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How to Harvest Rainwater for a Better Garden

Green Water Barrels Connected In Tandem Harvesting Rainwater From A Brick Farmhouse.

What kind of water do you use in your garden?  Many people use tap water and never see any harm to their plants.  Others might see discolored or unhealthy leaves.  A possible cause could be the chemicals in the tap water.  Discover how to do rainwater harvesting for the garden and avoid those problems.

rainwater falling from a metal roof.
Why should it go to waste? When you’re collecting rainwater you’re storing a valuable resource.

What are the Benefits of Harvesting Rainwater?

In another article, we discussed why rainwater is the best for plants.  We also have separate articles for hard water, softened water, and tap water.  Please refer to those articles for more information about water and plants.

We didn’t include personal hygiene or drinking in this list of benefits.  After running over roofs, gutters, and downspouts, rainwater isn’t safe to drink.

To make rainwater potable, you’ll need a way to purify it.

Here are the benefits of  collecting rainwater for the garden and other uses:

  • It saves energy, which equals less $$$ for the water bill. You don’t run the water meter or a pump when you use rainwater.  Other than the initial equipment cost, rainwater is free water.
  • In case of drought, stored rainwater comes in handy for keeping the garden hydrated. Large storage tanks can feed irrigation systems.
  • Storing rainwater is an obvious option for anyone who wants to get off the grid.  It also becomes necessary when surviving any major disaster.
  • If you’re willing to make the investment, you can purify rainwater so it’s potable water.
  • Filtering collected rainwater makes it usable in toilets or washing machines.
  • Many fire-fighting systems use collected rainwater.
  • When you collect water from a roof surface, it could contain organic matter.  Most of it is beneficial for plants.  It includes residue from leaves, twigs, or bird or animal droppings. Depending on the animal, the droppings could be a disadvantage.
  • Rainwater is soft and free of chemicals by nature.  This isn’t true in areas with uncontrolled air pollution.
  • Municipal water contains chemicals for purity, but their amounts and types vary.  It all depends on what the water needs to become pure.  The variations in the chemicals make their effects on people and plants suspicious.
  • Since its pH is a bit acidic, rainwater is ideal for plants.  Plants growing in somewhat acidic conditions have the best chance of absorbing nutrients.
  • Collecting rainwater reduces the runoff into storm drains.  Collecting all the rainwater from a roof is possible, but is it practical?  You’d need a lot of storage capacity to hold that much water!
  • Most residential systems need little maintenance.
  • Rainwater is excellent for washing vehicles, filling fish ponds, or watering animals.
  • Depending on the water system, rainwater could become your primary water source.
An AI image of a government bureaucrat.


Some misdirected local governments have placed regulations on harvesting rainwater. These people need real jobs.

Check the regulations in your area at

For more bureaucratic information, see the EPA Handbook for Rainwater Harvesting Policies.

Are There Any Negatives to Collecting Rainwater?

  • Rain isn’t always there when you need it.
  • Water from a roof could contain toxins.  For example, wooden shingles could contain fire retardants, and water from a copper roof can act as an herbicide.
  • Stormwater runoff, which is water that has made it to the drainage pipes, can have additional contaminants.
  • Depending on the animal, droppings picked up from runoff could be toxic.  A first flush diverter can allow most pollutants to wash away before sending water to the barrel.
  • Rainwater contains toxic dust in areas with excessive air pollution.
  • Installing systems with large cisterns can cost over $20,000.  If you need this kind of system, the costs are easy to recover.

Video: Rain Harvesting Plumbing: ‘Wet’ vs ‘Dry’ Delivery System

What are the methods for harvesting rainwater?

Any method for collecting rainwater will fall into one of these categories:

Dry Systems

A basic dry system is a roof, gutter, downspout, and barrel.  When the rain stops, the water completely drains from the roof, gutter, and downspout.

These systems also work well with several barrels or a large tank if they fit under a downspout.  Dry systems always have storage containers close to a building.

