"I grew tomatoes in the open field for years, so a greenhouse is the same thing, only faster." First year under cover: blossom end rot on a third of the fruit, a widespread gray bloom across the crop in October, and a yield lower than outdoors. In the open field, rain, wind and temperature swings absorb part of your mistakes. In a greenhouse there is no rain or wind: watering, ventilation, temperature and humidity are yours alone, and everything you do not control turns into a problem.
Quick glossary: VPD (vapor pressure deficit) is the shortfall in vapor pressure; in tomato and cucumber it drives transpiration and with it the delivery of calcium to the fruit, so the wrong VPD gives blossom end rot even at correct EC. CO₂ is carbon dioxide; under cover with good lighting its concentration falls below 300 ppm and becomes the limiting factor for photosynthesis. DLI (Daily Light Integral) is the daily dose of photons; tomato and cucumber in fruiting need a DLI of 25-35 mol/m²/day, more than most other crops.
VPD and Blossom End Rot in Tomato: Getting the Diagnosis Right
Blossom end rot on tomato fruit is the classic symptom that in 90% of cases gets diagnosed as "calcium deficiency", after which growers start raising calcium in the solution. It often does not help.
The mechanism: calcium travels with the water stream from the root to the fruit, carried by transpiration. When transpiration is weak, meaning VPD is low, not enough calcium reaches a fast-growing fruit. The bottom of the fruit has the weakest water flow, so it receives the least calcium of all.
Diagnosis: if blossom end rot appears at normal EC and normal pH, measure VPD first. A VPD below 0.6 kPa in the middle of the day under good light means calcium is not the cause. Raise VPD by lowering humidity or raising temperature, then check again a week later. At a correct VPD of 0.8-1.2 kPa, calcium delivery returns to normal with no change to EC.
CO₂: When It Really Limits Tomato and Cucumber
Under good light (DLI of 25 mol/m²/day and above), tomato and cucumber consume CO₂ fast, and in a closed loop without ventilation they pull the concentration down to 200-300 ppm within 2-3 hours of lights on.
At 200 ppm, photosynthesis is substantially limited even at correct temperature and light levels. CO₂ enrichment to 800-1200 ppm in a closed loop delivers a real yield increase of 15-25%, at a cost that pays for itself within a season in commercial production.
Check with a CO₂ sensor in the plant zone at midday with the vents closed: below 350 ppm, enrichment is justified; above 400 ppm, ventilation is coping and the effect will be minimal.
Lighting: When Natural Light Is Not Enough
In a Ukrainian winter, natural sunlight in a greenhouse gives a DLI of 4-8 mol/m²/day depending on region and cloud cover. Tomato in fruiting needs 25-35 mol/m²/day. Either you close that gap with supplemental lighting, or winter yields are considerably lower and the cycle stretches out.
With supplemental lighting, adding 15-20 mol/m²/day from lamps against a target DLI of 25 and above means a PPFD of 260-350 µmol/m²/s over a 16-hour lighting period. That takes about 105-140 W/m² with LEDs at a PPE of 2.5 µmol/J, and 150-230 W/m² with HPS lamps at a PPE of 1.5-1.7. The full cold-season picture is in winter growing in Ukraine.
Cucumber is less demanding on DLI than tomato: 18-25 mol/m²/day is enough for a good crop. Cucumber does want more moisture, though (its VPD optimum is 0.6-0.9 kPa), and below 60% humidity the leaf edges deform.
How to Train and Pollinate Tomato and Cucumber in a Greenhouse
Tomato is trained to a single stem on a string and pollinated with a vibrator or by bumblebees, because a greenhouse has no wind. Greenhouse cucumber is the opposite: parthenocarpic varieties set fruit without pollination, and insects harm it.
Side-shooting: every 5-7 days during active growth, removing shoots at 2-3 cm. Larger shoots leave a larger wound and a higher risk of infection. On drip irrigation, do not side-shoot right before or after watering: an open wound in a humid environment lets bacteria and fungi in.
Cucumber is a trellised crop: remove the lower lateral shoots and manage the fruit load. Keep no more than 4-6 fruits ripening on a plant at once.
A small greenhouse is pollinated with an electric vibrator: hold it for a few seconds against the stem of a truss with open flowers, not against the flower. From the first flower, repeat every other day at midday: above 80% humidity the pollen clumps.
Bumblebees (most often Bombus terrestris) pay off from 10,000 sq ft (930 m²) under one roof, while even the smallest hive over-pollinates a small greenhouse. The rate is 7-15 hives per hectare, or one hive per 7,200-15,400 sq ft (670-1,430 m²), placed in shade under the foliage, for 6-12 weeks. Check 20 flowers: fewer than 1-2 brown bite marks per flower means a new hive is due; too many, and flowers drop.
Greenhouse cucumber is mostly parthenocarpic: the fruit sets without pollination and without seeds. Once pollinated, it develops seeds, becomes misshapen, and ends up a cull. So keep insects away from cucumber and do not plant bee-pollinated varieties next to it: in a greenhouse shared with tomato, bumblebees will carry their pollen.
Three Mistakes That Cost the Most
Treating blossom end rot with calcium without checking VPD. Raising calcium in the solution at normal EC and low VPD gives no result and raises EC, which deepens osmotic stress. VPD first, then the decision. Temperature and humidity in the context of VPD are covered in their own article.
Not watching CO₂ under good lighting. At a DLI of 25 mol/m²/day and above in a closed greenhouse, CO₂ can fall to 200 ppm before midday, and for the next 6 hours photosynthesis runs below its potential. A simple CO₂ sensor in the plant zone, and decisions made on the data rather than on feel.
Not fitting a backup pump on drip irrigation. Drip is the standard for tomato and cucumber. If the main pump fails at 82°F (28°C) in a summer greenhouse and irrigation stops for 4-6 hours, you get critical water stress and a mass drop of fruit set. A spare pump or a UPS is insurance that pays for itself on the first incident.
How to Tell the System Is Set Up Right
VPD stays at 0.8-1.2 kPa for tomato and 0.6-0.9 kPa for cucumber during the day. CO₂ does not fall below 350 ppm in a closed loop. Blossom end rot is absent, or affects under 2% of fruit. Fruiting is even, with no sharp swings from week to week. The first and the fifth truss carry fruit of comparable quality, a sign that conditions held steady all cycle.
If you want to go deeper: VPD: vapor pressure deficit and why a plant "does not drink" at normal humidity explains why VPD is the central parameter for fruiting crops, and how it ties transpiration, calcium delivery and fruit quality into a single mechanism.