How to Harvest Rainwater in Your Vegetable Garden

, written by Benedict Vanheems gb flag

Rustic rain barrel

No matter where you live your kitchen gardening efforts are subject to the vagaries of weather. The clued-up grower has a number of means to smooth the peaks and troughs of rainfall, though sometimes even the most prepared of us can find the unpredictable nature of the elements a serious challenge. Here in the UK the southern half of the country has experienced two consecutive winters of abnormally low rainfall. The result was the official declaration of drought in many counties – swiftly followed by the wettest April on record!

Of course, for many of our North American, Australian and South African readers being cautious with water comes as second nature. Hot summers and long periods of raincloud-denuded skies have taught these gardeners to treat water as a precious resource. It’s only when something taken for granted becomes in short supply that you really begin to appreciate its value and the penny’s beginning to drop for us Brits.

Free from the Sky

Fresh, treated mains water isn’t all that great for the environment – or the garden. For a start it has to be extracted from somewhere. Then it has to be purified then stored, before finally being pumped into our homes. This requires (a) a lot of energy and (b) an inordinate volume of chemicals to treat the water. Extracting and storing this water often disrupts natural ecosystems and/or lowers water tables, which in turn can threaten fertile farmland. And when it does finally get to us, our fruits and vegetables don’t much care for it, preferring instead the natural balance and purity of highly oxygenated rainwater.

Water butt

Rainwater isn’t metered, it’s free from restrictions placed on mains water and, most crucially, our plants love it! It makes sense, therefore, to collect some of this natural resource for use on our crops. Even in areas of low annual rainfall it’s surprising just how much water it is possible to collect from a roof surface, particularly an expansive one such as the roof that covers your home.

Working out this potential requires a simple calculation. If you are metrically minded, start by multiplying the area of your roof in meters squared by the average annual rainfall for your area in millimetres. Multiply this by 0.75 to account for evaporation and rain bouncing off the roof and you have the number of liters per year you could be collecting. For imperial units, multiply your roof area in square feet by the annual rainfall total in inches. Now divide by 12 to convert to cubic feet and, as above, multiply by 0.75 to account for water loss. To convert cubic feet to US gallons multiply by 7.5 or for imperial gallons by 6.2.

No Ifs, Plenty of Butts!

The figure that pops out from your calculations will be a large one! The question now is how to catch and store it. For gardeners the best method is, of course, the rain barrel. These can be fitted to intercept a downpipe coming off a roof. A rainwater diverter will do as it says – divert the rain running down the pipe to your rain barrel. But don’t stop at one rain barrel; if you have the space link two or more together so that as one reaches capacity the water overflows to begin filling the next. Modern rain barrels don’t have to look utilitarian. There are plenty of eye-catching barrels that are features in their own right (and you don’t read that in a gardening blog very often!).

Every roof surface is a potential source of water. If you have a greenhouse or shed secure guttering to roof edges and have this feed directly into a rain barrel. Open-ended barrels and containers can be used in this instance but shade the water by covering it with fine-gauge shade netting. This will keep the water cool, reduce contamination, stop insects from breeding in it and prevent it becoming clogged with green algae. Covered barrels are essential for safety where younger children live or visit.

Rustic water butt. Image credit: Travels With a Dog and a Camera
Rustic water butt. Image credit: Travels With a Dog and a Camera

You can be imaginative with water what containers you use to collect rainwater – beer barrels given a second life to make an attractive showpiece, old bulk containers begged free from caterers and restaurants, or metal drums acquired from factories and recycling centres. Just about any water-tight container will do, so long as you can cover the open end to keep the water inside clean. Buy a rain barrel tap kit to fit at the base of the container for drawing off the collected water.

Using Rainwater

Whenever possible locate rain barrels and other rainwater storage containers uphill from the growing area. This way gravity will do the work of distributing it through any irrigation system you decide to connect to it. By the same token raise barrels up on purpose-made bases, cinder blocks or slabs to allow easy access to the tap at its base. Make sure the base is level and supportive as the container will get very heavy indeed when full.

