When to water: read the substrate, not the calendar

This is the practical skill the whole module is pointing at: deciding when to water. There are a few ways to tell, and the easiest one is free.

The finger test

Push a finger into the substrate and feel how dry it is. Iowa State's extension service calls this the easiest and most reliable way to tell when to water (Iowa State University Extension and Outreach, n.d.), and it costs nothing. How deep to check varies a little by source. One common rule is that when the top inch (2.5 cm) feels dry in a pot up to about 8 to 10 inches (20 to 25 cm) across, the plant probably needs water, and in smaller pots the top half inch (about 1 cm) will tell you (Davison, 1998). Others check to about two inches (5 cm) for most plants (University of Maryland Extension, 2023). Either way, don't judge by the surface alone: it dries ahead of the root zone, so feel below the top layer (PlantTalk Colorado, n.d.).

The depth also depends on the plant. Desert cacti and succulents are soaked and then allowed to get fairly dry before the next watering, and they need far less water than most houseplants (Trinklein, 2017; University of Maryland Extension, 2023). Plants with large or very thin leaves usually need watering more often (Davison, 1998). Some plants, such as ferns and prayer plants (Maranta), are kept evenly moist, with the surface never quite drying (Trinklein, 2017), and advice for everyday tropical foliage plants runs from letting the top inch (2.5 cm) dry to keeping the mix evenly moist. Our own rule of thumb, when in doubt: a plant adapted to bright light usually tolerates more drying than one adapted to shade.

Fig. 3. The finger test. Push in to about the first knuckle and feel whether the substrate below the surface is dry. The surface dries first, so feel below it.

The lift test

The second free tool is the pot's weight. As a plant uses water, the mix dries out and gets lighter, so lift the pot now and then and compare it with how it felt right after watering (University of Maryland Extension, n.d.). Water a plant thoroughly, let it drain, and pick it up. Remember that heavy, just-watered feel. A few days later, lift it again. A pot with dry mix weighs much less than one that still holds plenty of water (University of Maryland Extension, 2023). For a container too big to lift, push a stick or dowel to the bottom of the pot: moist mix sticks to it or darkens it slightly (University of Maryland Extension, n.d.).

Fig. 4. The lift test. Learn the weight of the pot right after watering, then again when it has dried out. The difference is the water. Use it alongside the finger test.

What about moisture meters?

Extension advice on the cheap probe-style moisture meters sold for houseplants is mixed. Some guides say they give a good rough sense of how moist the middle and bottom of the pot are (Davison, 1998). Others warn that they vary widely in accuracy and that fertilizer and the type of mix can throw off the reading (Shaughnessy & Pertuit, n.d.), or that they may not always be reliable (University of Illinois Extension, n.d.). If you use one, check it against your finger and the pot's weight until you know how it reads in that pot. We treat the finger and the lift test as the first tools, and a meter as a second opinion.