Houseplant Watering Computer

"Water weekly" is why houseplants die. The right question is: how fast does THIS pot dry out? — a function of substrate water-holding capacity, pot volume, plant size, light, temperature, and humidity. This skill computes the soil water balance per pot and turns it into an actionable answer: how much water, how often, and when to check next.

When to Use

• New plant, no idea how often to water it
• Repeated plant deaths (root rot = overwatering is the #1 killer)
• Changing seasons: the winter schedule isn't the summer schedule
• Repotting: substrate change changes everything
• Setting up self-watering pots / cache pots correctly
• Symptoms: yellowing, wilting despite wet soil, fungus gnats

Don't use for: hydroponics (different physics), outdoor gardens (rain dominates — use ET-based scheduling), lawns.

The Method

1. Inventory each pot: substrate mix, pot dimensions (top/bottom diameter + height), plant size class, light level, room temp/humidity.
2. compute — per-pot physics:
   • Substrate WHC (water-holding capacity) L per L of soil from the mix
   • Pot volume from truncated-cone geometry (tapered pots) or cylinder
   • Plant water use from leaf area proxy + light + VPD (vapor pressure deficit from temp/RH)
   • → Dryout time: days from saturation to the plant's wilt/watering point, with the usable reservoir in mL and per-watering dose in mL
3. check — interpret what you observe (finger test depths, moisture meter readings, weight) into wet/dry verdicts and next-check timing.
4. symptoms — route visual symptoms to over- vs under-watering and other causes; fungus gnat lifecycle breaks.
5. selfwatering — reservoir sizing and fill cadence for wick/cache pots; when NOT to use them (mines: always-wet substrate + rot-prone species).

Quick Start

# Monstera in a 24cm tapered pot, peat-based mix, bright indirect, 22C/45%RH
python3 scripts/watering_computer.py compute --substrate peat-mix --pot-tapered 24,18,22 \
    --plant large-foliage --light bright-indirect --temp 22 --humidity 45

# Check what I'm feeling in the pot right now
python3 scripts/watering_computer.py check --finger-depth shallow --feels dry --pot-tapered 24,18,22

# Diagnose: yellow lower leaves + fungus gnats + soil stays wet 10+ days
python3 scripts/watering_computer.py symptoms --yellowing lower --soil wet --gnats

# Self-watering pot reservoir sizing
python3 scripts/watering_computer.py selfwatering --pot-tapered 24,18,22 --substrate peat-mix --away-days 21

Stdlib-only Python 3. Offline.

Substrate Science (what drives everything)

The watering INTERVAL the plant experiences is substrate × pot size × demand — never a calendar. A peat mix in a 12 cm pot dries in ~3 days in summer; the same plant in a 24 cm pot takes 7–9.

Environment Multipliers (VPD)

Demand scales with warmth and dryness of air. The script computes vapor pressure deficit from temp + RH and multiplies plant water use:

• 18 °C / 70% RH → ×0.6 (winter, slow everything)
• 22 °C / 45% RH → ×1.0 (baseline)
• 26 °C / 30% RH → ×1.6 (heatwave, AC, radiator proximity)

Winter heating is the hidden killer: soil stays cold and wet (slow uptake) while air is dry — check less, water less, don't fertilize.

Common Pitfalls

1. Calendar watering. The #1 killer. Moisture-check before every watering for the first month with any new pot; the calendar emerges from the physics, per pot.
2. Watering on a schedule that ignores pot size. Same plant, doubled pot volume → interval nearly doubles. Small pots dry in days; big pots hold a hidden reservoir for a week+.
3. "Drainage fixes overwatering." Drainage holes move water OUT after saturation; they do nothing for a substrate kept continuously wet by too-frequent watering.
4. Even soak vs splash. A 200 mL splash on a 6 L pot wets the top 2 cm, salts accumulate in the root zone, and the bottom roots stay dry. Water to ~20–25% of substrate volume, until slight runoff.
5. Cold tap water on tropicals. Let it reach room temperature; some species are sensitive to chlorine — overnight resting resolves both.
6. Repotting into "bigger is better." Too large a pot = wet soil the roots never reach = rot. Go up ≤ 5 cm in diameter.
7. Self-watering pots as a cure for underwatering. They fix interval management, not light/substrate problems; rot-prone species need a timer-free setup or a very airy mix.

Verification Checklist

• Pot measured (top, bottom diameters, height) — tapered ≠ cylinder
• Substrate identified honestly (what's actually in the pot)
• Light level honestly assessed (what hits the plant, not the window)
• compute output dose is ~15–25% of substrate volume
• Finger/weight check performed before acting on any interval > 4 days
• Winter: interval stretched, fertilizer stopped

References

• references/water-balance.md — the physics: WHC, pot geometry math, VPD tables, plant demand classes, dryout-time derivation
• references/symptom-router.md — full visual symptom routing table, fungus gnat protocol, self-watering/cache-pot engineering