Introduction
In December, natural sunlight delivers just 3–4 mol/m²/day, while leafy greens need 15–17. Growth cycles stretch from 28 to 45 days. Unheated greenhouses in Kharkiv drop to −10°C on February nights. Power outages lasting 8 hours a day in some regions bring systems without backup power to a complete stop.
Key terms:
- DLI (Daily Light Integral): the daily photon dose; Ukraine's natural winter DLI is 2–5 mol/m²/day versus the 14–25 mol/m²/day crops actually need.
- Photoperiod: the duration of the light period; critical for some crops to prevent bolting or flowering regardless of DLI.
- UPS (Uninterruptible Power Supply): keeps systems running during outages — critical infrastructure where the grid is unstable.
Natural Winter DLI: Understanding the Baseline
Natural DLI by region and month in Ukraine:
- December / January: 2–4 mol/m²/day (Kharkiv, Kyiv) — 3–5 mol/m²/day (Odesa, Kherson)
- February: 4–7 mol/m²/day
- November / March: 6–10 mol/m²/day
Minimum DLI for leafy lettuce: 12–14 mol/m²/day. At a natural 3 mol/m²/day — even with correct EC, pH, and temperature — plants grow at roughly half the normal rate.
Winter Supplemental Lighting: Calculating What You Need
To reach a target DLI of 15 mol/m²/day when natural light provides only 3 mol/m²/day, you need 12 mol/m²/day from your fixtures:
12 mol/m²/day over a 16-hour supplemental period = PPFD ≈ 208 µmol/m²/s.
At a fixture efficiency of 2.5 µmol/J, that requires approximately 83 W/m². For a 20 m² area: 1.66 kW for lighting alone.
Strategies to reduce energy costs:
- Lower the target DLI to the minimum viable level (12 instead of 17 for lettuce)
- Use fixtures with the highest PPE rating (LEDs at 2.8–3.2 µmol/J)
- Grow shade-tolerant crops in winter
Temperature: Real Risks and How to Protect Against Them
Greenhouses: at −15 to −20°C outside with poor heating, interior temperatures can fall to 5–8°C overnight. At 5°C, roots stop actively absorbing nutrients regardless of correct EC.
Winter priority: temperature and humidity come first. Minimum night temperatures for leafy crops: 15°C. Basil and heat-loving varieties: 18°C. Cold-tolerant varieties (spinach, watercress) tolerate 10–12°C.
Solution temperature: cold spaces pull reservoir temperature down along with air temperature. Below 15°C, calcium and phosphorus uptake drops significantly.
Power Outages: A Systemic Problem Requiring Systemic Solutions
What stops without backup power:
- NFT and recirculation pumps: roots dry out in 15–30 minutes in summer heat, 1–2 hours in winter
- Lighting: daily DLI targets are not met
- Heating: temperature drops within 1–3 hours depending on insulation
- Automation and timers: reset when power is restored
Backup power by priority:
1. Pumps (critical): NFT and recirculation systems cannot run without them. A UPS on system pumps (500–1,000 VA) provides 2–6 hours of operation.
2. Heating (high priority in winter): a generator or high-capacity UPS maintains minimum temperature.
3. Lighting (medium priority): connecting supplemental lighting to backup power is expensive. Alternative: schedule lighting windows around known outage periods.
Three Mistakes That Cost the Most
Not adjusting winter production plans and expecting the same volumes on the same timelines. At 3 mol/m²/day of natural DLI with no supplemental lighting, cycles will be longer — plan for it.
Having zero pump backup on an unstable grid. NFT without pumps: 30 minutes in summer or 2 hours in winter means dead plants.
Not monitoring solution temperature in cold spaces during winter. "EC and pH are normal" at a solution temperature of 12°C means minimal uptake — plants are starving despite correct EC.
How to Know Your Winter Setup Is Working
Winter cycles run no more than 20–25% longer than summer cycles for comparable crops. Solution temperature never drops below 16°C even on the coldest nights. When outages occur, system pumps continue running for at least 2–3 hours.