How much, how often: it's a rate, not a number

Now for the question everyone came for. How often should you water? It depends, and by now you have the tools to know what it depends on. The size of the plant and pot, the light, the temperature, the humidity and other conditions all change how fast the mix dries out, so no general schedule works for every houseplant (Trinklein, 2017). Water leaves a pot through the plant, which transpires it through its leaves, and by evaporation from the surface of the mix and the walls of a porous clay pot (UConn Home & Garden Education Center, n.d.). Transpiration plus evaporation is called evapotranspiration (U.S. Geological Survey Water Science School, 2018), the whole-system water loss you met in Module I. Speed it up and you water more often; slow it down and you water less. Everything below is a list of the dials that move it.

• Light. Higher light can raise a plant's water use and leave the mix drier, and plants in lower light need watering less often (Criscione & Holliday-Goulart, n.d.). Move a plant from a bright window to a dim corner and expect to water it less.
• Temperature and humidity. Warm air and low humidity raise the rate of transpiration; cooler air and high humidity lower it (Criscione & Holliday-Goulart, n.d.). A plant kept by a cool window in winter may need less water.
• Airflow. Moving air usually speeds transpiration, because it replaces the humid air next to the leaf with drier air (U.S. Geological Survey Water Science School, 2018).
• The pot. Small pots dry out faster than large ones, because there's less water in them (Criscione & Holliday-Goulart, n.d.). Material matters too: water evaporates quickly through the sides of an unglazed clay pot, which needs watering more often than a glazed or plastic one (Shaughnessy & Pertuit, n.d.).
• The substrate. Fine-textured, peaty mixes hold more moisture than loose, porous mixes of bark, sand and perlite (Shaughnessy & Pertuit, n.d.). This is the air-filled porosity from Lesson 2 showing up in your watering can.
• The plant itself. A plant that's big for its pot needs watering more often than one that's small for its pot (Shaughnessy & Pertuit, n.d.). Moving a plant into a pot that's too large can leave its roots in mix that stays too wet (Criscione & Holliday-Goulart, n.d.).
• The season. Growth is slowest in winter, with cooler temperatures and less sunlight, so watch plants closely through autumn and winter to avoid overwatering and root rot (Criscione & Holliday-Goulart, n.d.).

You don't have to calculate any of this. You just have to know that the answer moves, and in which direction. Brighter, warmer, drier air, smaller pot, unglazed clay, coarse mix, big plant, summer: water more often. Dimmer, cooler, humid, bigger pot, glazed or plastic, fine mix, small plant, winter: water less. The finger and the lift test from Lesson 3 are how you read the result of all these dials at once, without doing the math.

Fig. 6. Frequency is a rate, not a number. These are the dials that turn it up or down. You don't calculate them; you read their combined result with your finger and the pot's weight.