Watering houseplants can be tricky to master. Just about everyone has under or overwatered a plant or two, so while it’s frustrating, know that you’re not alone. A moisture meter is a tool that can help demystify the process and help you better understand when your houseplants need water. Learn when and how to use a soil moisture meter.

What is a Soil Moisture Meter

A moisture meter measures the dampness in the potting soil. The device features one or two metal probes attached to a plastic casing with a display area that provides the reading. Most soil meters do not use batteries or an on/off switch. Many models also measure pH and sunlight for added convenience.

How It Works

There are several types of moisture meters, and while they do the same thing, they work differently—most meters work by measuring the ability of the soil to hold an electrical charge. The meters don’t measure the potting mix but rather any water in the soil. Wet soil holds a charge, while dry soil does not. 

The most common types of moisture meters use the following technology:

• Capacitance
• Time Domain Reflector (TDR)
• Time Domain Transmissometry (TDT)
• Standing Wave Sensors

Most moisture meters for houseplants use TDR or capacitance technology. TDT and Standing Wave meters are often used in commercial or professional applications.

Other Uses

Moisture meters have other uses beyond houseplants. These devices are also used for construction, landscaping, and home repair, so if you pick one up, make sure it is labeled as a plant moisture meter. Devices intended for other applications may have smaller prongs or rely on a sensor and will not work or will not be as effective at measuring soil.

Why Use a Moisture Meter for Plants?

All of the plant advice and care tips out there will tell you to feel the soil and water based on the dryness of the potting mix. Sticking your finger in the potting mix is great if you know what you’re doing. If you’re new to the hobby or picked up a new variety, getting the hang of when to water can be challenging. A plant moisture meter eliminates the guesswork, so you know precisely when a plant needs water and, perhaps more importantly when the plant does not need water.

Do I Need a Moisture Meter?

Maybe. A moisture meter isn’t necessary if all your houseplants are consistently healthy and properly watered. If you routinely struggle with some or all of your plants and they show signs of water stress, you should consider picking up a moisture meter for plants.

When to Use a Soil Moisture Meter

Take a moisture reading when you think the plant needs water. Plan on getting measurements every 7 to 10 days if you historically underwater plants. The type of plant, time of year, pot size, potting mix quality, available sunlight, and humidity all impact the rate at which the soil dries out. Consider all of these factors when deciding when to use the meter and measuring the dampness.

How to Use a Plant Moisture Meter

While each model is different, the general process is the same. Stick the probe into the soil and read the gauge. Avoid touching the bottom of the pot with the probe because that may impact the reading. The meter should give you a reading right away.

Completely submerge the prongs as deep as possible in the potting mix. If you only insert the prongs one or two inches, you’ll get a reading that only pertains to the top couple of inches. You can get by only inspecting the top few inches when gauging the dampness by hand. However, when feeling the soil to determine dryness, you also observe the plant for signs of water stress. The advantage of a moisture meter is that you can get a reading for the entire pot.

Multiple Readings

One reading is enough when measuring soil in a small pot that measures up to five inches across. Anything over five inches may need several readings throughout the container. The larger the pot, the more readings. Don’t rely on a single reading from one spot; instead, take multiple readings and average it in your head.

Protect the Roots

Be careful not to damage roots when inserting a moisture meter in the soil. Avoid placing the meter right up against the spot where the stem emerges from the soil. Ideally, you want to take a reading close to the roots but not so close that you potentially damage the roots. Gently insert the probe and immediately stop if you feel resistance.

Read the Meter

Some meters have a digital display, while others have a gauge. The gauge is usually broken down into different sections, identified as dry, moist, and wet, and there is often a numerical scale that further breaks down each level. A needle will point to the appropriate area based on the dampness detected in the soil.

Once you have the reading, use what you know about the specific plant to determine if it is time to water. Don’t water any plant if the meter reading is ‘wet.’ Plants with increased water needs, like Aglaonema and ferns, will need a drink when the meter reading is medium moist, while Pothos should be barely damp before watering. Wait until the reading is ‘dry’ before watering drought-loving plants like cacti, ZZ plants, and Kalanchoe. 

Pro tip: Feel the soil with your finger after getting a reading so you can understand what dry or damp soil feels like. As you get more confident, feel the soil first, make a guess, and then take a reading to fine-tune your skills.

Follow Up

A meter is great for determining when to water houseplants, but you can also use it to check how good of a job you did watering. After watering a plant, the potting mix should be wet so the root ball can soak up moisture. Take another moisture reading about 20 minutes after watering a plant. Check several spots to gauge the dampness throughout. If the meter gives you a ‘wet’ reading, you did well. If you find any dry or moist spots, saturate that area, and remember to work on that next time.

Plant Moisture Meter Buying Tips

A plant moisture meter can be a very helpful tool if you are starting out with houseplants or feel like you just can’t figure out when your Prayer Plant needs a drink. Using a meter can help you provide better care for plants while you figure out what ideal care entails. Learning how to use a moisture meter for plants is easy and will help your houseplants thrive.