(This article is written specifically for the CE markets)

April marks one of the most demanding periods for greenhouse tomato growers across the Central European markets, where weather variability and rapid crop development often collide. At this stage, plants are under maximum load—depending on the planting date, the 10th truss is already flowering, and the canopy is expanding rapidly. Combine this with the unpredictable weather typical for April, and you have a perfect recipe for stress.

The key to success during this critical phase? Maintaining an ideal vegetative–generative (V‑G) balance while keeping the root system in excellent condition. Achieving this requires a carefully controlled irrigation and climate strategy tailored to CE conditions.

Stage 1: Maintaining generative control (early april)

As fruiting intensifies, the goal is to slightly “hold back” the plant to prevent excessive vegetative growth at the expense of truss development. This is achieved by steering on the dry side:

• Slab moisture: Maintain at 55–60% during the day.
• First drain: Do not rush. Aim for a radiation sum of approx. 400 J/cm² or intensity of 550–600 W/m².
• Drainage volume: Target 30–35%.

Constant EC control in the slabs is essential. If drainage needs to be reduced, slightly lower feed EC by 0.2–0.3 mS/cm, especially on sunny days. If EC in the slab rises overnight, apply one or two extra or somewhat larger doses.

The most important stimulus: overnight moisture drop

The timing of irrigation start and stop determines the correct overnight drop in slab moisture—your main generative steering tool.

• From the last irrigation of the day to the first irrigation the next morning, slab moisture should drop by 8–10%. This applies to pink, beef and standard types of tomato.
• Include a 2% buffer after sunrise before the first cycle begins (i.e., 8% overnight + 2% after sunrise).

If this drop is smaller, the plant will steer vegetatively, negatively impacting truss quality.

Stage 2: Building a buffer for may (second half of april)

When the 12th–13th truss begins to flower, shift strategy to prepare for upcoming heatwaves. Transition to steering on a higher watercontent:

• Slab moisture: Increase gradually to 70–75%.
• Irrigation frequency: Based on light—approx. 3 ml of feed per 1 J/cm² radiation sum per m².

• Overnight drop: After sunny days, 8% is sufficient; after cloudy days, aim for 10%.

Maintain a delta between feed EC and slab EC of 1.5–2.0 mS/cm. Excessive EC harms root health throughout the season.

When and why to use a night cycle

Late-day sun can cause rapid drying after irrigation has stopped. If slab moisture drops by 4% or more within 3 hours of the last cycle, apply a night cycle (2–3 hours before midnight):

• Dose: Small, no drainage.
• Purpose: Replenish slab moisture without stimulating the plant.

Benefits

• Prevents EC spikes overnight.
• Avoids early-morning irrigation that causes excessive root pressure and fruit cracking.
• Allows safe irrigation start 2–2.5 hours after sunrise.

Summary: the golden rules of steering

• Start irrigation: After a 2% drop from sunrise OR 8–10% from last cycle OR radiation sum of 150–200 J/cm² OR 2–2.5 hours after sunrise.
• Stop irrigation: Always 3.5–4 hours before sunset.
• Night cycle: Only when justified, with a non-drainage dose.

Complement irrigation with proper climate settings:

• Night temperature: 16.5–17°C (with slight pre-night).
• Day temperature: 19.5–21°C, adjusted to external conditions.

Well‑controlled water management is more than irrigation—it’s active plant physiology steering, and in April it determines the success of the entire CE season.

CE Market Note

This irrigation guidance is designed specifically for Central European greenhouse conditions, where fluctuating spring weather and light levels require precise steering.

The information provided by Saint-Gobain Cultilene has been compiled with great care and is intended solely as a guideline for the user. Saint-Gobain Cultilene therefore cannot guarantee any errors and no rights may be derived from the information or advice provided. The user himself is responsible for checking the accuracy and suitability of the information and advice for application. No commercial use may be made of the information and advice provided by Saint-Gobain Cultilene, in whole or in part, without the written consent of Saint-Gobain Cultilene.