Which types you can use, if softened water is safe, and why rainwater is best

With droughts declared and hosepipe bans in effect in many areas, gardeners face a new challenge in how to make sure their plants and flowers – and wildlife – thrive.

It’s time to make more use of the watering can, and to conserve and reuse water – householders are being encouraged not to let it escape down the plughole but to keep it and use it in the garden.

However, many households have water-softening units fitted to reduce limescale on appliances and to improve the efficiency of household detergents – and advice can be confusing on whether softened water is safe for plant use.

Here is all you need to know about which water is best for your plants, whether and when to use softened water – and why rainwater (when we finally get it) is what your garden loves the most.

What is softened water?

Calcium and magnesium are the minerals in water that are mainly responsible for its hardness.

The most popular water-softening units exchange the calcium and magnesium ions for sodium ions. This leaves a trace of sodium in the water.

Certain types of water can be damaging to plants over time (Photo: Getty Images)

Can I use softened water on plants?

Although short-term use is unlikely to be very damaging, watering container plants or the garden with softened water is not advised over a long period of time.

This is because:

  • Potentially over time the sodium levels could build up within the soil or growing media and, although not harmful in itself, can damage the structure of clay soils. In practice in British conditions all sodium is eliminated by winter rains
  • Also, larger quantities of water used through the softening unit will increase the overall running costs

Consequently it is recommended that the outside tap runs off the mains rather than through the water-softening unit.

Which other forms of water can I use for plants?

Rain water is always the first choice; it is free from hard water elements and is the correct ph for the majority of plants, including acid-lovers such as rhododendrons and camellias.

The only downside is the limitations in the storage of rain water and that it usually runs out when it is most needed in the middle of summer.

Stored rain water is not recommended for use with seeds and seedlings in case it carries disease.

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Tap water will meet the needs of most garden plants but is relatively costly and is a finite resource and should be used carefully. Watering only when needed and soil level is the most economical method.

Ericaceous plants with lime-free organic mulches such as composted pine needles or composted bark will help retain moisture within the soil and keep the pH low.

Ideally, keep the use of tap water to a short spell over the summer months.

Purified water is available in a number of forms; including distilled and deionised. These waters do not contain any impurities such as minerals. Purified water is too expensive for wide-scale use but can be useful for plants that do best with rain water, such as orchids, when rain water is unavailable.

However, because they lack minerals, balanced fertilizers need to be added to meet plant needs. Although orchid specialists might have their own deioniser or reverse osmosis machines, most people have to source purified water from a car accessory shop or an aquatic shop.

Cost, transportation and storage limitations usually make them impractical for all but a few plants.

Water collected from tumble dryers and dehumidifiers is distilled and therefore suitable for use on plants.

Boiled water from the kettle can be used when cooled for plants. In hard water areas some of the calcium will be removed during the boiling process. Obviously this is an expensive way to water your ericaceous plants, but it may be worth considering as a stop-gap for watering a small number of containers when rain water is not available.

How can I collect rainfall?

Mains water usage in our homes and gardens is often highest when water availability is lowest – in hot, dry periods.

However, by adopting a few simple pledges, we can all make our gardens and outdoor spaces thrive on the rainfall we receive by collecting it and re-using it during dry spells. The RHS has advice here for more information.

Guy Barter is the Royal Horticultural Society’s Chief Horticultural Advisor. The RHS is a charity inspiring everyone to grow via its research, advisory, outreach, shows and gardens. For more information, visit: rhs.org.uk