Short answer: yes, rainwater works chemically for Kratky, because it is soft and low in minerals, much like distilled water. The concern is not the chemistry but what the water picks up from the roof and the barrel, especially bacteria. For leafy greens eaten raw I would not use roof-collected rainwater; for crops like tomatoes, where the edible part never touches the solution, it is a more reasonable choice.
Why rainwater appeals
Rutgers Cooperative Extension describes rain barrel water as usually soft, meaning free of dissolved minerals and slightly acidic, which suits nutrient uptake. For a Kratky grower with hard tap water, that means no extra sodium or bicarbonate, and a pH that does not creep up from alkalinity. It behaves much like distilled water once the nutrient is added.
What studies found in rain barrels
Rutgers notes that roof runoff can carry pathogens, zinc, lead and polycyclic aromatic hydrocarbons (PAHs), and that the first flush of runoff from a roof has been shown to have the highest levels of contaminants. Most home rain barrels do not divert that first flush.
In a New Jersey study of 12 rain barrels on asphalt-shingled roofs, sampled over four months, Rutgers found the overall water quality was very good:
- Lead and zinc were well below federal irrigation standards.
- PAHs were not detected in any sample.
- 9% of samples were above the single-sample irrigation limit for E. coli.
- Total coliform bacteria were present in every sample.
UConn Extension adds that the bacteria come mainly from droppings of birds and squirrels on the roof, and that rain barrel water should never be used for drinking, cooking or washing.
Why this matters more in Kratky
The usual advice for vegetable gardens is to apply rain barrel water to the soil and avoid contact with the parts you eat. UConn Extension quotes Dr. Mike Dietz recommending not using roof water on anything leafy that you are going to eat directly. In a Kratky jar, lettuce and herbs sit just above the water for weeks, and anything splashed or wicked up can reach the leaves. That is why I would not use it for greens eaten raw.
A still container is also a good place for bacteria to grow, and Penn State Extension notes that some Pythium species live in pond and stream water. I would keep any untreated surface water away from Kratky containers for the same reason.
If you do use rainwater
- Choose the crop. Fruiting crops such as tomatoes and peppers, where the harvest hangs well above the container, are a better fit than lettuce or herbs.
- Collect cleanly. Divert the first flush if you can, keep the barrel covered and in the dark, and clean it before the season. Rutgers recommends treating the water as routine when it will be used on edibles.
- Measure it. Check EC and pH like any other water. Rain is soft, so pH adjustments need much smaller doses.
- Mind the roof. The Rutgers study covered asphalt shingles. Other roofing materials were not part of it.
- Use it soon. Stored water grows algae and bacteria over time, especially in warm weather.
Rainwater versus distilled water
Chemically they are similar: low minerals, low alkalinity, very little buffering. The difference is that distilled water is clean by design and rainwater depends on the roof, the barrel and the weather. If what you want is soft water for leafy greens, distilled or RO water gives you the same chemistry without the bacteria question. See distilled, RO or tap water for Kratky.
Measuring rainwater before use
Because rain is soft, its EC is low and the nutrient supplies almost everything, as it does with distilled water. Rutgers describes rain barrel water as slightly acidic, so I always test pH after mixing rather than assuming it is in range. With so little buffering, I add pH adjusters in very small doses and wait between them. I also look at and smell stored water before using it. Any slime, green tint or sour smell means something has started growing in it, and I would not put it in a container.
Quick questions
Does boiling rainwater make it safe?
Boiling kills bacteria but does not remove metals or other dissolved contaminants, and it is impractical for large batches. For greens I would rather use a different water.
Is rain collected straight from the sky better?
Water caught in a clean container away from roofs and trees avoids the roof contaminants, but anything open will still collect dust and debris. Measure it and use it fresh.
Related guide: water for Kratky nutrient solution.
Sources
- Bakacs, M., Haberland, M. and Yergeau, S. Rutgers Cooperative Extension. Rain Barrels Part IV: Testing and Applying Harvested Water to Irrigate a Vegetable Garden (FS1218).
- Allen, J. UConn Home & Garden Education Center. Can I Water Vegetables with my Rain Barrel Water?
- Moorman, G.W. Penn State Extension. Pythium.