Besides helping growers to achieve their sustainability goals, efficient water management plays a crucial role in maintaining plant health. Realising or steering towards a certain drain volume or drain percentage per day should not be your goal. Instead, the goal should be optimising your irrigation strategy to provide your crop with precisely the amount of water and nutrients it needs. This will ensure the development of a strong and healthy root system, leading to consistently good plant growth, fruit quality and a higher total production. Grodan’s Andrew Lee explains how.
The role of drain volumes in root zone management
The root zone environment can be described as a crop’s ‘engine room’. A strong and healthy root system ensures that the plant receives an adequate supply of water and nutrients so that the crop can transpire, even on the brightest days. But in order to support generative steering for optimal plant balance, production and fruit quality, the root zone climate needs to be managed with precision.
Traditional drain approach vs precision irrigation
One factor in this is the drain – in other words, the water that drains out of the growing media after irrigation. Traditionally, high-tech growers have aimed to achieve a fixed drain percentage each and every day, in the belief that this is necessary to maintain stable root zone conditions.
“Consider why we need drain: primarily to combat for a lack of uniformity in the greenhouse, and secondarily to adjust for a nutrient imbalance or increasing EC. Historically, to manage EC, average 24-hour drain targets of >30% are not uncommon. But when uniform and steerable substrates are combined with uniform greenhouse temperatures, how much drain is really necessary?” says Andrew Lee, Green Knowledge Manager at Grodan.
“By definition, drain is a sign that you’ve over-irrigated – and over-irrigation makes the crop more vegetative rather than generative and can have a negative impact on root quality, especially in winter or with lit crops through the lighted season. This reduces the production potential and increases the risk of diseases.”
Instead of focusing on a fixed drain target every day, growers should think about what the plant really needs. “For example, plants require more water in periods of high radiation than on cooler, cloudy days. By controlling irrigation schedules, growers can optimise water and nutrient usage, and improve their cultivation results, with minimal drain volumes,” he adds.
Adopting precision irrigation towards generative growth
“Another mistake in irrigation management is steering on the drain EC, rather than on the EC in the growing media. In reality, EC in the drain can be far higher than that measured in the slab. Without this knowledge, growers can make incorrect decisions regards crop steering,” Lee points out.
Figure 1: Weekly average drip, slab and drain EC (mS/cm) during low-energy tomato growing trial under full LED (2024). Red solid line depicts average drain EC, green dotted line average slab EC and red dashed line average drip EC.
In a recent research trial, Grodan highlighted the differences between EC measurements in the root zone and in the drain (Figure 1). “The average drain EC was 8.1 mS/cm, whereas the average slab EC was just 4.4 mS/cm,” he states. “Admittedly, the average drain volume in this trial was around 8%. However, it highlights that focusing only on the drain EC can lead to over-irrigation, especially when generative steering is required. The intrinsic properties of Grodan’s stone wool growing media, in combination with our sensors in the substrate, give growers increased control over steering on WC% and EC for generative growth,” adds Lee.
Root zone management for generative steering
- Water content percentage (WC%): Maintain a lower average water content compared to the vegetative steering.
- Electrical conductivity (EC): Control water uptake using a higher EC in the irrigation water and substrate.
- Irrigation frequency: Apply larger volumes at a lower frequency (see example in Table 1).
Substrate volume 8.2/m2 | |||||||
| Session size 1% | 82 ml/m2 | 60 ml/dripper | |||||
| Session size 2% | 164 ml/m2 | 120 ml/dripper | |||||
| Session size 3% | 246 ml/m2 | 180 ml/dripper | |||||
| Session size 4% | 328 ml/m2 | 240 ml/dripper | |||||
| Session size 5% | 410 ml/m2 | 300 ml/dripper | |||||
| Session size 6% | 494 ml/m2 | 360 ml/dripper | |||||
*average stone wool volume in Netherlands for tomato growers in 8.3 l/m2
Benefits of generative steering
Generative steering offers several benefits in terms of both root quality and plant quality. Enhanced root quality is achieved by managing WC% during the night to encourage deeper and more extensive root systems, supporting healthy fruit production. Precise irrigation management prevents waterlogging, reducing the risk of root diseases, while higher EC levels ensure efficient nutrient uptake, leading to healthier plants.
Precision irrigation in low-heat strategies
It is particularly important to apply generative irrigation strategies in full-LED lit crops, especially when growing with an overall lower radiant heat (energy) input, according to Lee. “If you put excess moisture into the greenhouse system, you have to use extra energy to extract it again in terms of humidity control,” he explains. “The combination of a strong, generative plant with a controlled leaf area index (LAI), means you can avoid pushing moisture into the air with a precise irrigation strategy.”
The results of this approach? A good, well-developed plant with healthy roots that last right through to the end of the cultivation cycle. “Maintaining root quality through the lighted winter season can give growers a huge advantage during the final stages of the cultivation by avoiding production losses in May and June. This is beneficial for their yield and bottom line,” says Lee.
Conducting research into precision irrigation systems
“In our low-heat tomato cultivation trial 2024-2025, we demonstrated that you don’t need to chase a certain drain volume. By aligning the irrigation strategy start and stop times with lights on and lights off to realise a set decrease in WC% overnight, and by matching the volume and frequency of irrigation with crop demand (ml per mol of light), we achieved an average drain of just 13%. This was similar to the results from the 2023-2024 season, when average drain volumes for the lighting season were a mere 8%. However, even with these low drain volumes, we were still able to keep the EC in the root zone stable,” he comments. “This demonstrates that if you can understand and control the uptake, you can also precisely align the amount of water you give.”
In fact, by adjusting the irrigation to the measured WC% and EC in the slab over the course of the day, it was possible to achieve zero drain in the first eight weeks. “In other words, the plants absorbed everything. The EC in the slab remained stable during that period, which gave us confidence that we were offering sufficient water and nutrients. And at the end of the cultivation cycle, we still maintained high-quality roots. I can imagine a future in which the irrigation set-point control will be based on plant uptake rather than searching for drain volumes,” he concludes.
