Stem topple is a serious problem in the flower bulb sector. Bent stems are unsaleable and are therefore something to avoid if at all possible. It has long been known that this problem is associated with transpiration from the plant, and for many years the advice has therefore been to ventilate well during the forcing phase so as to prevent the RH from rising above 70%. Researchers have tested whether this is really necessary throughout the whole forcing period, since less transpiration means much lower energy usage.

The main energy gobblers in tulip forcing in two- or three-tier greenhouses are light, transpiration and, in particular, heat loss to the outside. Researchers Jeroen Wildschut and Martin van Dam of Wageningen University & Research in the Netherlands investigated the potential for light reduction a while back. A tulip doesn’t need photosynthesis light or grow light because the bulb itself is a plentiful source of carbohydrates. Only steering light is needed to keep the plant standing upright and to enable it to develop good colour. In 2005 these findings paved the way for the current system of multi-tier cultivation, a method that enables forcers to expand their production area vertically. This system leads to energy savings of as much as 40-50% per crop, not least thanks to the high utilisation rate.

In Next Generation multi-tier cultivation, bulbs are forced in six or more tiers in well insulated cells. The facility is no longer a greenhouse but more like a warehouse lit with low-energy LED lights. The different climate requirements needed in each growth phase are met by compartmentalising the forcing area.

Ventilating costs energy

Following on from their research into light reduction, Wildschut and Van Dam set their sights on transpiration and ventilation. Wildschut: “Moving from a single tier to a multi-tier system cut energy consumption by 50%. In this new situation, ventilating to remove moisture uses the most energy as the system that warms up the outdoor air and then removes the moisture runs day and night. Our research shows that you can save roughly another 50% by ventilating less. By reducing transpiration, in other words by removing as little moisture as possible, you quickly start saving energy.”

In multi-tier systems with two or three tiers, the researchers have observed that energy is consumed at a rate of 300 MJ per 1,000 bulbs. The figure for rooms with six tiers and balanced ventilation is just 180 MJ per 1,000 bulbs – an encouraging outcome. However, as the researchers emphasise, savings are all well and good but they should never come at the expense of quality. So the project started off by looking into the transpiration needs of the plant in each growth phase.

Susceptible phase

Van Dam: “In the first phase of the project, we explored the susceptibility of some of the main cultivars to topple. Leaf and stem topple, along with fading and too lightweight or too short plants, are the result of poor transpiration and calcium deficiency in the cells. Calcium deficiency causes the cell membranes to become more permeable, so the cells burst and moisture leaks out, forming the point at which the stem or leaf topples over. We know that the growing parts of the flower absorb calcium as the stems elongate rapidly in the growth phase. During this phase, transpiration draws calcium from sources including water to wherever it is needed in the plant via the sap flow. Sufficient transpiration is therefore crucial in this phase.”

Also noteworthy is the fact that toppling doesn’t actually happen in this phase per se. The flowers can be standing proudly upright at the time of harvesting but the symptom can rear its head at the auction or even in the vase. “Our research shows that the degree of susceptibility to topple is clearly cultivar-specific. For example, excessive RH was found to have no effect at all in the cultivar Barcelona. Strong Gold and Seadov turned out to be much more susceptible: the number of plants affected by topple at 83% RH was very high. The middle growth phase was found to be the most susceptible to high RH.”

Low RH in middle week only

The researchers established which phase was most susceptible to topple by dividing twenty “pin trays” of tulips between two compartments. The RH in the first compartment was kept at about 100%, with below 70% in the second. The researchers switched the trays round every two days. As soon as some plants started to display signs of toppling, they were logged and removed. This trial was carried out in three forcings. The same picture emerged in each cycle. Out of the roughly 20 days the bulbs spend in the greenhouse, it is the middle week that determines whether the plant will be susceptible to topple later on.

In their report, the researchers state that the tulips need to be able to transpire for between 30 and 65% of the forcing period. Therefore, during this short period (which can be between three and nine days, depending on the cultivar used) the grower can influence calcium uptake and should keep the RH sufficiently low. The tulip is not affected by high RH during the first and last weeks of forcing. Wildschut: “By keeping the RH below 70% in the middle week only, when the tulips are at their most susceptible to topple, you can therefore save a lot of energy without making the problem worse.”

Adjusting the cultivation strategy

Last January, Van Dam shared the findings of the research with attendees at the Day of the Tulip industry event. “I could see that there was a lot of interest among the audience. Obviously growers aren’t going to change tack straight away, but it is important for them to think about it. They will only start making adjustments to their own cultivation strategy if they believe that it works. That’s fair enough – and it’s also why it will be important to follow up on this research. We want to be able to offer practical guidance. That’s the only way our efforts up to now will actually lead to a reduction in energy consumption and therefore in CO2 emissions in the flower bulb sector.”

Once the researchers secure financing for the follow-up research, they plan to investigate transpiration in the phase most susceptible to topple in more depth. The latest research revealed that the RH doesn’t have to be low 24 hours a day at this time. They expect that it should be possible to switch the ventilation system off for between three and 10 hours a day in this phase. If that is the case, growers could save up to as much as 47% of the energy they use on dehumidification by allowing the plants to transpire for between 18 and 21 hours a day during the susceptible phase. Now that’s definitely worth investigating.


Ventilating less can save energy. However, if the RH is allowed to get too high, this can result in leaves bursting open or stems collapsing (toppling). Researchers looked into what effect reducing transpiration in different growth phases during forcing would have. They discovered a distinct period in which tulips are more susceptible to topple and in which adequate transpiration is essential in order to maintain quality. In the days before and after this period, growers can quite safely allow the RH to increase slightly without making the problem worse.

Text: Jojanneke Rodenburg.
Images: Studio G.J. Vlekke.