Wet Systems

These systems are usually more complex than dry systems.  This is the option when storage tanks need a location away from buildings.

Rain from a downspout flows through underground pipes to the storage tank.  At the storage tank, the pipe comes out of the ground and connects to the top of the tank.

For water to flow into the tank, the pipe connecting to the top of the tank must be lower than the rain collector. A practical way to get this is to install the tanks in a lower-lying area than the collecting system.

These systems are “wet” because the underground pipe never drains completely.  That can lead to stagnant water in the underground pipe.

Video: How to Install and Use a Rain Barrel

This video shows the basics of installing a rain barrel. We need to point out a few issues with it:

  • The presenter made an unstable base for the barrel since there are only two bricks on the top layer. Ensure you don’t do that. One gallon of water weighs about 8.34 pounds. So the weight of a filled 50-gallon barrel would be: 8.34 lbs./gal. X 50 gallons = 417 lbs. That isn’t the kind of weight you want to get off balance!
  • The original downspout drained into the backyard. That isn’t always a good idea, but sometimes a homeowner doesn’t have much choice. We owned a very nice house where the previous owners let the backyard downspout drain into the yard for 30 years. It made a swamp every time it rained. In a few days, I dug a trench and had the downspout tied into the drainage system. Problem solved.
  • For a better installation, the barrel overflow should run to the drainage system.
  • The barrel has an uncovered mesh screen. This is excellent for keeping out mosquitoes, bugs, animals, and debris. Unfortunately, it won’t keep out algae. The barrel needs a solid cover that blocks sunlight. Remove the cover to collect rainwater.

In spite of the above issues, the video is still a good introduction to installing a rain barrel.

How to Size a Household Rainwater Collection System

Information in the table below is for supplying an entire household with rainwater.  If you plan to use water to irrigate the lawn or large fields, then you’ll need more water than shown in the table below.  Watering small gardens won’t change these amounts.

If you use rainwater only for gardens or other small projects, you’ll need less water than the table shows.

Household SizeWater Storage Needed (gallons)
Cabins for occasional use1200 – 2500
1 – 2 persons2500 – 3000
3 – 5 persons5500 – 6000
6 or more8000 – 11,000 +

How to Calculate the Monthly Rainfall for Collecting and Storing

To make this easy, we created a calculator for harvesting rainwater. You only have to enter the numbers and the calculator does all the hard work. Give it a try, we’re sure you’ll like it!

How to Calculate the Amount of Water Needed for a Garden

Like rainwater harvesting, we also created a calculator for garden water. This is the easy way to discover how many gallons of water your garden needs weekly and daily.

On average, gardens need about two inches of water every week during the hot,dry summer months.  During moister conditions, the garden needs one inch or less.

For the fun of it: How much rainwater do you need to store if it doesn’t rain all summer?

Let’s take a look at a “worst case” situation to see how much rainwater needs stored to get through a hot, dry summer.

Suppose we have a garden that’s 20 feet long X 10 feet wide.  That garden has an area of 20 ft X 10 ft = 300 square feet.  The formula we’ll use is:

Garden area in square feet X inches of water X 0.623 = gallons of water needed. See the garden water calculator for more details and also how to get the answers using the metric system.

So, 300 square feet X 2 inches of water every week X 0.623 = 374 gallons of water every week.  As a quick rule, remember that 100 square feet with 1 inch of water equals 62.3 gallons.

Our long, hot dry summer lasts from June to August.  That would be 92 days. Definitely a “worst case” situation.

92 days / 7 days per week = 13.14 weeks.  13.14 weeks X 374 gallons per week = 4,914 gallons of water needed for the summer.  Since the summer is dry, the collection system must store that much water from the spring rains.

If you don’t have that much rainwater stored, most gardeners will have to use tap water instead.  We’re fortunate that most areas of the US and Canada don’t have summers without rain for that long!

A green plastic rain barrel beside a watering can on the back patio.
A basic residential rain barrel.

