Unless you’re incredibly lucky, water is your biggest consumable cost on an allotment after the rent. And if you are lucky enough to have mains water on site at all, you’re probably queuing up with twenty other plot holders at a shared standpipe, lugging a watering can back and forth until your vegetables shrivel up in July. Watering is one of those things that you accept is always going to be annoying until you realise that it doesn’t have to be like this.
Rainwater harvesting seems like an elitist, complicated way of collecting water. In reality it just means capturing rainwater where it falls and storing it for later use. Our complete guide to sustainable gardening covers the basics, but you need something a bit more specific when working on shared sites, with limited budgets and the unique restrictions of allotment life.
Want to know how many people are involved in community gardening? Over 2.5 million people across the UK got hands in the soil as part of a community gardening scheme in the last three years (Royal Horticultural Society). Sounds a lot right? Here’s more – over a quarter of community gardening sites run on annual budgets of less than £500 (Royal Horticultural Society). To accommodate demand, 76 percent of councils have actually decreased the size of their standard allotment plot in recent years (LocalGov). That’s smaller plots, tighter budgets and more competition for access to whatever water is available.
Harvesting rainwater isn’t just good for the environment, it’s becoming essential.
## What’s The Water Situation On Your Allotment?
It’s helpful to take stock of what’s already available before you start harvesting rainwater. Most sites fall into one of three categories when it comes to water supplies:
Sites with mains water, either individual taps or communal standpipes. Sites with communal standpipes shared between a few plots. Sites that rely exclusively on water deliveries or rainwater collection.
Mains Water Sites
Assuming your site rents by plot rather than charges per unit of water consumed (many urban sites do still operate like this), you’re still paying through the roof rent for water you could be collecting yourself for free. You’re also likely restricted on how much you can actually use during dry spells when everyone’s fighting over the same dwindling supply.
Shared Standpipe Sites
This is where rainwater harvesting can make the most practical difference. It’s time consuming lugging watering cans back and forwards, not to mention a bit of a drag when you have to queue up and chat to your neighbour every time you want to fill up during the evening watering rush. With harvested water stored on your plot you don’t have either problem.
No Mains Supply
Full rainwater collection systems aren’t just good for the environment, in some cases they’re the only option. Rural sites or newer community gardens may not have mains water connected at all. You’re entirely reliant on what you can carry to your plot, or collect from the sky.
Demand fluctuates based on weather conditions and the time of year, but a good rule of thumb is that flower beds require roughly 20 to 25 litres of water per square metre every week during hot weather periods (Franktons). A standard half plot measuring 125 square metres with beds covering half the area could need up to 600 litres of water per week watering everything during peak summer months. That’s a lot of trips to the communal standpipe.
Collecting rainwater makes environmental sense but it also frees you from the pressures of peak time water use on sites with shared supplies. The Royal Horticultural Society (RHS) advocates harvesting rainwater as a simple way to save on water bills and help the environment (National Allotment Society). Smarter watering also makes you less reliant on everyone else’s conscientiousness – if people start complaipping about your watering can tapping them on the shoulder in the evenings then you’ll still have water storage on your plot.
## Harvesting Rainwater On Your Allotment Plot
Rainwater harvesting can be as simple or as complex as you make it. The following sections provide tips and examples of everything from basic rainwater collection at the shed, to large scale water butt installation projects designed to make you totally independent of shared water supplies.
Shed Basics
Unless your shed came with guttering as standard, this will be your first job. Your shed roof is your biggest area for collecting rainwater. A standard sized 8×6 shed will collect about 150 litres of water from a 25mm rainfall event in its gutters alone, even if you’ve added no other collection surfaces. Over the course of a typical year, that shed will collect between 2,000 and 2,500 litres depending on your location and annual rainfall.
Plastic guttering is easy to find and very cheap to install yourself. Half round guttering with a diameter of 68mm or 76mm is fine for most sheds. You’ll need guttering brackets every 60cm or so, a slight incline run towards your downpipe (1 in 60 is plenty) and a leaf guard to stop debris blocking your downpipe. Guttering a standard shed properly should cost you between £30 and £50 to buy the parts.
Cast iron is more durable and looks smarter once its installed, but you’ll pay 2 or 3 times as much and need stronger fixing brackets. Only consider this if you’re planning on staying on the same plot for decades, and your site allows more permanent installations.
Water Butt Sizing and Positioning
The standard water butt holds 200-220 litres and slots neatly under a downpipe. If your plot rules allow it, pop your butt on top of a concrete slab or small wooden frame so it sits slightly higher than the tap on your guttering. This lets you slide the spout of your watering can underneath the tap to fill it up.
Some sites have restrictions on water containers over 220 litres as this can be a structural hazard (BDAC Allotments). Refer to your site rules if you’re considering something larger than a standard container. These limits are partly due to structural limitations, but also because containers that are too big create perfect mosquito breeding habitats if the water isn’t used quickly enough.