While collected rainwater is a lot better for plants than mains water, there are very minor risks to human health. Bird contamination from gutters, along with other detritus will find its way into your store of water. This needn’t be a problem so long as caution is exercised when eating salads and other crops recently watered with collected rainwater. Use rainwater to water the soil, not the plant and take care to thoroughly wash your produce after harvest.

Going Further with Collected Rainwater

Heavy water users can exhaust a series of rain barrels within a matter of days, particularly during hot weather. There are two options to further bulk up the amount of water you have available. The first is to install ever-bigger storage containers. Thousand gallon plus tanks can be fitted under a driveway, out of sight but offering an almost limitless supply of water, while evaporating away the worry of your stash of rainwater running dry.

GutterMate rain diverter
GutterMate rain diverter

The other solution is to use a proportion of household ‘grey’ water – that is waste water collected from baths, showers and kitchen sinks. This is well worth exploring and further information on this can be found in our article on Using Greywater. You can also reduce the amount of water your plants require by adopting Dryland Gardening techniques.

Whatever water you manage to save or reuse will equate to fewer gallons taken directly from our mains supply. Growing food should help to reduce our environmental impact and harvesting your own water will bring you another step closer to truly growing in harmony with nature.

By Benedict Vanheems. Photo of at top of page credit: Lee Jordan

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Show Comments


"I notice a definite improvement in my veg and flower gardens when they get a good rainwater soaking (as opposed to water from the city supply). This spring my son installed two rainbarrels for me at the top slope of my vegetable garden and I have been happily harvesting raindrops from the roof of our home, here in Virginia. Our winter was virtually devoid of snow and I was concerned about our extremely warm early Spring...too hot and quite dry...but May has been quite wet! I had considered incorporating a grey water system into my gardening efforts but had read various articles about potential health hazards associated with its use and decided to opt out. Anything we gardeners can do to improve our small (or large) plots of earth while protecting the environment is a step in the right direction! "The Gardenmuse""
DR-T on Saturday 26 May 2012
"I completely agree DR-T - we should all do our bit to tread lightly where we can, especially when we're growing our own food. Keep on harvesting the wet stuff!"
Benedict Vanheems on Sunday 27 May 2012
"In Colorado it is illegal to have a rainbarrel on one's property if your source of drinking water is a city's municipal system, or if you're on a communal well that serves more than three households. Colorado water law is incredibly strict about rainwater, in that it assumed that any raindrop that falls in the state is allocated to one of the users downstream (ie., California, Nevada, Kansas, Nebraska and so on). Almost all of the water in this state flows out of it (the Green River in NW Colorado is the exception - it originates in Wyoming, crosses out state and flows into Utah). The only people who are allowed to gather rainwater (after applying for a permit, per barrel, with the state) are those living in a rural area served by one well on their property. The water laws here are draconian, and in a state with almost no humidity and very little rainfall, one would think that allowing homeowners to use what little rainfall we do get would be a no-brainer. However, thanks to the downstream rights holders, every raindrop in Colorado, no matter where it lands, is spoken for by either a city, an agricultural entity or someone downstream. I'm one of the few in the state that isn't forced to pay exorbient prices to a city for drinking water. However, there hasn't been enough rain in our area to fill a bucket under the downspout, let alone a rain barrel. I actually believe the scientists when they say that the Great Plains will someday be a desert at the rate it's going. As it is, we've installed drip lines everywhere to cut down on what we draw from the well. I also wouldn't put it past the gas meter reader to report an illegal rain barrel to the state. Be sure of what the laws in your area allow as far as water use and water rights before going ahead with a cistern or a rain barrel."
CJ on Tuesday 29 May 2012
"How can I get a system of storing water on an allotment that has no water. I have no shed so can't collect from a roof. Any iseas???"
