The same irrigation timer, the same EC 2.0, the same 16 hours of light — all year round. In February the plants look sluggish and the cycle drags. In August leaf margins are burning and roots are browning. Not because the parameters are wrong — because they are right for one season and wrong for another. Heat, light, humidity, and plant activity shift between summer and winter by a factor of several times — and the optimal parameters shift with them.
Quick glossary: Seasonal adjustment — planned correction of EC, pH, lighting schedule, irrigation, and temperature based on seasonal changes in the external environment and plant activity. EC (Electrical Conductivity) — nutrient solution concentration; the optimal level depends on temperature and transpiration — in summer at higher transpiration the EC optimum is lower, in winter at lower activity it is higher. DLI (Daily Light Integral) — the daily photon dose; in winter the natural DLI in Ukraine is 2–5 mol/m²/day, in summer 20–40 mol/m²/day.
Why Seasonal Adjustment Is Not Fine-Tuning — It Is a Basic Requirement
The difference in conditions between December and June in Ukraine for a greenhouse or grow room producer:
| Parameter | December | June | Difference |
|---|---|---|---|
| Natural DLI | 3–4 mol/m²/day | 30–40 mol/m²/day | 8–10× |
| Outdoor temperature | −5 to −15°C | 25–35°C | 40°C |
| Natural day length | 8–9 hours | 15–16 hours | 7 hours |
| Plant transpiration | 30–50% of summer | maximum | — |
In December a plant absorbs roughly half the water and nutrients it does in June under the same system conditions — because transpiration is low and activity is constrained by a short day. The same EC 2.0 that is normal during active summer growth is too concentrated at low winter transpiration and accumulates in the substrate.
EC and pH: Seasonal Corrections
Summer (June–August):
- High transpiration: the plant absorbs more water relative to nutrients → substrate EC may rise. A small reduction in feed EC of 0.2–0.3 mS/cm below standard is often justified.
- Higher solution temperature lowers DO → increased Root Rot risk → solution temperature control is the priority.
- pH is more stable in warm solution but can shift faster under heavy transpiration — more frequent monitoring.
- Recommended EC for leafy crops: 1.2–1.6 mS/cm in summer (lower end of the range).
Winter (December–February):
- Low transpiration: the plant absorbs little water → risk of salt accumulation in the substrate at unchanged EC. Reduce the drain percentage or increase irrigation frequency to flush.
- Low root activity in cold solution → even a correct EC is not absorbed efficiently. Solution temperature of 18–20°C is the priority.
- EC can be raised 0.2–0.3 mS/cm above standard — a more concentrated solution at lower uptake — but only if drain EC remains within normal range.
- Recommended EC for leafy crops: 1.6–2.0 mS/cm in winter (upper end of the range).
pH seasonally: pH is adjusted not by season but when source water changes. In many regions winter and summer water differ — especially when rain or snowmelt dilutes alkalinity. Check source water EC and pH at each seasonal transition.
Lighting: What to Adapt and Where
Greenhouse in winter: natural DLI of 3–4 mol/m²/day. Without supplemental lighting, the cycle doubles in length. With supplemental lighting — calculate the requirement based on the gap between natural and target DLI. Run supplemental lighting through the entire dark period plus supplement to target PPFD during daylight hours.
Greenhouse in summer: natural DLI can exceed the crop optimum (30–40 mol/m²/day against a lettuce norm of 15–17). Shading at 30–50%, or switching off supplemental lighting entirely plus shading — to keep DLI below the upper limit. Excess light without matching CO₂ and temperature → photoinhibition.
Grow room year-round: 100% artificial lighting — theoretically no seasonal effect. But in summer: lamp heat adds to an already warm external environment → overheating risk. In winter: lamp heat helps maintain temperature → reduced heating load. It can sometimes make sense to reduce lamp output in winter if temperature is already at target without them.
Irrigation and Drainage: Seasonal Schedule Adjustment
Transpiration in summer is 2–3 times higher than in winter. The same irrigation timer:
- Summer in heat: substrate dries quickly, plant is stressed between irrigations
- Winter in cold: substrate stays constantly wet, DO is reduced, Root Rot risk rises
Winter: reduce irrigation frequency or volume. Check substrate moisture content between irrigations — it should drop over a 2–4 hour cycle just as it does in summer. If it is not dropping, the next irrigation is unnecessary.
Summer: increase irrigation frequency or shorten intervals. First irrigation earlier in the morning (the plant is active and transpiring from the start of the light period), last irrigation closer to the end of the light period.
For more on temperature and humidity and their effect on transpiration — and therefore irrigation demand.
Three Mistakes That Cost the Most
Not adjusting EC and irrigation frequency at the transition from summer to winter. The same EC and the same timer in September–October as transpiration declines → gradual salt accumulation in the substrate and waterlogging. The problem builds slowly and invisibly until it becomes obvious in November as osmotic stress. Review the regime at each seasonal transition — do not assume "if it's running, don't change it."
Not recalculating cooling capacity at the seasonal transition. An air conditioner that coped in winter (lamp heat only) may not cope in summer (lamp heat + outdoor temperature + wall transmission). Insufficient cooling in summer becomes apparent within a week through overheating and Root Rot.
Not checking source water parameters at the seasonal transition. Spring snowmelt or autumn rain changes the EC and alkalinity of tap or well water. The same recipe mixed on water with a different EC gives a different result. Measure source water EC and pH at the start of each new season — or monthly with an unstable source.
How to Know Seasonal Adjustment Is Working Correctly
Cycle length is stable year-round (±20%) — it does not double in winter. Drain EC stays within normal range in both winter and summer — no accumulation in either season. The production plan accounts for seasonal differences: winter growing specifics are built into the plan in advance.
For deeper understanding: EC in Hydroponics: What Electrical Conductivity Measures and How to Control It — explains why the same EC has a different effect at different temperatures and transpiration rates, and how to account for this when making seasonal recipe corrections.