Positioning Multiple Containers
Joining multiple standard containers together gives you lots of storage without exceeding limits on individual water butt sizes. Water flows from the first butt into the second when it becomes full, then on to a third if your system has three containers. You can effectively create one large container without exceeding rules that limit plot holders to single 220 litre containers.
Setup your containers at slightly different heights so that gravity forces the water from the first butt to fill the second, then the third. Having your containers staggered by 10-15cm is plenty. Use official butt linking kits rather than knocking together some pipework with bent wire. Overflows will cause just as many problems for your neighbour as yours if you don’t.
Large Scale Rainwater Collection Systems
Larger water butts provide better value per litre stored and are more efficient, but you need to check some sites allow them. Birkdale allotments added 20 large water butts around their site (United Utilities). Each of these large installed water butts stored 1000 litres of rainwater captured from roofs (United Utilities). These aren’t DIY jobs – industrial water butts need proper foundations and may need planning approval on some sites.
Rainwater Harvesting Mistakes
Mistake #1: Ignoring the benefits. It really is that simple. I see allotment plots with full grown trees hammered into the ground next to sheds with no guttering. Read this article, make a plan and then just do it.
Gutters need a slight incline so that water flows towards the downpipe when it rains. If they’re flat, or sloped the wrong way, your gutters will quickly fill up with rainwater and then overflow. mosquito heaven. Install guttering with a minimum fall of 1 in 60 towards your downpipe, and use a spirit level to check. Don’t guess.
If you position your butt where it will be regularly frozen solid by winter sunlight, it will crack when the water inside expands. Water butts aren’t cheap. Rotate between two containers so you can drain and store the other during winter. Alternatively fit a small water butt heater or ensure it’s tucked away in a corner that never sees sunlight.
When your storage is full, somewhere has to deal with the excess water. If you don’t divert it away from your plot, you’ll have swampy ground next to your containers and risk flooding your shed or your neighbours. Install proper overflow pipes that feed into your drainage or soakaway.
Standard household guttering requires a minimum 68mm diameter downpipe. Smaller diameter pipe creates a bottleneck during heavy rainfall and you’ll still have overflowing gutters. It doesn’t make sense to collect rainwater if you can’t efficiently drain it from your roof.
The first bit of water that runs off your roof carries the most crap. Birds dropping, dirt, debris – its all washed into your storage container with the first rainfall after a dry spell. Install a simple first flush diverter to stop the first 10-15 litres of rainflow entering your storage containers.
In my experience allotment theft is commonplace. Water butt? Primitive currency on sites. Empty out one Saturday and you’ll likely return Monday to find yours empty or swapped with ice. Fit locks, bolt everything to the ground, or make sure removing your system requires tools a thief isn’t willing to spend. Factor the costs in from day one.
Rainwater Harvesting Systems Explained
For rainwater harvesting to be practical and efficient your system components need to work well with your plot conditions. Soil type, rainfall levels and the size of your shed will determine the size of collection surface you have, and drive your storage requirements.
Calculating how much rainwater you can harvest
A shed sloping roof is going to collect significantly more rainfall than an allotment build raised bed garden bed. Knowing the limits of your system helps you decide how much storage you really need. A simplified version of the rainwater harvesting calculation is area of collection surface (metres squared) x annual rainfall (mm) x 0.7 (loss factor) = water collected in a year (litres).
Example – A typical UK garden with a 6×3 shed and raised beds covering 30 square metres (3m x 3m x4 sides) has 42 square metres of collection surface. If we assume the garden is in Manchester and receives 700mm of rain per year, it would collect:-
42sqM x 700mm x 0.7 = 20,580 litres/year.
Assuming thats your sheds roof and raised beds are constantly full of rainwater that never spills over or evaporates you could effectively supply two medium households annually. In reality these numbers help us establish how much storage we need to capture and store rainfall when it does happen.
Storage
Rainfall isn’t constant so we also need storage to bridge the gaps. A useful way to measure storage is to look at how many weeks of watering your storage can provide during peak water demand. On our example allotment plot water requirements might be 500 litres per week in hot weather.
Our storage needs would be calculated as –
500 litres x 4 weeks = 2,000 litre storage
500 litres x 6 weeks = 3,000 litre storage
Most domestic rainwater harvesting systems have between 2500 and 4000 litres of storage. Using the figures from our example garden we can see that even a medium sized rainwater harvesting system is only going to fully cover the watering needs of a small allotment plot for around 4-5 weeks in peak summer if we rely exclusively on rainwater.
To meet those peak watering requirements you’d have collected roughly 180 litres of rainfall every week when the ground was actually dry enough to put it to good use.
Plot Watering Needs Vs Rainwater Harvesting Potential
While effectively replacing mains water isn’t impossible, for most allotment and small gardens harvesting rainwater lets you supplement the mains rather than completely replace it.
Filtration
Your rainwater harvesting system doesn’t need to filter water to potable standards. Mesh or leaf guards are normally sufficient for stopping junk from blocking your system. If you’re using rainwater for tasks other than irrigation a sand filter or small settling tank could be useful.
