At Grodan, we have amassed several decades of cultivation experience using stone wool growing media. In fact, we pioneered the development of hydroponic growing methods in the 1960s. Today, our soilless root zone management solutions are used for a wide range of cropsin Controlled Environment Agriculture (CEA) across the globe. As part of our commitment to continuous improvement, we have repeatedly pushed the boundaries, running countless trials to gain an even deeper understanding of how our stone wool growing media can be utilised to support plant health and development, and hence commercial success for growers.
The importance of precision irrigation
Time and again, these trials have underlined the importance of a precision irrigation strategy: giving the plant the right volume of water containing the right mix of nutrients at the right frequency. This creates and maintains optimal conditions in the growing media to stimulate root development and maintain root health. By striking this careful balance in the root zone, high-tech growers are consistently able to achieve higher yields of up to 15% (see Table 1) by steering the irrigation system compared to their current practices (benchmark is a commercial grower on stone wool) leading to more uniform and better-quality crops. Moreover, thanks to our soilless growing media ensuring a clean start, they reduce the disease pressure for growers throughout the crop cycles.
| Strategy | Weight (cleaned) |
| Optimised irrigation strategy | 504 g |
| Reference stone wool growers strategy | 435 g |
Table 1. Mean values of fresh weights (cleaned) of reference stone wool grower’s strategy and optimised irrigation strategy
Significant improvements
Supported by our research-based advice over the years, countless growers of crops such as tomatoes, peppers and cucumbers have significantly improved their results by switching to stone wool. In view of the rising worldwide demand for lettuce, we are keen to apply our existing irrigation expertise to help high-tech lettuce growers achieve similar gains. Since 2023, to deepen our understanding of how lettuce performs on stone wool, we’ve been conducting trials with a leading research centre for lettuce: the Proefstation voor de Groenteteelt, PSKW, Belgium together with Ben Vanvoorden, researcher at PSKW.
Irrigation frequency matters
Before starting the trials, we hypothesised that a soilless lettuce crop would need more frequent irrigation than in the traditional way of growing because of its structural characteristics that makes the cultivation on stone wool growing media very easy to control and administer water and fertilisers in a directed and controlled manner. The question was, how much more exactly? In the nutrient film technique (NFT) systems that are commonplace in the high-tech lettuce sector, it’s not only the irrigation volume that matters, but most of all the frequency of watering. Therefore, we chose to focus on this factor.
Many growers using stone wool apply a traditional irrigation strategy, meaning they keep their irrigation frequency the same over 24 hours. This is somewhat understandable, because they don’t want to risk the crop drying out irretrievably. But we want our growers to get the most out of our stone wool. Therefore, we decided to combine our long-standing experience with the expertise at the research centre to investigate various irrigation strategies based on adjusted irrigation frequencies using stone wool growing media.
This led to us testing three strategies:
Strategy 1 (traditional irrigation strategy): Normal irrigation frequency during the day, limited irrigation during the night
Strategy 2: Increased irrigation frequency compared with Strategy 1, both during the day and at night
Strategy 3: Very high irrigation frequency in the morning, higher frequency than Strategy 1 during the day, limited irrigation during the night
Trial results
After studying and comparing the trial results, we identified that Strategy 3 gave the biggest improvement in terms of both root development and root health. The trial showed that a too high frequency during the night (Strategy 2) increased the risk of root rot and decreased root growth. To improve root quality, and to minimise tip burn, rot and yellow leaves, a higher frequency during the day is advised. However, a higher irrigation frequency (night or day) can actually increase the risk of yellow leaves and especially rot (Botrytis) on the oldest leaves.
The three-stage irrigation approach in Strategy 3 – very high irrigation frequency in the morning, high frequency during the day andlimited irrigation during the night– was most favourable in terms of root quality. This strategy produced more, longer and healthier roots with less browning (see Table 2 and Figure1).
| Strategy | Root Health | Root development |
| Reference stone wool growers strategy | 6,88 | 6,78 |
| Optimised irrigation strategy | 7,11 | 7,43 |
Table 2: Mean root scores of a Reference stone wool grower’s strategy vs Optimised irrigation strategy. Scores go from minimum 1 to maximum 9 (The higher the better).
Figure 1: Root growth and quality at harvest time of a Reference stone wool grower’s strategy (left) and Optimised irrigation strategy (right).
Resilient from the start
As a result of this trial, we are now even better equipped to help lettuce growers identify the optimal irrigation strategy specifically for stone wool. But, as in all crops, if you want to achieve the highest possible yield it’s essential to get the plants off to the best possible start. Based on this reasoning, we also included a propagation trial in our research programme to explore how our stone wool can optimally prime young lettuce plants for the growing phase. For example, it is important to determine how an adapted irrigation scheme can prevent a steep decrease of moisture level in the stone wool block, which can cause a shock to the plant in an early stage.
One clear benefit of Grodan’s stone wool in the propagation phase is that it allows growers to start from a known baseline. They can be sure that the plant will only get what they give it, which puts them fully in control. But because of the non-organic nature of stone wool, precise management of the electrical conductivity (EC) is crucial, particularly in the early stages of the crop. This aspect is already being researched in running propagation trials.
Validating the potential
We have already learnt so much about how our stone wool can improve the strength, health and resilience of lettuce crops. Based on the findings from our previous trials, it even has the potential to accelerate the rate of growth and therefore shorten the cultivation cycle. That’s why, later this year, we will validate all our learnings about propagation, irrigation and EC in further trials with Proefstation voor de Groenteteelt, PSKW, Belgium.
Expert advice and support
The results from our trials will continue to lead to new insights as the basis for fine-tuning our support to growers. High-tech lettuce cultivation involves so many factors that there is no ‘one-size-fits-all’ approach to precision irrigation. Therefore, our crop consultants are on hand to provide tailored advice to each grower working with stone wool, taking account of their specific situation including location and climate, seasonal changes and their technical setup. With expert guidance right from the start, every grower can switch to clean lettuce successfully.