Amanda squires on Friday 1 June 2012
"CO has updated the rainwater laws and now allows rain barrels and collection as long as the water is being distributed back into the ground on your property. You can look up the new regulations for our out of date state in several locations online."
TB on Friday 1 June 2012
"I am an avid rain barrel user, but for edibles I use only water I collect from my metal roofs as I understand water collected off ashpalt shingles may not be advisable. I do use it for ornamentals. Does anyone know better about this?"
Anna Jean on Friday 1 June 2012
"TB - the laws updated in 2009 allow people with private wells (mostly rural residents who do not have to pay a municipal entity for their water) to divert rainwater onto their property for their own use. However, if you're in a city, you are required to pay the city utility for the water they provide AND you are not allowed to have rainbarrels. Here are the articles and websites I used for reference: And with Colorado having a budget shortfall for the last three years, I wouldn't put it past any municipality to start enforcing the 'fine' letter of the new water laws. So if you're on a municipal water system, I'd think twice about putting in a rain barrel just yet. "
CJ on Friday 1 June 2012
"Has anyone had experience with using pumps on your rain barrels. My plot is flat as a table, and although my barrels are up on stands the pressure is not enough to use a hose for watering the garden. Any thoughts?"
SS in SATX on Friday 1 June 2012
"Great Article on saving and using rainwater. I recently heard of a giant plastic bladder that cna hold a huge amonth of water and cna be fitted under a back yard sun deck. Have you looked into this storage system? Also do you reccomend a pump of hand or electric rather than relying on gravity? "
Mark Hausammann on Saturday 2 June 2012
"Ben's article is very interesting, thankyou. I am Australian in Belgium, so catching and using rainwater is 2nd nature ... I have installed 2 @ 11,000 litre tanks under the ground. One I use for my vegetable garden but it tends to get very smelly, I suppose it is methane generated by leaves etc in the water which persist going into the tank even though I have a filter. Is the smell harmful to the plants? My second 11,000 litre tank I use for flushing the house toilets but also for the flowers and garden around the house. Because that water also smells (and to keep the smell out of the house) I have put a small swimming pool chlorine dispenser in it ... the smell has gone, but will the small amount of chlorine in the water now affect the garden plants? "
Roger on Sunday 3 June 2012
"PUMPS IN RAIN BARRELS Yes, you can put a pump in a rain barrel, I tried that with an immersion pump and it works fine ... the only thing is that the rain barrel will empty in a few minutes as the pumps are usually pretty powerful (in terms of litres/minute) and to get any sort of water pressure you need a powerful pump. I would suggest that you install a drip system. To do that, firstly, you empty your barrel and raise it with some bricks to a level where it is above the level of your ground. It only needs to be a foot or so above the highest point of the ground you want watered. Then instead of using a pump just use gravity to allow the water to flow into a black polypipe drip system connected to a tap in the bottom of your barrel (or tank). Lay the polypipe and drippers along the ground near your plants and let the water drip for several hours. Slow, but very effective. I have installed this sort of arrangement with a small 1000 rainwater tank and it works just fine."
Roger on Sunday 3 June 2012
"PUMPS IN BARRELS - Agreed Roger. For those with several water butts linked together, or a particularly large water collection system, special water butt pumps are widely available. This can save considerable to-ing and fro-ing with the watering can, although as Roger points out, watch out for it draining your precious collected rainwater quicker that you'd anticipated!"
Benedict Vanheems on Wednesday 6 June 2012
"COLLECTING RAINWATER ON THE ALLOTMENT. Amanda asked about collecting rainwater on an allotment (where mains water is often unavailable). If you haven't got a roof to collect rainwater then one possible idea is to dig a trench into the ground and line this with tarpaulin. Extend the tarpauling over the ground either side of the trench so that the water flows into it. You will then have a ready trench filled with rainwater for scooping out as needed with a watering can. This does, of course, take up some space! Where possible in such a situation, the first priorty should be to add plenty of organic matter to the soil to improve its water retention properties. That way you will need to do less watering."
