The Logbook: Six Numbers, Two Minutes a Day — and One Real Logged Week, Annotated

A cannabis grow journal is not a diary of plant photos and hopeful notes. It is the record that tells you what changed before the leaves changed. In a compact cabinet, a small shift in light height, reservoir temperature, fan speed, or nutrient strength shows up fast — and if you didn't write the input down, you are left guessing at the cause.

The journal doesn't need to be elaborate. It needs six numbers, taken the same way at the same time, and two minutes a day. An honesty note: we don't yet have a run of our own logged on the record, so the real logged week later on this page comes from a public grower journal, credited and linked. Nothing below is presented as ours.

The Six Numbers, in Order

These are the same six variables our cabinet numbers guide opens with, in the same order, so every page in the season speaks one language:

1. Light — fixture setting and hang height every day; a PPFD spot-check at the canopy once a week.
2. Air temperature — at canopy height, from a sensor placed where the number means something.
3. Humidity — read together with temperature against the VPD chart.
4. Reservoir temperature — the root zone's daily stand-in; a dissolved-oxygen reading if you own a pen.
5. EC — against the one feeding chart.
6. pH — from a calibrated meter, held in the 5.5–6.2 band.

The targets live on those pages. The journal's job is different: it records where you actually were, so the targets can be compared against reality.

What the Extension Services Actually Recommend

Hydroponic record-keeping isn't a cannabis invention, and the people who teach commercial growers are consistent about it. Oklahoma State's extension guide says to check pH and EC daily, at the same time of day, adjusting EC first and pH second. Cornell's hydroponics specialists recommend that deep-water growers check EC and pH every day, because the solution changes little hour to hour but meaningfully week to week. Missouri's beginner guide says pH and EC must be tested and recorded daily.

Two other points from extension sources are worth stealing. Purdue's guide argues that the value of testing is the trend — graph the readings over time instead of reacting to single numbers. Auburn's crop-records guidance suggests a weekly photo of a representative plant, taken from a fixed distance or with a ruler in frame, so growth is comparable week to week.

To be straight about the evidence: we found no study that tests whether daily logging by itself catches problems earlier. What cannabis research does show is how much these numbers move — which is the case for writing them down:

• In a 2024 comparison of fertigation systems, a recirculating solution's pH drifted from 5.9 to about 6.95 within each roughly week-long cycle between solution changes.
• In a 2025 study, plants kept at canopy humidity of 78–98% produced about 9.9 g of dry flower each, versus 33.8 g at 37–58% — and the researchers caught it with an hourly data logger.
• Utah State's hydroponic cannabis trials measured EC and pH before every daily refill. Research-grade growing is logged growing.

The Two-Minute Routine

One row per day, same point in the light cycle, after the cabinet has settled — not right after you open the door.

Write measurements, not interpretations. "Canopy 80 °F" is a measurement; "too hot" is a guess that may be wrong at a different humidity. And when you change something, log the exact change and the reason: "EC raised from 1.4 to 1.6 mS/cm after three days of falling EC and pale new growth" is a case file. "Fed more" is not.

One Real Week, Annotated

The best public example we found is a home grower's journal on the Growers Network forum: ZenMaster's first hydroponic run, a recirculating eight-bucket DWC system under T5 lights, with dated entries in June 2020. The numbers below are theirs, summarized; the annotations are ours. (Their TDS readings are ppm on their own meter; the scale isn't stated, which is exactly why we log EC.)

• June 7 — fresh reservoir mixed: TDS 498 ppm, pH 5.48.
  A low starting pH, just under the 5.5 floor. Logging it means the next reading has context.
• June 9 — pH had climbed to 7.4 after a day and more of recirculation; corrected to 5.75 by adding a gallon of pH-adjusted water. TDS 472 ppm, air 81 °F, humidity under 50% until a humidifier lifted it to 65%.
  A two-point jump in a day is too fast to be normal uptake drift. Because it was logged, the grower knew the size of the swing instead of discovering it later as yellowing leaves.
• June 10 — inside the tent 82 °F and 65% RH with lights on; outside the tent 78 °F and 50%. Only one of the two light sets was running, because running both pushed the tent to 88 °F.
  Here the log made a hardware decision for the grower: the cabinet could not shed the heat of both fixtures. That is a light-and-airflow problem, exactly the interaction our heat stress guide is built around.
• Over the following days — TDS rose from 472 toward 707 ppm with no nutrients added, which the grower put down to evaporation; the humidifier ran dry and RH fell to 56%.
  Rising concentration with falling water level means the plants were drinking water faster than they were taking up nutrients. The fix is a plain-water top-off, not more nutrients — and only a log shows the two trends side by side.

A second public journal, Tygrow78's DWC grow, shows the healthy version of drift: on January 19, 2020, pH started at 5.5, drifted to 6.1 within 36 hours, then held between 5.8 and 6.1. A slow climb inside the band is the plant feeding; that is information, not an emergency.

Paper, Spreadsheet, or App

The format matters less than consistency. A spreadsheet graphs EC, pH, temperature, and humidity for you; a notebook kept beside the cabinet gets filled in because it's there; an app works if you actually open it. Keep measurements, actions, and observations in separate columns so a critical change never hides inside a long note. If you'd rather see the daily check demonstrated, the hydroponics retailer Hydro Select has a short, Creative Commons–licensed lesson on logging canopy temperature with a min/max thermo-hygrometer — the min/max reading shows what happened while you weren't in the room.

The Weekly Review

Once a week, read the last seven rows and mark any change in direction: humidity creeping up at night, reservoir level falling faster, pH swings widening, the canopy getting closer to the light. Then compare those shifts against the actions you logged. That is where the journal earns its space — and where most troubleshooting problems turn out to have started three days earlier, in a change you made or a reading you skipped. Pair it with the grow checklist for the things that don't fit in a row.

Six numbers, two minutes, every day. The next time a leaf tells you something is wrong, start with the last three rows of the log, not the leaf.