Amateur botanists and gardeners with other than 'green' thumbs can rest easy away from their plants now that automatic, precise soil monitoring is here. Garden products maker Cabinotier, a three-year-old Tokyo-based start-up, has developed the Sustee, a portable sensor that says when to add water to indoor potted plants.

"The Sustee was actually born out of concern over how to relieve stress," says Cabinotier project assistant Fumiya Asa. "Humans have a natural relationship with plants, and studies have shown that potted plants can play a big role in helping to defuse stressful environments. Keeping plants healthy is obviously a concern, but surprisingly, not a lot of people know a great deal about proper plant care."

The Sustee is principally designed to address root rot - a real, and serious condition that affects many potted plants, due overzealous watering. Overwatering can cause roots to die in two ways: a lack of oxygen and the proliferation of soil fungus.

Root rot can be lethal. Treating it is equivalent to radical surgery; plants must be removed entirely from their containers, washed thoroughly, and have the affected roots cut away before being repotted with fresh soil.

The patented Sustee keeps the soil's water content in ideal balance by using the "pF curve," which measures the relation between soil moisture suction and soil moisture content. Cabinotier's design flows from trial-and-error experiments showing that a pF curve of 1.7-2.3 is an effective water moisture level for virtually any plant.

The company makes two versions of the unit. The M size is for plant containers between 4.7 inches and 7 inches in diameter, and the L size is for pots of 8.3 inches in diameter and above. The device's casing is made of polycarbonate; colors come in white, red, black, and green.

Using the device is fairly easy; owners simply insert it vertically into the soil at the base of the potted plant. The color of the on-board meter registers blue when the plant is sufficiently watered, and gradually turns white as the soil dries out, signaling that it's time to add more water.

"The Sustee was designed for both amateurs who may not know what they are doing as well as for more experienced plant owners who may not have much time on their hands," says Mr Asa. "Tending plants is not rocket science, but it is a bit of a science if you want to obtain optimum results."

The device is best suited for potted flowers such as roses, begonias, and carnations, and for small coffee, blackberry, and olive trees, not to mention bonsai plants.

The pen-like device has garnered some respected accolades, including Germany's Red Dot design award, Hong Kong's Design for Asia award (both in 2014), and a Japanese Good Design award (2015). Customer reception among online retailers has also been strong, where the units sell for about 680 yen each.