What water should I use, and what pH should it be?

For most home growers, tap water is fine. The catch is that you don't know your tap until you test it, and a pH number only means something once you know what you measured. So test it, don't guess. Set your input pH for the mix you grow in, then read the plant before you chase any number.

A pH reading is a clue, not a verdict.

Is my tap water OK?

Usually, yes. Three things are worth knowing about it.

Chlorine or chloramine. Letting water sit out for a day clears chlorine, but not chloramine, and plenty of city water uses chloramine. Check your water utility's report to see which one yours uses. If it's chloramine, vitamin C or a chloramine rated filter takes it out.

What a ppm pen tells you. A ppm pen reads total dissolved solids: how much is in the water, not what it is. A high number tells you there's a lot in your water. It doesn't tell you the water is hard, and a middling number doesn't prove it's soft. Know which ppm scale your pen uses, too, because two pens can show different numbers for the same water.

Temperature. Use it at room temperature, not ice cold straight from the faucet.

What a pen can't see is the pH of your tap. That takes a pH pen or test, and it's worth doing once on plain tap water so you know where you're starting.

What pH should my water be?

Match the number to what you grow in, and test the water you're about to give her, after the nutrients are mixed in, not the plain tap.

• Soil and peat mixes: roughly 6.2 to 6.8.

• Coco and hydro: roughly 5.8 to 6.2.

These are lanes to steer inside, not one magic number. If you grow in amended organic soil and just add water, the soil buffers its own pH, so the input number matters less.

Make sure the pen is telling the truth

A pen that's off can invent a problem that isn't there. Calibrate your pH pen with two buffers, pH 4.0 and 7.0, before you trust a reading that's about to change what you do. Three uncalibrated meters aren't three opinions. They're zero measurements.

Why your pH numbers don't match

Your pen says one thing, your runoff another, a soil probe a third. Most of the time none of them is lying. They're answering different questions.

• Input pH tells you what you gave the plant.

• Runoff pH tells you what came out after the water moved through the pot and picked up whatever was in its path.

• A probe reads one small pocket of the pot.

The mix pushes back against change, and the roots shift the pH around themselves as they feed. So runoff a couple tenths off your input is normal. What matters is a big swing, or a drift that keeps going the same way over a few waterings.

The disagreement isn't a malfunction. It's the diagnosis.

What not to do

Don't chase runoff. Push the input down because runoff read high, push it back up next time, and the roots ride a pH roller coaster. Keep your input steady and watch the plant over days, not minutes. Runoff is a clue. It's not a steering wheel.

Don't blame the water first. When a plant looks off, read her in order: the pattern on the leaves, then the room and the pot weight, then the root zone, then your inputs. Water and pH come after the plant has told you where to look. Then make one small move and give it time.

When it's something else

• Yellow leaves, and you think pH is the cause: see Yellow leaves: is it a deficiency, and is this too much?

• A high runoff number: see My runoff number is high. Should I flush?

• Drooping, and you're not sure it's the water at all: see How often should I water, and why is she drooping?

• Wondering whether she needs more food: see How can I tell when my plant wants more food?

Keep going

Watch the episode: Your pH Tools Aren't Lying, They're Measuring Different Things on MyRemy Live.

Watch the episode: Stop Chasing Runoff pH on MyRemy Live.

Test the water, set the pH for your mix, and let the plant call the next move. The whole system is written down in the free Stability Playbook. Get the free Stability Playbook

More answers: all grow guides. Every shelf guide and maker feed chart: Grower Resources.