How Smart Gardeners Collect Rainwater Automatically

Without water, all the hard work we put into our gardens would go to waste. Summers – where I live at least – are getting hotter, and that means our plants are at increased risk of shriveling up.

But why use expensive treated tap water, when you can just collect it from the sky – for free – with these three quick and easy projects? We’re talking bigger storage, better collection, and a virtually free supply of the best possible water for everything you grow. Let’s get to it!

Why Use Rainwater?

But first, why is rainwater is so much better for your plants than tap water?

Tap water, especially in hard water areas, has a pH of around 6.5 to 8 – ever-so-slightly acidic to mildly alkaline – and it’s loaded with dissolved calcium and magnesium. Over time, watering plants with tap water raises the pH of your soil, and it can start to lock out key nutrients too. For acid-loving plants like blueberries, azaleas and rhododendrons this can cause real problems, but even for everyday crops such as your tomatoes, brassicas, or squash plants, opting instead to use slightly acidic, mineral-free water - that stuff that falls from the sky – is simply better for plants.

Rainwater has a mildly acidic pH of around 5.5 to 6.5. It’s naturally soft. It contains a little dissolved nitrogen absorbed from the atmosphere, so actually acts as a gentle, natural feed. And, of course, it costs absolutely nothing.

The challenge, though, is collecting enough of it, then storing it so it’s available when we need it most: in the heat of summer, when regular rains can be frustratingly absent. The best time to set up a rainwater harvesting system is during the winter, when it’s wetter – but, as they say, the second-best time to start is right now!

How Much Rainwater Storage Do You Need?

A typical water barrel holds around 60 US gallons (225 liter)). It sounds like a lot, but it drains quickly in a dry summer. In my garden, for example, I use two watering cans to water one of my small raised beds. Each can holds about 10 liters (2.5 gallons), and with 18 beds to water, you can see that it won’t take long to empty a water barrel!

Calculate how many beds or growing areas you need to water, and how much water you might need to do that. If you’ve got, say, a greenhouse full of tomatoes or young seedlings, and perhaps some pots on the patio too, it soon adds up.

To keep plants quenched we’ll want to collect as much rainwater as possible, from every possible roof surface, so look around and see what’s available in your garden – your house roof, obviously, but other buildings too. In my garden that means harvesting rainwater from the roof of my outbuilding and shed.

Project 1: Water Barrels

If you have a pitched roof, both sides will shed water, which means the perfect opportunity to collect plenty of rainwater by popping a water barrel on either side.

The first job is to install some guttering on each side, running alongside the dripline. When installing guttering aim for a very gentle slope. To achieve this, screw in the top bracket – the uphill one – roughly half an inch (1-1.5cm) higher than the lowest bracket, closest to what will be the water barrel. You can use a spirit level to help check that you do indeed have a slight downhill drop.

With the highest and lowest brackets in position, add a taut stringline between the two. This will serve as a guide for the remaining brackets, ensuring they follow the right slope. Make sure that the cup of each bracket sits snug on the stringline for consistency.

Next, add the guttering. Clip on a stop end to close off the highest point of the gutter. If you’re going for a long length of guttering, use a union piece to connect separate sections. At the lower end, push on a gutter stop end outlet to channel the water down into the barrel, and then fit the downpipe. A gutter guard or mesh over the top of the guttering is a great, inexpensive investment that keeps gutters cleaner for longer, or use one over the downpipe to prevent debris being washed into your water barrel.

Raise your water barrel up off the ground on a base. You can use a purpose-sold one, or stacks of bricks or slabs would do the job too – anything that’s sturdy and that ensures enough clearance to get a watering can under the tap. Add a tight-fitting lid to keep out debris, light, and any insects such as mosquitoes looking for a breeding ground.

Project 2: IBC

But what if you’ve got a bigger roof space and want to go large and take your rainwater harvesting to the next level? This is where the IBC comes in.

IBC stands for Intermediate Bulk Container, and these things are extraordinary value for rainwater harvesting. A standard IBC holds around just over 260 US gallons (1,000 liters), which is four times the capacity of a standard water barrel.

Brand new IBCs can be pretty pricey (although still cheaper per gallon of water saved than rainwater barrels), but shop around and you can often pick up second-hand ones for anywhere upwards of around $50 (£30). Always go for a food-grade IBC. You’ll usually see the words ‘food grade’ or an outline of a fork and glass symbol on it. Non-food grade second-hand containers may have previously held chemicals which, for obvious reasons, we really don’t want anywhere near our edibles.

