Professional tips from the producer

Ornamental horticulture

The optimum temperature is 19°C. To reduce length growth, a day temperature of 18°C and a night temperature of 20°C is recommended. At the end of cultivation, it is advantageous to lower the temperature by 2 to 3°C. This leads to better colouring and a more stable structure.

Additional light is an advantage for the early sets. However, light should not be provided for more than 16 hours per day (at least 8 hours of darkness). The optimum is 14 hours with a daylight sum of 60 klxh, which will shorten the cultivation time accordingly for at least the first 30 to 40 days of cultivation.

CO² fumigation can shorten the cultivation time by a further 8% (on average). An optimum concentration is 900-1100 ppm during the light hours. Fumigation is not necessary at night as it has no effect.

At the start of cultivation, it is important to get N into the plant, as it has almost no N left in the substrate due to overwintering. It is important to ensure a high proportion of nitrate, as this is directly available to the plant. It is recommended to fertilise in a balanced or N-K emphasised way. With the blue varieties, it is important to ensure that fertilisers low in phosphorus are used, as phosphorus has a negative effect on the availability of AL sulphate.

We also recommend fertilising with iron at the start of cultivation, as early hydrangeas in particular can very quickly suffer from iron deficiency.

The average pH value of blue hydrangeas is 4.2 and we therefore recommend a one-off fertilisation with AL sulphate of 1.5% (15g/L water) within the first 3 to 4 weeks. (Please only use AL sulphate concentrations, not potassium alum).

Please spray the hydrangeas once against botrytis immediately after forcing, as early forcing can lead to rotting when the plants are new in the sap. Furthermore, treatment against snails is particularly beneficial for white hydrangeas.

In addition to Alar, Desmel has also proved effective as a good inhibitor in recent years, but it is important to pay attention to the concentrations. In hydrangeas, Desmel is authorised against leaf spot, which is very important due to the new PS law. Concentrations of 0.02 to 0.03% can be used to regulate length growth without any problems. The frequency and possible increase in concentration depends very much on the crop and climate management and the weather. We therefore recommend a slow approach to higher concentrations. Furthermore, you should stop using Desmel shortly before the petals are visible, as it can lead to flower discolouration.

We hope this has given you a good start to the hydrangea season. If you have any further questions on the above or other topics, please do not hesitate to contact us. Please understand that these are only recommendations and that we cannot accept any liability in the event of damage.

Length growth can of course also be restricted using cultivation techniques. Andreas Pellens has written his master’s thesis on this subject. Read the “Professional tips from the grower” further down on this page.

Tree nursery

To produce compact and stable plants, it is advisable to open the vents as much as possible. The cooler the hydrangeas are cultivated, the more stable the structure and the more intense the colouring. However, frost must be prevented.

The ventilation temperature should be adjusted to the average night-time temperature. The maximum difference should not exceed approx. 5-8°C. This means that if the average night-time temperature is around 5°C, the ventilation should be opened during the day from 10-12°C at the latest. If the difference is too high, we will unnecessarily promote length growth and the hydrangeas will become softer in structure.

It is also important to ensure that unnecessary moisture is ventilated out. If the vents remain closed during a cold period, it is advisable to ventilate them 1 to 2 times a day if the humidity is high, as otherwise the plants will also grow soft and long.

The substrate should have a coarse structure and be very air-permeable. All major substrate suppliers have ready-made hydrangea mixes, we recommend using these. It is best to ask one of our sales representatives directly, who also offer substrates and know the requirements for a hydrangea substrate.

The substrate should be fertilised with approx. 3.0kg/m³ Osmocote Exact Standard (15-9-12-2.5Mg) 5-6M and 200g/m³ Micromax (trace elements).

For blue hydrangeas, the pH value should be adjusted to approx. 4.5-4.8 and possibly 1kg/m³ Al sulphate should be mixed in or the hydrangeas should be dipped in an Al sulphate solution before potting.

AL sulphate must not be used for red, pink and white hydrangeas and the pH value should be adjusted to 5.5.

Top-up fertilisation should only be carried out in consultation with an ICL consultant, as water quality can also play a major role.

