"Lettuce is undemanding — it will grow in almost anything." But it is precisely this reputation that leads growers to cultivate lettuce under conditions where it "kind of grows" — and end up with a product showing tip burn, bitter taste, uneven sizing, and a short shelf life. Lettuce is genuinely undemanding when it comes to agronomic tricks. It is very demanding when it comes to stability: consistent EC, consistent temperature, and consistent airflow. One week of poor ventilation and the entire batch has necrosis on young leaves.
Quick Glossary
- Tip burn — die-back of the edges and tips of leaves in lettuce and some other crops; caused by calcium deficiency at the growth points due to weak transpiration, not by a lack of calcium in the solution
- EC (Electrical Conductivity) — nutrient solution concentration; leafy crops have a narrower working range than fruiting crops — excess EC quickly produces bitterness and stress
- DLI (Daily Light Integral) — the daily photon dose; 14–17 mol/m²/day is sufficient for lettuce without excess
Solution Parameters: A Narrower Range Than It Seems
Leafy lettuces and greens tolerate EC deviations less than fruiting crops. At elevated EC, excess salts pass directly into the leaf mass — and it turns bitter.
EC for leafy crops:
- Seedling stage and first week after transplanting: 0.8–1.2 mS/cm
- Active vegetative growth: 1.2–1.8 mS/cm
- Final week before harvest: can be raised to 2.0 mS/cm for denser leaves and better flavour
- Maximum without pronounced bitterness: 2.2–2.5 mS/cm depending on variety
pH: 5.8–6.2 — a narrower optimum than for most crops. Above pH 6.5, iron and manganese are locked out and chlorosis appears on young leaves. Below pH 5.5, calcium and magnesium availability drops and tip burn risk increases.
Solution temperature: 18–22°C. Above 24°C, dissolved oxygen drops and the root weakens. Especially critical in summer for NFT where the root is continuously in the flow.
Lighting: Neither Too Little Nor Too Much
Lettuce is a relatively low-DLI crop. Target range: 14–17 mol/m²/day. Below DLI 12 — stretched, loose leaves, long cycle, poor quality. Above DLI 20 — temperature in the canopy rises, bolting risk increases in susceptible varieties, and tip burn intensifies.
PPFD for lettuce: 200–350 µmol/m²/s at a 16-hour photoperiod delivers 11.5–20 mol/m²/day. The optimal point is 250 µmol/m²/s at 16 hours = DLI 14.4 mol/m²/day.
Photoperiod: 16 hours is the standard. At 18 hours or more, some lettuce and spinach varieties begin to bolt even without temperature stress. Coriander and rocket are especially sensitive — maximum 14–15 hours for the vegetative phase.
Tip Burn: The Most Common Problem and How to Avoid It
Tip burn is not a calcium deficiency in the solution. It is a calcium deficiency at a specific growth point caused by weak transpiration. Calcium moves with the water stream — and when transpiration is weak it does not reach the youngest, fastest-growing leaves where water flow is minimal.
Causes of weak transpiration in leafy lettuce under protected cultivation:
- High relative humidity (above 80%) with no air movement
- Low VPD at normal absolute humidity
- No circulation fans in the canopy
Solution: do not increase calcium in the solution — it will not help. Instead, improve ventilation and reduce humidity in the zone of young leaves. Horizontal circulation fans between rows are the primary tool for tip burn prevention.
Bolting: Why It Happens and How to Prevent It
Bolting is the transition to the generative phase: the plant elongates, leaves become smaller and bitter, and a flower stem emerges. In leafy crops, bolting means the end of commercial value.
Bolting triggers for leafy crops:
- Long photoperiod (more than 16 hours for sensitive varieties)
- Temperature stress — a sharp swing of more than 8–10°C between day and night
- Water stress from drying out or excessive EC
- Genetic predisposition of the variety to early bolting
Variety selection is the primary factor: "slow bolt" or "heat tolerant" lettuce varieties are fundamentally different from standard ones in their bolting resistance. When growing in summer or at temperatures above 24°C — slow bolt varieties only.
Three Mistakes That Cost the Most
Growing all leafy crops under identical parameters. Spinach and basil are different crops with different EC, photoperiod, and temperature requirements. Basil needs 24–26°C and a 14-hour day; spinach thrives at 16–18°C and is prone to bolting under long days. One set of parameters for both is a compromise that is optimal for neither.
Ignoring tip burn and continuing the cycle because "it doesn't spread." Tip burn appearing is a sign that conditions are already wrong. It does not spread to neighbouring plants like a disease, but continuing the cycle under the same conditions produces progressively more affected leaves and reduces marketable quality at harvest. When you see it — correct the conditions.
Not accounting for source water EC when mixing the solution. Tap water at EC 0.6 plus fertilisers to a total EC of 2.0 means 1.4 mS/cm coming from fertilisers. The same "EC 2.0" from soft water at EC 0.1 means 1.9 mS/cm from fertilisers. The difference in nutrition is significant, especially within the narrow working range of leafy crops.
How to Know the Cycle Is Going Well
- Young leaves have no edge necrosis
- Outer and inner leaves are uniformly coloured with no chlorosis
- Plants across the entire growing area are uniform in size and density
- No bitterness when tasted, even at the end of the cycle
- Harvest timing matches the variety's stated cycle within plus or minus three days