Moisture Meters

If you like to be told when to water your plants by using a mechanical aid, there are a number available from nurseries and garden centres. Called moisture or water meters and gauges, they will give you accurate information about watering.

Their use is not limited to plants kept inside the house — they can be used very successfully in sunrooms, conservatories and greenhouses. You can also use them out on the patio for assessing the watering needs of plants in tubs, troughs, window-boxes and hanging baskets.


Some water meters are sophisticated and quite elaborate, but most are simple to use. Some incorporate other aids to good growing such as a meter for measuring the light intensity orfor testing the acidity or alkalinity of the potting mixture. If you have one that combines all three you will find it very simple to use–justflick the central switch and you will be able to use it for each purpose.

What do they look like?

Most have one or more spikes or probes which can be pushed intothe potting mixture. Almost immediately you do this a reading is given on the dial. This will tell you whether the plant needs water or not. The most common dial is a numbered one with a pointer that swings up the scale of figures. Others have differently colou red areas, with orwithout guide words such as ‘dry’, ‘moist’, or ‘wet’ . Normally the higher the figure given at a reading, the wetter the compost is.

Over-watering is the cause of most house plant failure, so take the guesswork out of plant watering and use a moisture meter. Several different varieties are available that will show you exactly when your plants need water.

Using a water card

Push the pointed end of the card into I the potting mixture to the level indicated.

It is the tip of the probe that registers the moisture content, so this needs to be fairly close to the rootball. The dial of the meter will record the level of wetness or dryness and you should act accordingly. Some meters are issued with booklets showing various plants and the appropriate meter reading at which watering is necessary.

Indicator cards

Long-pronged meters obviously are unsuitable for checking water needs of smaller plants in small pots. For these, test the water content using a much simpler aid – a small strip of treated card known as a ‘water signal’. These strips or pins change colour when they are in contact with the potting mixture. They look like plant labels . To use them push them into potting mixture (inserting the card midway between the plant stem and the pot rim) to a depth of 2.5-3.8cm (1-11/2 inches).

  • Leave them in the potting mixture all the time. When you inspect your plants note the colour of the card. Water the plants when the card has reverted to its original colour.
  • Wait a while to see if the specified areas on the card changes colour. When they do, act accordingly.
  • The spikes of the water meter vary in length but the most useful for testing the wetness of potting mixture in larger pots are those which are 15-20cm (6-8 inches) long.
  • Probed meters are particularly useful. They can reach down into the middle and lower levels of the potting mixture, where the bulk of the plant’s roots are. Using a probed meter, therefore, can give you a better indication of the state of the mixture than you would have by just feeling the surface with your fingers.

Using a probe

  1. Insert the probe vertically mid-way I between the pot rim and the plant stem. In larger pots place it nearer to the stems. Take the reading.
  2. Withdraw the probe and wipe the L probe clean with a soft cloth or tissue before moving to the next plant or packing the meter away.
  3. The meter probes or probe should be pushed into the potting mixture about half way between the plant stem and the pot side, and should go into half the depth of the pot.


Will the prongs of the moisture meter damage the roots of my plants?

As the prongs are pushed into the mixture they will do a little damage to the plant’s roots, but the holes they make allow air to enter the mixture. This aeration will outweigh any minor root damage.

Does the kind of potting mixture I use affect the reading of the mixture?

Yes. Mixtures that contain a lot of sharp sand or grit–the sort used for cacti and other succulents, give a confused reading. The probe registers differently when in contact with a quick draining material, such as grit, to the way it does when in contact with a water-retentive material such as peat or leaf mould.

Should I act immediately if a reading shows that the mixture is getting dry?

This depends on the kind of plant. Plants that prefer permanently moist mixtures should be watered straight away. Some, like cacti and other succulents, prefer a period of a little drying out between applications, so water them in the next day or two after taking the reading.

Should I react to a reading 74‘from a meter or a water card in the same way during the winter as in summer?

No. Plants that take a winter rest and those that prefer to be drier in the winter should be given less water during that period, even if the reading shows it is dry.

Do’s and Don’ts


  • Make sure you push the moisture meter probe about half-way between the pot and the plant stem. It should go down to about half the depth of the pot.
  • Withdraw moisture meter immediately when you have taken a reading. Always wipe a moisture meter probe clean and keep it dry until you check the next plant. Pack it away in its box between watering sessions.


  • Leave the meter in the compost or let it remain in contact with moisture for any length of time. You should leave the probe in the mixture only for as long as you need to get the required reading, then remove it.
  • Let indicator cards get wet – it will give a false reading.

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