"I grow all year round and change nothing." December: natural sunlight gives a DLI of 3-4 mol/m²/day against the 14-17 that lettuce needs. The cycle stretches from 28 to 45 days. In an unheated greenhouse in Kharkiv, February nights drop to 14°F (-10°C). Some regions lose power for 8 hours every day, and a system without a UPS simply stops. Winter in Ukraine changes the agronomy itself, well beyond a lower temperature: the conditions differ, and every parameter needs a different approach at the same time.
Quick glossary: DLI (Daily Light Integral) is the daily dose of photons a plant receives; in a Ukrainian winter natural DLI is 2-5 mol/m²/day, against a requirement of 14-25 depending on the crop, and up to 35 for fruiting tomato. Photoperiod is the length of the light period; for some crops it is critical for preventing bolting or triggering flowering, whatever the DLI. A UPS (uninterruptible power supply) is a device that keeps equipment powered when the grid goes down; for production on an unstable power supply it is a critical piece of infrastructure.
Natural DLI in Winter: Where Your Starting Point Really Is
The most important thing to understand is that in winter a greenhouse plant receives 4-7 times fewer photons from natural sunlight than in summer. And that is before any clouds.
Natural DLI in Ukraine by region and month:
- December and January: 2-4 mol/m²/day (Kharkiv, Kyiv), 3-5 (Odesa, Kherson)
- February: 4-7 mol/m²/day
- November and March: 6-10 mol/m²/day
The minimum DLI for leaf lettuce is 12-14 mol/m²/day. At a natural DLI of 3 mol/m²/day, a plant grows twice as slowly as in summer even with correct EC, pH and temperature. A 28-day cycle becomes 45-55 days with every condition the same except the light.
What this means in practice: either supplemental lighting that tops the light up to the crop's minimum DLI, or a production plan built around the longer cycle, or a smaller planned volume with more time per cycle. "Changing nothing" is itself a choice: a longer cycle for the same result.
Supplemental Lighting in Winter: Working Out What You Need
Winter supplemental lighting is worked out from the gap between target and natural DLI: with a target of 15 mol/m²/day and a natural DLI of 3 mol/m²/day, the lamps have to supply the missing 12 mol/m²/day, which over 16 hours works out to a PPFD of about 208 µmol/m²/s and a real load on the electricity bill. The DLI and PPFD calculator works this out for any outdoor DLI and glazing loss.
12 mol/m²/day over 16 hours of supplemental lighting = PPFD ≈ 208 µmol/m²/s.
With lamps at a PPE of 2.5 µmol/J, that takes about 83 W/m². Over 20 m² it comes to 1.66 kW for supplemental lighting alone, which adds substantially to the electricity bill.
Ways to cut the cost:
- Lower the target DLI to the minimum that is still enough (12 instead of 17 for lettuce)
- Use lamps with the highest PPE (LEDs at 2.8-3.2 µmol/J)
- Grow more shade-tolerant crops in winter
- On a small scale, start with supplemental lighting only for the most demanding crops
Temperature: Where the Real Risk Is and What Protects You
Greenhouses: with 5 to -4°F (-15 to -20°C) outside and weak heating, the temperature inside can fall to 41-46°F (5-8°C) at night. At 41°F (5°C) the root stops actively taking up nutrients even at correct EC. Chilling injury below 50°F (10°C) causes irreversible tissue damage in warm-season crops such as tomato.
The winter priority: temperature and humidity come first. The night minimum for leafy crops is 59°F (15°C), and 64°F (18°C) for basil and other warmth-loving crops. If you cannot hold that minimum, either insulate the cladding further or switch to more cold-hardy crops (spinach and garden cress tolerate 50-54°F / 10-12°C).
Solution temperature: in a cold room, the solution in the reservoir cools along with the air. Below 59°F (15°C), uptake of calcium and phosphorus drops substantially. An aquarium or industrial heater in the reservoir is the cheaper way to keep the solution temperature up in a cold room.
Power Cuts: A Systemic Problem That Needs a Systemic Solution
In many regions of Ukraine, power cuts range from a few hours a week to 8-12 hours a day in the peak winter months. For production that depends on pumps, lighting and equipment, this is a systemic risk.
What stops during a power cut with no backup:
- Pumps in NFT and recirculating systems: roots dry out within 15-30 minutes in hot weather and within 1-2 hours in winter
- Lighting: the day's planned DLI is not reached
- Heating: the temperature falls within 1-3 hours depending on insulation
- Automation and timers: their settings are lost when power comes back
Backup power by priority:
- Pumps (critical priority). NFT and recirculating systems stop without their pumps, and a UPS on the system pumps keeps them running through a cut. Pumps usually draw 20-100 W. Runtime is set by the battery capacity, and the VA rating of the UPS says nothing about it: 2-6 hours at that load needs roughly 50-700 Wh, which means a UPS with an external battery or a power station. A typical office UPS rated 500-1000 VA with a built-in 7-9 Ah battery holds 100 W for only 30-45 minutes.
- Heating (high priority in winter). A generator or a high-capacity UPS to hold the minimum temperature.
- Lighting (medium priority). Putting supplemental lighting on backup power is expensive because of the high load. The alternative is to plan the lighting schedule around the cuts and move lighting hours to the times when there is power.
A practical approach on an unstable supply:
- Build published outage schedules, where they exist, into the lighting timer
- Do not plan critical operations such as harvesting and packing for expected outage hours
- A UPS on the pumps at the very least: a fairly affordable measure that protects against the fastest losses
Three Mistakes That Cost the Most
Not adjusting the production plan for winter and expecting the same output in the same time. At a natural DLI of 3 mol/m²/day with no supplemental lighting, the cycle gets longer. Plan a smaller volume over a longer cycle, or budget for supplemental lighting to hold the volume. "Changing nothing" while keeping the same sales plan ends in a product shortfall or missed deliveries.
Having no backup at all for the pumps on an unstable power supply. NFT without a pump for 30 minutes on a hot day, or 2 hours in winter, means plants dying from drying out. A UPS for the pump is one of the cheapest forms of insurance relative to the value it protects.
Not checking solution temperature in a cold room in winter. "EC and pH are normal" at a solution temperature of 54°F (12°C) still means minimal uptake, and the plant starves however correct the EC is. A thermometer in the reservoir is a mandatory part of winter monitoring.
How to Tell the Winter Regime Is Set Up Right
The winter cycle runs no more than 20-25% longer than the summer one for comparable crops, a sign that supplemental lighting and temperature are held at a sufficient level. Solution temperature does not fall below 61°F (16°C) even on the coldest nights. During a power cut, the system pumps keep running for at least 2-3 hours.
If you want to go deeper: Backup power: UPS units, generators and priorities for production explains how to calculate the backup capacity your specific equipment needs, and what to connect first when the budget for backup is limited.