Growing tomatoes in a greenhouse works best with daytime temperatures between 70–82°F, nighttime lows at 62–64°F, and 60–70% humidity with consistent pruning and watering.
A greenhouse gives you control over the environment for reliable fruit set. But that control cuts both ways — the same glass that traps warmth can cook pollen and breed fungus if you miss the narrow ranges tomatoes demand. The difference between a full harvest and flowers that never set fruit comes down to a handful of daily dials.
Temperature, Humidity, and the Pollination Window
Tomato flowers fail when daytime temps climb above 85–90°F or nighttime lows stay above 70°F. Pollen becomes sterile above about 90°F. Mississippi State University gives the optimal day range as 70–82°F, with nights between 62–64°F. A natural night drop lets the plant rest and supports fruit development. Humidity matters almost as much: optimal relative humidity is 60–70%. Below that, pollen dries out; above it, fungal diseases find a foothold. Ridge vents, side vents, and a circulation fan on a thermostat keep both in the useful band. If you are setting up a new greenhouse or upgrading, our tested roundup of the best greenhouses for tomatoes will help you pick a structure that vents well.
Spacing, Support, and Pruning Rhythm
Cramped tomatoes produce more leaves than fruit. Alabama Extension recommends 3.5 to 5 square feet per plant, with about 16 inches between plants in the row and 5 feet between rows. Mississippi State describes double rows roughly 4 feet apart, making efficient use of floor space while keeping the canopy open. Indeterminate varieties need a trellis, stake, or string system. Prune at least once a week, removing small suckers by pinching for a clean snap; woody suckers leave wounds that invite infection. Strip the oldest leaves from the bottom 12–18 inches of the stem as the plant grows to reduce shading and humidity near the soil.
Watering, Media Chemistry, and the Numbers
Greenhouse tomatoes are grown in a soilless mix. The media needs even moisture without saturation — overwatering pushes out oxygen and invites root diseases. Alabama Extension says to irrigate enough to get 10–25% leachate (water draining out the bottom) during daytime. Night watering invites pythium and root rots. Maintain pH between 5.5 and 6.5 and electrical conductivity (EC) at 2.0–2.5 mS/cm. Test runoff weekly and adjust feed strength to keep targets; high EC burns root tips, low EC produces pale growth and poor fruit set.
Four Mistakes That Kill a Greenhouse Tomato Crop
- High heat without ventilation. Above 90°F, pollen dies and flowers drop. Open vents and run fans before hitting that ceiling.
- Overwatering. Soilless media holds less oxygen; saturation drowns roots and feeds fusarium and pythium. Stick to daytime watering with a leachate check.
- Skimping on pruning. Every sucker left produces a thick canopy that blocks light and spreads disease. One pass per week keeps the plant open.
- Ignoring humidity. Above 70% RH, fungal spores germinate; below 50%, pollen won’t stick. Keep a hygrometer and ventilate in response.
| Condition | Target Range | Why It Matters |
|---|---|---|
| Day temperature | 70–82°F | Photosynthesis and flower viability peak here |
| Night temperature | 62–64°F | Rest period supports fruit set; above 70°F causes flower drop |
| Relative humidity | 60–70% | Pollen sticks; fungal spores stay dormant |
| Media pH | 5.5–6.5 | Nutrients stay available; roots absorb properly |
| Media EC | 2.0–2.5 mS/cm | Balanced salt level; not too strong or too weak |
| Plant spacing | 3.5–5 sq ft per plant | Adequate airflow; lower disease pressure |
| Irrigation leachate | 10–25% | Flushes excess salts; confirms media is wet through |
FAQs
Why are my greenhouse tomatoes flowering but not setting fruit?
High daytime heat above 90°F sterilizes pollen, and humidity above 70% makes it stick too well to anthers. Improve ventilation to stay in the 70–82°F day range and 60–70% humidity. If nights stay above 70°F, open vents earlier in the evening.
Do I need to pollinate greenhouse tomatoes by hand?
Air movement from a circulation fan is usually enough. If fruit set is poor despite good conditions, gently shake plants at midday when humidity is in the 60–70% sweet spot. Bumblebee hives are used commercially but are rarely necessary for home-scale production.
Can I grow tomatoes in a greenhouse during winter?
Only if you can heat to keep nighttime lows above 60°F and provide supplemental lighting for at least 12–14 hours daily. Without heat and light, growth stalls, flowers drop, and fruit won’t ripen. Most home growers without a heated, lit greenhouse wait until early spring.
References & Sources
- Alabama Cooperative Extension System. “Greenhouse Tomato Production.” Provides targets for temperature, spacing, media pH, EC, irrigation, and pruning.
- Mississippi State University Extension. “Greenhouse Tomato Handbook.” Specifies optimum day and night temperatures and relative humidity.