Tap Water vs Rainwater: Does It Matter for Your Plants?

Use rainwater for pot-grown blueberries and other acid-loving plants, and tap water for everything else. Hard tap water carries dissolved calcium and magnesium carbonates, and every watering adds a little more to a pot of ericaceous compost, neutralising its acidity the way antacid neutralises stomach acid. One watering does nothing measurable. Twice a week for a whole season can push compost that started at pH 4.5 well past the 4.5 to 5.5 a blueberry needs, and the plant then struggles to take up iron and manganese: the leaves yellow between the veins while the veins stay green, usually on newer growth first, and growth stalls. It looks like a nutrient deficiency because it is one. The good news in Scotland is that most tap water here is soft, so the drift is slower than in hard-water parts of the UK, but your own supply decides how much it matters.

What's in tap water

Mains water is treated before it reaches your tap, and two things from that matter in the garden, very differently.

Chlorine and chloramine are added in small amounts to keep water safe to drink. Gardening forums often blame them for killing soil life, but extension research doesn't back that up at drinking-water levels. At Colorado State University, water chlorinated at 5 parts per million affected organisms only in the top half inch of soil, and after 126 days of continuous heavily chlorinated water, populations were back to normal two days after it stopped. Chlorine binds readily to soil particles, which is why it doesn't travel far. You can ignore chlorine.

Hardness and alkalinity are the issue: dissolved calcium and magnesium, mostly as bicarbonates, picked up as water passes through rock on its way to the treatment works. Hardness furs up a kettle, and it makes water mildly alkaline. It matters in the garden because it is cumulative. One can of hard water won't shift a compost's pH. Fifty cans over a season, on a plant in a fixed volume of ericaceous compost with nowhere for the carbonates to wash away, will.

If your household has a water softener, don't use that water on acid-loving plants. A softener swaps calcium and magnesium for sodium, so it removes the hardness without lowering the pH. Softened water is fine for short-term use but not recommended long term, because sodium can build up in soil and reduce the nutrients available to plants. An outside tap that bypasses the softener is the simple fix, or use rainwater.

What's different about rainwater

Rainwater is naturally soft and low in minerals, and its pH suits most plants, acid-lovers like rhododendrons and camellias included. It is usually mildly acidic (around pH 5.5 to 6.5) because falling rain absorbs a little carbon dioxide and forms a weak carbonic acid, the same chemistry that makes fizzy water slightly acid. It has no added chlorine because it was never treated. The mineral argument is the reason RHS advice is to use rainwater from a butt in preference to tap water on blueberries. Collecting it also saves treated mains water in general: in dry south-east England a small shed roof can collect 2,500 to 3,500 litres a year, enough to fill 10 to 15 standard 240-litre butts (RHS).

Who needs to bother

Pot-grown acid-loving plants, most of all. Blueberries are the clearest case, needing acid soil or compost at pH 4.5 to 5.5, which in a pot means a peat-freeCompost made without peat, which is harvested from a slow-forming, carbon-storing wetland habitat that peat extraction damages. Full definition → ericaceous compost in a container of at least 30cm across for a young plant and 45 to 50cm for a larger one. Tap water is usually alkaline in hard-water areas, so used regularly it raises the compost pH. A small pot forces the same drift into a small volume of compost, so it happens faster and shows sooner than in open ground. Water from a butt rather than the tap, unless it's a drought and the butt is empty. The same applies to pot-grown rhododendrons, camellias and azaleas. A shrub planted in naturally acid, free-drainingSoil that lets water pass through quickly rather than pooling, so roots aren't sitting in wet ground after rain. Full definition → ground has far more soil to dilute and buffer the alkalinity, and on a soft water supply tap water is a small worry there. Check both the plant's situation (pot or ground) and your local water before assuming you need rainwater at all.

Most vegetables and garden plants don't need it. Potatoes, brassicasThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition →, beans, bedding plants and lawns grow happily on tap water. A small pH nudge that would slowly wreck a blueberry's acid compost is irrelevant to plants that are happy in neutral to slightly alkaline soil, which describes most vegetables.

The practical rule: know which of your plants are the acid-loving exceptions, use rainwater for those when you can, and don't haul butt water round the whole plot on principle. A cheap pH test kit is a more direct check than guessing from the water type. Test an ericaceous pot before and partway through a season, so drift is caught while it's small.

If the leaves have already yellowed

Yellowing between the veins is usually a manganese, iron or magnesium deficiency, and on acid-lovers in alkaline conditions it is typically iron. Switching to rainwater stops more alkalinity going in but won't remove what's there, so the recovery is slow on its own. A sequestered (chelated) iron feed sold for ericaceous plants, applied at the rate on its own label, treats the immediate deficiency. For a badly drifted pot, replacing the top third to half of the compost with fresh ericaceous mix usually resets it much faster than waiting for rain to reverse months of build-up.

Is rainwater ever the worse choice?

Rarely, but it isn't risk-free. Rain picks up whatever is in the air and on the surface it runs off, so water from near a busy road or an industrial area, or off an old or unusual roof, may carry more than water from a clean roof. For most domestic garden use that is background-level and nowhere near enough to outweigh the pH advantage for an acid-lover. The bigger practical risk is what happens in the butt afterwards. Algae, debris and stagnant water build up in a butt that is uncovered or never emptied, and that is a storage problem rather than a rain-versus-tap one; setting up a water butt covers how to avoid it.

Bath or washing-up water (grey water) is not a substitute for rainwater on edible crops. Use it fresh, within 24 hours, from a watering can only, and preferably on ornamentals rather than edibles (RHS).

Rainfall across Scotland varies enormously, from under 700mm on the East Lothian, Fife and Moray Firth coasts to over 3,500mm in the west Highlands (the Scotland climate page has the regional figures), so how easily a butt refills depends on where you are: on the driest east coast a butt can run empty in a dry spring, while in the west it refills quickly. A full butt is also a hedge against restrictions: hosepipe ban rules explains how a Scottish Water Shortage Order differs from an England-and-Wales hosepipe ban.

Is your Scottish tap water hard or soft?

Most drinking water in Scotland is classified as soft, because the chalk and limestone that make water hard are uncommon here. That makes tap water less of a worry for a Scottish blueberry pot than for the same pot in much of England, and it means you may be collecting rainwater for plants that would cope on tap water. Harder supplies exist, though: limestone areas and some borehole supplies tend to give harder water, while granite, schist and peat catchments tend to give soft. The local geology and water source decide, so your postcode tells you more than your region.

To check yours, put your postcode into Scottish Water's water quality search to find your supply zone, or phone their helpline on 0800 077 8778 and ask for the hardness of that zone. Don't assume you're safe because you live in the Highlands or on the west coast, and don't assume you're in a hard-water area because you live in the Lothians or Central Belt.

The bottom line

A water butt is worth the effort when two things stack up: the plant is a pot-grown ericaceous one, and your supply is hard enough to drift the compost. Check the plant, check the zone, then decide. If your water is soft, a yearly pH check on the pot may be all you need.

Common questions

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