Plot Drainage
If you have poor drainage on your allotment its worth addressing before you start harvesting rainwater. Stagnant water left next to your plot buildings creates just as many problems as the water you’re seeking to harvest. Rainwater harvesting can actually make existing drainage problems worse by preventing runoff.
Real Life Examples and Evidence
Research from rainwater harvesting installations shows plots can easily meet 60-80% of their annual water needs from rainfall. As rainfall varies greatly from year to year these figures will change accordingly.
A study of rainwater harvesting systems on UK allotments found that where 400-600 litres of storage was provided it acted as useful supplementary water but was not enough to completely replace mains watering during hot dry spells. Larger systems with storage capacities of over 1000 litres were sufficient to provide total irrigation independence in some cases for several weeks at a time.
Rainwater harvested from roofs is almost always suitable for irrigation without treatment. A word of caution, water quality does degrade if left to stand for prolonged periods in hot weather.
Demand for water is usually highest just after people arrive on site and during the evening when everyone starts watering at the same time. Plot holders with rainwater harvesting dont add to peak time demand on shared supplies.
Annual maintenance should hopefully catch most potential problems. Check for leaks during dry spells, clear debris from your gutters and test your overflow pipe after every winter storm.
Rainwater Harvesting Systems On Different Plot Sizes
Standard Full Plots
Got a large plot and big shed? You have no excuses not to harvest rainwater and may even want to look at multiple collection points. Use gutters on outhouses or hook animal feeding troughs onto your rainwater storage barrels.
Half Plots and Small Plots
Smaller roof area means less rainwater collects but you also don’t have nearly as much vegetation to water. Focus on maximising your water storage and think about linking multiple containers. Place containers so you minimise how far you have to carry watering cans.
No Sheds
You can still collect rainwater without a shed. There are plenty of temporary or portable rainwater collection systems. The simplest is to throw a tarpaulin over an area of ground to collect water, then channel it into a water butt. These systems are harder to secure and often don’t stand up to the British winter.
Community Gardens
Community gardens have slightly different requirements than single allotment plots. Collecting water from a larger roof area then distributing it around the site with decentralised water storage can reduce setup costs and makes maintenance easier.
Restricted Sites
Some allotment & Community gardens try and ban water butting due to “safety” concerns. Sites with strict rules about storage will still have limitations on tank sizes or positioning. Installing systems that get confiscated will achieve nothing and waste your money. Talk to site management to try and find a solution everyone is happy with.
If your site rules forbid water butts that can’t be turned upside down, install temporary systems that you take down or fill with soil during winter months.
Benefits of Rainwater Harvesting On Allotments
Money saved on water bills. Some sites are moving away from charging for water completely, but if yours does rainwater harvesting will still cut your bill. They also remove some of the pressure from standpipes during peak time demands.
Healthier plants. Rainwater is soft, naturally free of chlorine and always at ambient temperature. You’ll almost always see healthier plant growth using rainwater for watering.
Less heavy lifting. There’s no nicer way to put it, watering is bloody hard work. With a water butt onsite you’ll make life that little bit easier.
Free yourself from hosepipe bans. Stored water means you’ve got irrigation covered during dry spells when water use restrictions come into force. If you’ve correctly sized your system it could keep you going for weeks.
It’s good for the planet. Taking stress of demand for mains water and alleviating surface runoff are two big benefits of rainwater harvesting. It also helps reduce pressure on our combined sewers when its raining heavily.
Plot holder happiness. More water butt systems means less friction over access to shared resources. Sites limited by their mains supply can support more plot holders if everyone harvests rainwater.
Installing Your Rainwater Harvesting System
Weekend Job – Install guttering on your shed and connect a single water butt.
Month 1 – Add one or two more water butts using a butt linking kit. Install first flush and think about filtration.
End of Growing Season – Evaluate how you got on. You’ll probably want to add more storage_capacity if you’ve been able to use rainwater throughout the summer. Consider improving security if someone nicked your spare watering can. Look at adding a tap timer to water more efficiently.
Your budget will vary depending on how many storage containers you decide to install and whether you go for a basic rainwater collection system or full blown water butt installation with multiple linked containers. Using the table below as a guide, most allotment friendly rainwater harvesting systems should cost between £200-£300.
Item
Cost (£)
Comments
Guttering kit enough for 6m shed
25-40
Plastic half round guttering. Kits include all brackets and fittings needed.
Standard water butt
30-50
220 litre capacity with tap and stand.
Downpipe kit
15-25
Includes necessary bends, brackets and ground level connection kit.
Linking kit (per butt connection)
8-15
Connects two standard water butts together.
First flush diverter
20-35
Helps improve water quality.
Security camera/recycling lock/etc
15-30
Related article: Ten tips for preventing allotment crime.
Total
115-195 Basic system. Add more containers for comprehensive coverage.
Total rainwater collected during peak summer months in the UK would comfortably supply most allotment plots. Thinking about water storage and planning ahead should help you capture most rainfall when it happens.
As your plot matures you’ll use less water anyway. Rainwater harvesting is a marathon not a sprint. Start small and add storage gradually.
