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Most beginners are warned away from underwatering — but overwatering is the mistake that quietly kills more greenhouse crops than anything else, especially under glass where there is no rain to leach the buildup. Add high humidity with poor airflow and you have the exact conditions where grey mould, damping off, and root rots take hold.
This is the third beginner lesson in the GrowHaus quick-read series. It pairs naturally with the climate lessons already on the site — ventilation and heating — because watering and humidity are three sides of the same problem. Get any one wrong and the other two skew with it.
The rules below come from my own 10×14 greenhouse in a temperate zone, plus the same physics every greenhouse responds to: water evaporates, relative humidity rises, plants must transpire, and air must move. If you take nothing else away, take this: water less often than you think, check humidity with a real sensor, and open a vent before you water.
Need a refresher on airflow and how vents actually drop humidity? The ventilation guide walks through ridge vents, side vents, fans, and shade cloth, and the temperature thresholds that should drive when you crack a window open.
Greenhouse watering frequency
There is no universal "water every X days" rule — but there is a baseline grid that works for almost every beginner. The table assumes a closed polycarbonate greenhouse with mixed sun exposure; if you're running a glass greenhouse with full sun all day, expect to water closer to the hot-season end of each row.
| Plant stage | Spring (10–20°C) | Summer (20–32°C) | Autumn (5–15°C) | Winter (0–10°C) |
|---|---|---|---|---|
| Seedlings / cuttings (small cells, ≤7cm) | Daily light mist | Twice daily | Daily | Every 2–3 days |
| Young transplants (7–15cm pots) | Every 2 days | Daily | Every 3 days | Weekly |
| Established leafy crops (lettuce, herbs) | Every 2–3 days | Every 1–2 days | Every 3–4 days | Weekly |
| Fruiting vegetables (tomatoes, peppers, cucumbers) | Every 2 days | Daily, often twice daily in heatwaves | Every 2–3 days | Every 7–10 days — go easy |
| Container fruit trees (citrus, figs) | Every 3 days | Every 1–2 days | Every 4–5 days | Every 10–14 days |
| Seedlings in deep root trainers / soil blocks | Every 2 days | Daily | Every 3 days | Weekly |
The single biggest beginner error is watering on a fixed schedule (e.g. "tomatoes get watered every morning"). Plants don't read the schedule — humidity, sun, and stage all change the actual need. Always check the medium first, water deeply when you do water, and let it dry back enough that roots reach down for moisture.
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Humidity ranges for common crops
Different crops evolved in different climates — they want different humidity. Lettuce thrives in cool damp air; a tomato prefers it drier and won't set fruit above 90% RH. Grouping plants with similar humidity needs on one bench cuts your watering AND your ventilation work in half.
| Crop | Day RH | Night RH | Same-bench neighbours |
|---|---|---|---|
| Lettuce, Asian greens, spinach | 60–75% | 70–85% | Mâche, chard, rocket, parsley |
| Tomatoes | 60–70% | 50–65% | Peppers, aubergine, basil |
| Cucumbers | 70–80% | 75–90% | Courgettes, melons (separate from tomatoes) |
| Peppers (sweet, hot) | 60–70% | 55–70% | Tomatoes, aubergine, basil |
| Herbs (basil, coriander, dill) | 55–70% | 55–70% | Peppers, tomatoes |
| Strawberries | 60–70% | 65–75% | Lettuce, herbs (avoid wet leaves on hot days) |
| Seedlings / cuttings (propagation) | 80–95% | 85–95% | Keep them in their own dome until hardened off |
| Citrus in containers | 50–65% | 55–70% | Bay, figs, olives — Mediterranean bench |
Why night humidity matters more than day: spores of botrytis (grey mould) and other fungal diseases germinate on wet leaves and high humidity from dusk through dawn. Aim for a relative humidity drop of 10–15% overnight if you can — either by venting before sunset or by leaving a low extractor running. A $15 min/max hygrometer that records overnight highs is the simplest way to actually see the problem.
Ventilation tips that fix humidity
Ventilation is the cheapest humidity control you have. Heaters and dehumidifiers are loud, expensive, and need electricity. Open vents use free physics — air exchange drops RH within minutes. The four habits below are what I do every single day the greenhouse is closed:
Even on overcast days, interior humidity climbs fast once plants start transpiring. Crack a roof vent or side vent before you water. The earlier you open it, the less humidity builds under glass.
A single roof vent on a 90°F afternoon won't drop humidity enough. Open one vent low on one side and another high on the opposite side — cool dry air enters low, warm humid air escapes high. This is what keeps RH in the 60s.
Close vents 1–2 hours before sunset so the greenhouse traps some heat, but only after humidity has dropped. Closing too early traps the daytime humidity overnight, which is exactly when fungal disease spreads.
Watering late in the day is the single most common cause of disease in a beginner's greenhouse. Wet leaves + cool air + still conditions at night = botrytis and damping off by morning. Always water before lunch.
A bigger temperature gradient means stronger natural convection, which moves humid air up and out instead of letting it pool around plant leaves. If roof and floor read the same temperature, your ventilation setup is too gentle.
Stuck with a closed greenhouse all day? A small 4-inch USB or 12V fan running on a timer during the heat of the day is enough to keep humidity from spiking in a 6×8 kit. The full ventilation guide covers passive ridge vents, side vents, exhaust fans, and shade cloth — pick the method that matches your greenhouse type and budget.
Dig deeper with the full guides
This quick read is part of the Climate group. The full ventilation and greenhouse-setup guides go deeper on airflow, sizing, and humidity control for your specific kit.
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Common Questions
Related guides
Greenhouse Ventilation Guide
8 ways to keep your greenhouse cool and dry — passive ridge vents, side vents, exhaust fans, shade cloth, and more. Pairs directly with humidity control.
🌱Best Greenhouse Plants for Beginners
12 beginner-proof crops grouped by humidity and water needs so you can build benches that share the same climate.
⚠️Avoiding Beginner Mistakes
Overwatering, overcrowding, forgetting airflow — the 6 mistakes that wipe out a first crop and how to avoid each one.
🏗️Setting Up Your Greenhouse
10 steps from delivery day to first plants — placement for airflow, base prep for drainage, and where to put vents for cross-draught.