would be less effort and cost to just plant the wildflowers in the ground. The other problem is that our rainfall is heavily winter-oriented. So anything you plant has to either grow only in the season when the rain falls, or be able to cope with long months in a dry pot. Many of our local wildflowers will, but not all, so there’s more work involved in getting it looking good. A vegetable raingarden will look fine in winter here, think anything you can plant in autumn – lettuces, carrots, marigolds, basil or even tomatoes. But I can’t imagine growing anything across summer be it perennial or annual. Well, I can imagine it, but I can also imagine either watering it daily for four months, like any other garden bed, or having a planter full of brown sticks. So I’m not convinced that this tech is a useful design for an average Perth house. Carefully planned for sets of townhouses though, in our increasingly popular high-density developments that have very little soil access to drain roof runoff, they might still have a place.

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