First, level off the area where you want to site your IBC tank. It needs to be level because a full IBC is seriously heavy – we’re talking a literal tonne of water, so the base absolutely needs to be as solid, flat and stable as we can get it. You can site it on paving slabs, compacted hardcore or a well-laid gravel base. Avoid soft or uneven ground.

The tank is made of semi-transparent plastic, and if light can get through to the water inside there’s a good chance we’ll get algae growing, which will turn all that lovely rainwater bright green. We need to block out as much of the light as possible, which will also help to stop the plastic from degrading too quickly.

You can buy IBC jackets specifically for the job, or use a black tarpaulin or shade cloth wrapped tightly around your IBC. Alternatively, paint the outside of the tank with two coats of a black waterproof exterior paint. Whichever method you use, no light in means no algae. Job done!

If you think an IBC is a bit unsightly, there are options. You could try masking it with a natural screen of hedging, or perhaps climbing plants trained up a trellis attached to the tank.

With the tank positioned and prepped, connect it to the downpipe coming off the guttering with a rainwater diverter kit. They’re cheap to buy and don’t take long to fit. Run a flexible hose from the diverter down into the IBC’s inlet port at the top of the tank. Make sure the lid or inlet cover is fitted with some mesh or a fine filter to catch leaves and debris before they enter the tank.

Many IBCs have a tap or spigot near the base, which is ideal for connecting a hose or filling a watering can. Make sure the tank sits high enough on its base to get a watering can beneath the tap.

One more thing that can be worth doing is to fit an overflow pipe near the top of any tank or water butt to direct water away from the building’s foundations and towards a drain or, even better, a soakaway or garden pond. When the tank fills – and it will likely fill quickly after heavy rain – you need that overflow going somewhere sensible rather than pooling against the wall.

Project 3: Add Even More!

This final little project is helpful if you already have guttering in place and an existing water bareel or tank and just want to add more collection potential. So long as you have the space, all that’s needed is an additional water barrel or tank placed nearby, and a linking kit. This just fits towards the top of your existing water barrel, and then the other end connects to the top of the next.

As water reaches the top of the water barrel, it simply overflows into the adjacent one. And you can keep this going on and on by adding even more barrels or tanks. Nice and easy!

Maximize Your Rainwater Harvest

It’s fun to work out how much rain you can potentially harvest. Using my garden as an example, I now have two water barrels and that big old IBC, which together come to roughly 1,450 liters – a big jump up from what I was collecting before. In a dry summer, that will make a big difference!

If you know the average annual rainfall total where you garden, you can work out the potential for rainwater capture. In my climate for example, we get about 31in (78cm) of rain a year. At that level, even a modest 6 x 8ft (2 x 2.5m) roof typical of, say, a standard greenhouse or garden shed could yield up to 800 US gallons (3,000 liters), when allowing for losses. So you can see how a bigger roof such as the one on your home could collect 10,000 liters or more annually.

It’s so worth collecting rainwater, so it’s surprising that some US states have historically restricted this. But in recent years laws have relaxed, and rainwater harvesting is now actively encouraged across almost all states, with some even offering tax incentives to do so. The exceptions are Colorado, where residential collection is limited to 110 gallons over two rain barrels, and Utah where residents can collect up to 2,500 gallons without a permit. But, of course, laws are subject to change, so please check the up-to-date situation in your area.

Roofs often get cluttered with moss, ivy, and other overgrown plants so it’s worth clearing some of it off if you can easily and safely access your roof before installing any water collection tank or barrel. If you have roof tiles, be careful not to disturb them as you work. And, of course, when working on any roof, please make sure you’ve got someone on hand to hold your ladder and keep an eye out for your safety!

Check any existing gutters too – you might be surprised to see that they are leaking precious water. It’s a common problem on older outbuildings and sheds and it’s well worth sorting out problems like this before committing to a big storage solution. Tightening up a loose bracket or replacing the odd section of guttering is a simple fix.

It’s also worth clearing out any debris in your gutters such as leaves, moss, and bird droppings, and dumping it in your compost heap. If organic matter like this ends up in your water tank it can potentially create a less-than-lovely rotten egg smell, and it could block the spigot or tap at the bottom of the barrel. And as mentioned before, adding a gutter guard is worth doing.

Maximize the quality of the rain you’ve harvested by keeping barrels and tanks clean. Once a year – usually in autumn once the season winds down – empty everything out and give them a good scrub with warm water. This prevents sediment build-up, keeps things nice and fresh, and means your system is ready to fill again, just in time for the return of wetter winter weather.

Whether you start with a single water barrel or go all-in on a big IBC, every gallon you collect is a gallon you won’t have to pay for – and, of course, a gallon of genuinely great water for anything you grow.

Let me know your current setup or plans in the comment below. Stay cool...and wet!