The Equipment Needed to Collect Rainwater

These systems are easy to construct and need little maintenance.  Here are the possible  components for a home system:

  • The rainwater catchment system.  Another name for this is the collecting system, and a roof is the obvious player here.  More complicated ways to collect rainwater are possible, but the roof is a no-brainer.
  • Channeling.  This would be gutters and downspouts.
  • Leaf screens.  Leaves are a pesky problem, no matter what.  Use the screens to prevent leaves from staying in the rainwater.  Then, you’ll have to clean the screens anyway after they clog.
  • Gutter guards.  These are more practical than leaf screens but have a higher initial cost.
  • First flush diverter.  The first flush diverter lets the initial rainwater bypass the barrel.  That allows contaminants to wash from the roof.  That way, they won’t enter the rain barrel.
  • Primary filtration.  Filtration ahead of the barrel keeps out other particles.
  • Storage.  This could be one or more barrels, large containers, or cisterns.  Many collapsible containers are available that are easy to store during the winter.
  • Overflow.  Barrels and containers for rainwater have a fitting at the top for overflow.  You can connect several barrels so the overflow goes to the next barrel in line as one fills up.  You could also connect the overflow back to the downspout to drain the excess rainwater.
  • Delivery system. Depending on the circumstances, water delivery could be gravity-fed or pumped.  Of course, you won’t need any of this if you’re only filling watering cans from the barrel.
  • Purification.  Optional unless you plan to use the rainwater for drinking.
  • Accessories.  These are any other parts needed to complete the setup.
  • Barrel covers.  Extra rainwater is excellent for your garden, but you don’t want to use it to start a mosquito farm!  Covers protect the water from all sorts of uglies.

Some Vital Information About Rainwater Barrels

You can find some important tips about rain barrels and tanks in our article about Rainwater Harvesting Systems for Home Gardens.

What Does It Take to Gravity-Feed Rainwater?

  • Gravity-feeding always needs the water source to be higher than the spot where you want the water to go.  From the clouds to the rain barrel, water is gravity-fed.
  • For every 2.31 feet of vertical drop, water gains 1 PSI of pressure.  Water for hose or sprinkler irrigation needs 20 PSI, which is the pressure a pump supplies.
  • To get 20 PSI from a gravity-fed system, the water has to drop 46 feet.  That situation leads us toward drip irrigation for the garden.  Gravity-feeding water from rain barrels to a soaker hose is as easy as it gets.

What Do I Need to Know About Using a Water Pump?

  • The pump should be an on-demand pump designed for rainwater.
  • Always use a filter on the pump feed line.  Cleaning a clogged pump isn’t the best way to spend a pleasant summer afternoon.
  • The pump should be able to run “dry.” That means it won’t suffer damage if it isn’t moving water.  The ability to self-prime is another essential feature of the pump.
  • Pumps that run on 12-volt batteries with solar panel rechargers are available.  That way, you could avoid using the 120 VAC household supply.
An old rain barrel full of water from a downspout.
An old country rain barrel. As simple as it gets.

Final Thoughts

My first experience with rain water collection was at my grandparent’s house.  Grandpa always kept a rain barrel at one of the downspouts at the back of his garage.

My mother and aunts said they always liked to wash their hair with rainwater from the barrel.  That would have been in the 1930s and 40s.

A wild group of children (including myself) came around in the 1950s and 60s.  We would help by using a step stool to fill the watering cans, which we then carried to Grandpa’s garden.

Once we had done that, we used the rain barrel for what we thought was its most important job:  filling up our squirt guns.

Collecting rainwater at home can save money and provide a good water supply for many projects.  With some imagination, you can even use it for fun…

Bob Styer

As a child, I hated gardening. That was mainly because Dad expected us to work in the garden every so often even though we thought play was more important. Over the years, though, I've developed a real appreciation for growing things. Whether you're growing plants for food or to enjoy their beauty, gardening can make your life better. Seize the moment!

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