Ben Vanheems on Wednesday 6 June 2012
"RAINWATER FROM ASHPHALT ROOFS. Anna Jean asks above whether it's ok to use rainwater collected off of ashphalt shingles. The answer is yes - this should pose no hazard as any potential contaminants will be filtered by the soil as it is watered on. As with any fresh vegetable or salad, wash before use to be extra sure."
Benedict Vanheems on Wednesday 6 June 2012
"Does anyone know where you can get hose connectors for UK plastic water butt taps? Then I could connect my seep hose to one when I'm on holiday."
Helen on Sunday 17 June 2012
"CJ, take a look at what mulching with wood chips could do for your garden: It seems to have done a wonder in a garden under the rain shadow of the mountains out West. My husband and I put this technique into practice this year in part of our Midwestern garden, and despite the severe lack of rain this summer, our veggies are doing amazingly well without additional watering. We're still praying for rain, though!"
Pamela on Friday 13 July 2012
"Pamela - we're already mulching, driving to Aurora to get large truckfuls of free mulch from the city (60 miles round trip makes the mulch less than free, but what the heck), and the majority of our shrubs have a rock mulch around them, with drip lines beneath the rock in some cases. The other problem with rain barrels in Colorado, especially this year, is that if there is zero rain, a rainbarrel becomes just another yard decoration. We finally got some real, soaking rain on the 8th, but not a lot since then. What we did manage to collect has already been used on the vegetable garden. I agree with you about praying for rain, though. We need the high pressure system over us to get out of the way and let the monsoon moisture from the Gulf reach the front range. The moisture that comes in from the Pacific, more often that not, ends up as rain in the mountains while the Plains bake in the heat. My only consolation about the current drought and heat is that because we didn't have the normally chaotic spring weather, just early warmth, I have a ton of fruit on my apple, pear and what I believe to be peach trees. Nothing's ripe yet, but I've been watering deeply via the drip lines and spraying everything with copper fungicide and neem oil, trying to keep things from drying out or getting eaten by bugs. In a regular year, April and May frosts and June hail would have killed the blossoms on them and I'd maybe get about ten apples a tree. This year, it's more like one hundred."
CJ on Friday 13 July 2012
"Weather and gardening is such a strange thing. What we give up in one area, we gain in another. This year many of us lost our fruit because of early warm weather and blossoming, just to have our normal sub-freezing temperatures return. I only had a vague idea about the policies of water "ownership" in your region of the country. What a tough spot to be in; we have it so easy in comparison. Cheering you on from Illinois!"
Pamela on Friday 13 July 2012
"The issue with Colorado water law and the strict interpretation of who gets what amount water and how much that entity is allowed to use stems from the fact that Colorado is the point of origin for a lot of rivers upon which many, many other states depend for their agricultural and municipal water needs, as well as recreational and environmental protections. There is never enough water in Colorado for all of the people and institutions that need it or want more of it, and that includes the places downstream. If our farmers, ranchers, industries and cities used all of the water that fell here for their own interests, we'd be fine, but Utah, Arizona, California, New Mexico, Kansas, Nebraska and Wyoming might have a few nasty things to say about it. And do about it, in the federal courts. I think it's hard for people outside of Colorado to understand because in many of the places downstream from us, especially the arid states like Arizona and New Mexico, collecting water is encouraged. However, there aren't as many places downstream that rely on the water that THEY don't use, or must release back into the river. Our water treatment facilities (and Utah's because I've had the opportunity to tour facilities on both sides of the Divide) are pretty amazing as far as returning as much clean water back to the rivers as possible, because they have no other choice; a city can use it's allotment of water, but they have to send a certain portion of it downstream once they've used it, rather than hoarding it. For instance, Aurora. CO gets to use the water that other cities, farther west, release back into the tributaries that supply its reserviors and deep wells. I can understand