Gardeners have known for generations that plants prefer rainwater. But what makes it so much better than tap water?
The Chemistry of Rainwater
Rainwater is naturally distilled — it contains no dissolved minerals, chlorine, or fluoride. When it falls through the atmosphere, it absorbs carbon dioxide, making it slightly acidic (around pH 5.6). This mild acidity helps release nutrients in the soil, making them more available to plant roots.
Tap water, by contrast, is treated with chlorine and often contains high levels of calcium and magnesium (the minerals that cause hardness). These raise the pH and can lock up essential nutrients over time.
What This Means for Your Garden
Acid-loving plants like rhododendrons, azaleas, blueberries, and camellias suffer in alkaline tap water. Rainwater keeps the soil pH where they need it.
Seedlings and young plants are particularly sensitive. Tap water chemicals can damage tender roots. Rainwater gives them the best start.
Compost and soil biology thrive with rainwater. The beneficial bacteria and fungi that break down organic matter prefer the conditions that rainwater creates.
Indoor plants often develop white residue on pots and leaves from tap water minerals. Rainwater eliminates this entirely.
How to Make the Switch
Installing a water butt is the most practical way to collect rainwater for your garden. A single 200-litre butt will cover most of your watering needs during spring and summer. For houseplants, fill a watering can from the butt and let it settle before watering indoors.
Your plants will thank you.