"I've been growing tomatoes in the open field for years — in a greenhouse it's the same, just faster." First year in the greenhouse: blossom end rot on a third of the fruit, widespread grey mould in October, yields lower than outdoors. In the open field, rain, wind, and temperature swings absorb many mistakes. In a greenhouse, everything you don't control becomes a problem.
Quick Glossary
- VPD (Vapour Pressure Deficit) — the driving force of transpiration; in tomato and cucumber it determines calcium delivery to fruit; incorrect VPD causes blossom end rot even when EC is normal
- CO₂ — carbon dioxide; in a sealed greenhouse with good lighting, concentration drops below 300 ppm and becomes the limiting factor for photosynthesis
- DLI (Daily Light Integral) — cumulative daily photon dose; tomato and cucumber in fruiting require 25–35 mol/m²/day — more than most other crops
The Key Difference Between Greenhouse and Open Field: You Control Everything
In the open field there is rain — natural moisture and substrate flushing. There is wind — natural air circulation and humidity reduction after rain. There is a large temperature and light cycle the plant "knows." Mistakes are absorbed by this natural regulation.
In a greenhouse: no rain — irrigation and humidity are entirely your responsibility. No wind — ventilation and air movement are yours to manage. Temperature and humidity — yours alone. No fans installed means stagnant air. No CO₂ enrichment means concentration drops to 200 ppm at peak photosynthesis. No VPD monitoring means blossom end rot.
VPD and Tomato Blossom End Rot: The Correct Diagnosis
Blossom end rot on tomato fruit is a classic symptom that in 90% of cases is diagnosed as "calcium deficiency" — and more calcium is added to the solution. It often doesn't help.
The mechanism: calcium moves with the water stream from root to fruit through transpiration. With weak transpiration (low VPD), calcium does not reach the fast-growing fruit in sufficient quantity. The distal end of the fruit, which has the lowest water flow, receives the least calcium.
Diagnosis: if blossom end rot appears with normal EC and normal pH — measure VPD first. VPD below 0.6 kPa mid-day under good light means the cause is not calcium. Raise VPD by lowering humidity or increasing temperature — and recheck after one week. With correct VPD (0.8–1.2 kPa), calcium delivery normalises without changing EC.
CO₂: When It Actually Limits Tomato and Cucumber
Tomato and cucumber under good light (DLI 25+ mol/m²/day) actively consume CO₂ and in a closed system without ventilation can drop concentration to 200–300 ppm within 2–3 hours of lights on.
At 200 ppm, photosynthesis is significantly limited even with correct temperature and lighting. CO₂ enrichment to 800–1200 ppm in a closed system delivers a real yield increase of 15–25% — with a payback period within one season at commercial scale.
Testing the need is simple: a CO₂ sensor in the plant canopy at midday with vents closed. If below 350 ppm — enrichment is justified. If holding above 400 ppm — ventilation is sufficient and enrichment will have minimal effect.
Lighting: When Natural Light Is Not Enough
In winter in Ukraine, natural DLI inside a greenhouse is 4–8 mol/m²/day depending on region and cloud cover. Tomato in fruiting needs 25–35 mol/m²/day. The gap must be covered by supplemental lighting — otherwise winter yields are significantly lower and crop cycles extend.
Cucumber is less demanding on DLI than tomato: 18–25 mol/m²/day is sufficient for good yields. However, cucumber is more moisture-sensitive (optimal VPD 0.6–0.9 kPa) — below 60% humidity, leaf edges deform.
Training and Crop Management
Greenhouse tomato is grown as a single-stem cordon on a trellis with regular removal of side shoots. Without training — excessive vegetative mass, poor air circulation within the canopy, and a sharp decline in fruit quality.
Side shoot removal: every 5–7 days during active growth; remove shoots at 2–3 cm — larger shoots leave bigger wounds and increase infection risk. Do not remove shoots immediately before or after irrigation.
Cucumber: trellis crop with removal of lower lateral shoots and management of fruit load. No more than 4–6 fruits simultaneously on one plant during active setting.
Three Mistakes That Cost the Most
Treating blossom end rot with calcium without checking VPD. Raising calcium in the solution with normal EC and low VPD has no effect — and raises EC, compounding osmotic stress. VPD first, then decision.
Not monitoring CO₂ under good lighting. At DLI 25+ mol/m²/day in a closed greenhouse, CO₂ can drop to 200 ppm by midday — plants then photosynthesise well below their potential for the next 6 hours. A simple CO₂ sensor in the canopy — not guesswork.
No backup pump on drip irrigation. Primary pump failure at 28°C in summer with no irrigation for 4–6 hours causes critical water stress and mass fruit drop. A backup pump or UPS for the pump — insurance that pays back on the first incident.
Signs That the System Is Configured Correctly
- VPD 0.8–1.2 kPa for tomato and 0.6–0.9 kPa for cucumber throughout the active day
- CO₂ does not drop below 350 ppm in a closed system
- Blossom end rot absent or below 2% of fruit
- Consistent fruiting without sharp fluctuations week to week
- First and fifth truss deliver comparable fruit quality