how, back East, where humidity is much higher, rain and snow are more plentiful and the laws governing who owns how much water much less draconian, none of what Coloradoans collectively have to consider here when doing something as simple as drilling a well or putting in a rainbarrel makes much sense. However, with what the terrible heat wave has done to this year's Midwest corn crop, I think the states to the east of Kansas are getting a small taste of what Colorado's Front Range arid conditions are like and why we hassle over every tiny drop of water that falls here. Someone, somewhere down the line has their finger on that drop and they want it and they're willing to sue over it. Or worse. Mark Twain said it best when he described how the settlers out West treated water and water rights: "Whiskey is for drinking; water is for fighting over." "
CJ on Friday 13 July 2012
"I have been thinking for quiet a long time...that biggest hurdle to rain water harvesting is storage...currently most Indian cities are getting water by taking out underground water....usually a pump is attached to deep bore wells. Now my point of view is that if unused rain water is pumped back to the same underground water bed, this will definitely keep at least balance on underground water level...kindly suggest...???"
Rajesh Pagaria on Sunday 25 November 2012
"Hello Rajesh. Yes, in theory if you are watering crops with collected rainwater and returning unused rainwater back to the underground water table then this should keep things in balance. By watering crops with collected rainwater you are merely delaying the point at which the rain begins its journey to the underground water table - the rainwater will make its way there eventually! Watering with rainwater is always preferable to using treated tapwater - both from a water resources perspective and with regards the energy used to treat and pump the mainswater. "
Ben Vanheems on Thursday 29 November 2012
"I'm thinking of collecting rainwater from my conservatory roof but I'm sure I heard someone say years ago that it is illegal to collect the water from your house roof. Is this true/still the case? (UK law of course)"
John on Sunday 4 August 2013
"Hi John. I've never heard of any such law. Water butts are actively encouraged and promoted by local councils and other organisations as a way to reduce water use. I'd say go for it!"
Benedict Vanheems on Monday 5 August 2013
"We took reclycled 55 gal soda drums and put a spigot on the bottom. They set in steel frames (or use blocks to raise) and are situated at the end of a gutter. When there is no rain, we fill the barrel with water from hose. The lid is a wood frame with metal mesh to keep out animals and large objects. A timer (about $35) is placed between the spigot and soaker hose. The timer is set to run for 10 minutes every other day and the barrel is still 1/4 full at end of week. The filter in timer needs rinsed out periodically and soaker hoses adjusted to focus on plants that need more. We are only at this property on weekends and during the dry Arizona summer, our plants did great. Zuchinni, tomatoes and peppers enough to give away. "
Catherine on Wednesday 25 March 2015
"Hi Catherine. This sounds like an ingenious set-up - very good idea! Glad it's yielding clearly fantastic results. Thanks for sharing this."
Benedict Vanheems on Wednesday 25 March 2015
"I have had to locate my waterbutt at the end of my garden in the apex of two fencing panels as my wife won't allow the butts on the house downpipes, any ideas how I can increase the surface area of the rain collection properties? I have no garden shed or greenhouse:("
colin Fowler on Monday 2 April 2018
"The only way you can collect lots of water is from a roof of some sort. Are you sure you can't convince your wife to let you put one on the house downpipes? Properly sealed up with a cover, butts pose no health risk."
Ben Vanheems on Tuesday 3 April 2018
"to Colin Fowler - I don't know why your wife won't allow the use of water butts on your house down pipes, it could be safety or it may be the look of them, they are not the prettiest things to have around especially if you have a very pretty house. It is possible to disguise the butts with trellis which can then be planted with ivy or a climber, just allowing access to the tap. Or just boxing them in with something that can tie in to your house appearance. With a little ingenuity it should not be a problem. If you wanted to run soaker systems off it that may mean hoses all over the place and she may not like that but even if you have to carry watering cans at least you are getting the water. "
Pegs Sanderson on Thursday 14 March 2019

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