Water is an essential need in the garden. Rainwater harvesting for use in the garden helps us to use water that would otherwise go to waste. We have been collecting rainwater for many years in different ways and places. Here we share some of our experiences.
Rainwater is free, fairly clean water that is a gift from the sky. Even though the water is free, the harvesting of it might not be legal at your place. Usually, however, if you only use the water in the garden and not in the house, it is not a problem. If in doubt, check with your local municipality.
The rule of thumb is that you get 0.6 gallons per 1 square foot, so one inch of rainfall on a 1,000-square-foot roof produces about 600 gallons of water. That is a lot of water.
The quality of the water does depend on the quality of your roof. However, for use in the garden, it seems that there are only two materials that are doubtful and not very common: Wood shingles that are treated with fire retardants, and copper because it is naturally a herbicide and not good for the garden (source: Blue Barrel Systems).
The rain barrels
To start you can use any barrel to catch water. As long as it holds a good amount, we would suggest starting with 45-gallon barrels, it is good to go. Depending on your roof size, for a 45-gallon barrel, you will need a good overflow for the barrel.
A drain hose makes a good overflow. You can use a longer one to lay along a food forest. For that you would plug the far end of the hose, place it along the plants that you want to irrigate, and drill enough holes on top of the hose to have the same throughput as the hose itself.
Our 40′ x 20′ garage roof collects roughly 500 gallons from 1″ rain. To catch all of it we installed a two 275 gallon totes on top of each other. The discharge of the top tote is connected via a plastic tube to the top inlet of the bottom tank.
Dirt, algae, and bugs
Rain barrels can be real collecting system not just for rain but also for dust, dirt, and bugs. If using a simple barrel, keep the lid intact, just make a hole big enough for the downspout. It will help so much to prevent debris from accumulating in the barrel, not to mention all the mosquitoes that will breed in there.
If the barrel has no lid or is an open barrel, you can install netting on top of it. IKEA netting curtains work well for this.
Even a closed barrel should have a strainer on the inlet side. We used a paint strainer attached to the downspout extension hose.
Algae will eventually grow anywhere where it is bright and moist. However, it will not grow overnight. We have used our barrels for many years now without significant algae growth. A dark barrel will prevent the growth much better than a transparent. Painting or wrapping a barrel can also help. We also find that having the barrels on the north side of the house is better than on the south side where the sun is much more intense all day.
Preparing the rain barrels for winter
Water expands in frozen state, leaving it in the barrel when temperature drops below freezing, will damage the barrel. Emptying the water harvesting barrel is a must.
In the fall trees, shrubs and perennials need to be watered. This way they better survive the cold winter. This is convenient because water barrels also need to be emptied. It is all part of preparing the garden for winter.
From our own experience, emptying alone is not good enough. If you don’t divert the discharge of the downspout, then it might happen that during a warm weather spell the winter months, snow will melt and water will again collect in the barrel. This way we lost one of our 1000 litter water tanks. It had been emptied in the fall, but not disconnected from the rain gutter. Some water got into the tank, and the heavy frost caused the tank to burst. So yes, we learned that it is also important to disconnect the water barrel from the gutter in the fall.
For a few years we stored our water tanks in the garage. It was amazing because the water never froze and winter collected water was ready to use during our often dry springs. If space is available to collect water in a building in a cold climate, it is a great option. The water also served as a much-needed thermal mass.
Using harvested rainwater in the garden
Just to collect rainwater without making it easy to be used does not help much. We collect the water not to just have it, but to use it.
Every barrel in our place has a faucet in the bottom. We also like to attach a short house to it for easier filling a watering can.
To use gravity better, it also helps to elevate the tank for increased pressure. The water flow this way is much faster and it’s more enjoyable to use.
Our two water tanks system have also a connection to a pump and a garden hose. We use a repurposed hot tub Flo-Master pump and a hose long enough to reach the farthest corners of the garden. During dry times this is really convenient. Also, if there is a lot of rain water it is easy to pump it over to the geodesic dome greenhouse water tank.
Rainwater from a rain barrel is much better for the garden than a tap or well water that is cold and maybe even has chlorine in it. The rainwater usually has a good temperature to use in the garden. It is worth it, not just for the cost of water but also for the quality to install a good rainwater harvesting system for use in the garden.