In this article
No. Water does not expire, and rain barrel water does not go bad in the way milk goes bad. What it does is go stagnant, which means algae, bacteria, a sulfur smell and sometimes mosquito larvae. None of that stops it being fine for watering a lawn, flower beds or ornamental plants. What it does mean is that rain barrel water is never drinking water, for you, your kids or your pets, no matter how clear it looks.
Safety note: do not spray collected rainwater onto the leaves or edible parts of vegetables. Pour it at the base of the plant instead. The roots take what they need and leave the rest behind.
Why the water changes but does not spoil
Think about the trip that water takes. It lands on your roof, runs across shingles, through a gutter full of leaf litter and grit, down a spout, and into a barrel. It picks up plant matter, bird droppings and bacteria on the way. Then it sits in the dark and warms up.
Used within a couple of weeks, nothing much happens. Left for a month or more, three things tend to show up: a rotten egg smell, green or brown algae, and mosquitoes. All three are unpleasant. None of them make the water unusable for the job it was collected for.
Why your rain barrel stinks
That sulfur smell is bacteria breaking down organic matter in water with very little oxygen in it. It is the same process that makes a neglected vase of flowers smell foul. The water is still safe to put on plants. If the smell bothers you, the fix is upstream: less debris going in, less sunlight hitting the barrel, and using the water often enough that it never sits long.
How long can water sit in a rain barrel?
Two weeks is the number you will see quoted most often, and it is a reasonable target rather than a deadline. There is no point at which the water flips from good to bad. How fast it turns depends on heat, sunlight, humidity and how much organic matter came in with it. Plenty of people leave water in a barrel for months and use it with no trouble.
The genuinely useful rule is simpler: the more often you actually use the water, the fewer problems you will have. A barrel that gets drained and refilled every week rarely smells and rarely breeds anything. It also does more for your water bill, which is the reason most people put one in. If you are chasing outdoor water savings generally, the same question comes up with pressure washing and your water bill.
There is one exception. If something has died in the barrel, drain it, clean it and start again.
Four things that keep the water clean
These are the ones that matter for water quality specifically. For choosing, siting and plumbing a barrel in the first place, I covered the whole setup in the ultimate guide to rain barrels.
Use a dark, opaque barrel
Algae needs light. A clear or pale barrel lets sunlight reach the water and you get green slime and smells much faster. Dark and opaque is the single easiest thing to get right, and it costs nothing if you are still choosing a barrel.
Put it in shade
Same principle, applied to placement. A barrel on the north side of the house, or under a tree, stays cooler and darker than one baking on a south wall all afternoon. Cooler water grows less of everything.
Screen the inlet, but not too finely
A mesh screen over the barrel opening keeps out leaves, seeds and adult mosquitoes looking for somewhere to lay eggs. There is a trade-off in mesh size. Very fine mesh clogs with pollen and grit and needs cleaning constantly. Medium holes are the sweet spot for a rain barrel.
You do not need a purpose-built one, though a round aluminum rain barrel screen saves cutting something to fit.
Close the gap at the downspout
If there is an open gap between the end of your downspout and the barrel, every insect and windblown leaf in the yard has an invitation. Connecting them directly with a downspout adapter removes the entry point entirely, and it costs a few dollars.
Clean gutters help too, for the obvious reason that whatever is sitting in your gutter is what gets flushed into the barrel next time it rains.
Mosquitoes are the one problem worth fixing straight away
Algae is cosmetic. Mosquitoes are not. A rain barrel is close to a perfect mosquito nursery: still, warm, sheltered water. Even with a good screen, eggs get washed in from the gutter.
The standard answer is a larvicide tablet, sometimes sold as a mosquito dunk. The common ones use a soil bacterium that targets mosquito larvae specifically, so the water stays usable on plants. Check the label on whichever you buy, drop it in, and it keeps working for weeks.
If mosquitoes are a broader problem in your yard and not just in the barrel, there is some overlap with the advice in keeping flying insects away from an outdoor party.
Cleaning the barrel
Once or twice a year is enough for most people. Some clean whenever the barrel starts to smell or shows algae, which is fine too.
- Empty the barrel completely.
- Use white vinegar or baking soda for a routine clean, or diluted bleach if you want to go further.
- Scrub the sides if you can reach them. If you cannot, slosh the solution around and flush it out with a hose.
- Check the spigot and clear it if it is gummed up.
Rinse well afterwards, and put the first barrel-full of water on the lawn rather than on anything you plan to eat.
Should you add bleach to stop the smell?
A capful of chlorine bleach is a common suggestion for a smelly barrel, and it does suppress bacteria. Whether you want to do it depends on what you water. If the barrel feeds a vegetable garden, a lot of people would rather deal with the smell than add chlorine. If it waters the lawn and the shrubs, it is a non-issue. If you do add it, wait 24 hours before using the water so the chlorine can dissipate.
Questions people ask
Can you drink rain barrel water if you boil it?
Treat the answer as no. Boiling kills organisms but does nothing about what washed off your roof: shingle granules, metal from flashing, whatever the birds left. Rainwater intended for drinking needs a proper collection surface and a filtration system, not a barrel under a gutter.
Is green water in my rain barrel a problem?
Only cosmetically. Algae means light is getting to the water. Move the barrel into shade or switch to an opaque one. The green water itself is fine on plants and lawns.
Can I use rain barrel water on my vegetable garden?
Yes, poured at the base of the plants rather than over the leaves and the parts you eat. Wash the produce as usual before eating it.
Does rain barrel water freeze and crack the barrel?
It can. In a climate that freezes, drain and disconnect the barrel before winter and leave the spigot open. A full barrel that freezes solid will split, which is a more expensive problem than any of the ones above.