If the technical equipment allows the hydrangeas to be fertilised with liquid fertiliser using Dosatron or EC-controlled fertilisation, we recommend fertilising at the start of cultivation (after rooting in the container) with Universol white (15-0-19-2Mg) approx. 1-2 times per week with 1.0g/l as required. Universol white can also be replaced by Universol orange (16-5-25-3.4Mg) after every 2-3 fertilisations.

It is recommended to dissolve 200g of Tenso Iron (Yara) in the stock solution for every 25kg of Universol to ensure the iron supply and maintain a strong foliage colour.

If you do not have the technical equipment, we recommend topping up with Osmoform (19-5-13-2Mg), approx. 1.5-2.0g/l, as a top-up fertiliser (after rooting in the container).

If necessary, foliar fertilisation can also be used. In this case, we recommend spraying with Peters Professional Foliar Feed (27-15-12) in several applications of 1.0g/l at the start of the crop. At the end of the crop you should then switch to Peters Professional Combi-Sol (6-18-36-3 +SpE).

This year, the PH value of the blue hydrangeas is 4.2, so we recommend a one-off fertilisation with AL sulphate of 1.5% (15g/L water) before potting the hydrangeas (please only use AL sulphate concentrations, not potassium alum!). It is also possible to dip the hydrangeas in a stock solution before potting, just make sure that the foliage and buds are rinsed off with clear water.

Normally, no plant protection is required for hydrangea forcing. Only aphids and botrytis can be a problem. Pirimor granules are quite well tolerated against aphids and Switch and Teldor have proved effective against botrytis. However, compatibility must be tested individually, as the plant’s stage of development is decisive.

This recommendation has been developed in co-operation with ICL. If you have any questions about substrate fertilisation and top dressing, please contact your responsible ICL consultant.

We hope this has given you a good start to the hydrangea season. If you have any further questions on the above or other topics, please do not hesitate to contact us. Please understand that these are only recommendations and that we cannot accept any liability in the event of damage or any legal consequences.

To prevent the hydrangeas from growing too long, you can also top dress them up to once a week with Alar (0.1-0.4%) or Desmel/Tilt (0.02 ? 0.03%). The frequency and possible increase in concentration depends very much on the crop and climate management and the weather. We therefore recommend a slow approach to higher concentrations. Furthermore, you should stop using Desmel/Tilt shortly before the petals are visible, as it can lead to flower discolouration.

Managing director and master gardener Andreas Pellens

Managing director and master gardener Andreas Pellens

Inhibiting length growth also works with special cultivation techniques. Read more about this in our professional tips for growers.

Inhibition of length growth

The possibility of using cultivation techniques to regulate the growth of hydrangeas is often a question of economic efficiency and the question must be asked whether the success achieved is in relation to the labour required. Unfortunately, this was often not the case with most variants. However, in view of the customer’s desire for greater environmental protection, these options should not be ignored.

The following measures have been used in the past to reduce length growth. We will explain to what extent they are practical and can be used.

1. temperature control

Before I go into more detail about temperature management, it is worth mentioning that the forcing date plays a decisive role, especially when it comes to temperature management. When choosing a strategy for early forcing (start of forcing in mid-November to the end of December), you should bear in mind that most varieties do not require the use of inhibitors.

This means that no strategy should be chosen that has an inhibiting effect on length growth, as this would result in too short a crop, which in turn would not generate such good yields. It should also be mentioned that strategies such as negative diff (raising the temperature during the night) for late forcing (forcing begins in mid-April) are not economically and energetically justifiable.

When it comes to temperature management, we have to differentiate whether we want to try to achieve a compact and strong structure of the plants only by lowering the general average daily temperature, or whether we want to try a so-called temperature strategy such as “Diff” or “Cool Morning”.

1.1 Influence of “Cool Morning” on the length growth of Hydrangea macrophylla

In the past, people have already considered temperature strategies to reduce the length growth of hydrangeas. This also gave rise to the idea of testing the effectiveness of “Cool Morning”. Many of the trials at that time did not achieve any significant results, although “Cool Morning” has advantages over “negative diff”, namely energy savings and year-round applicability. This in turn is necessary for late forcing in order to be able to dispense with inhibitors. (Röber and Bauer 1995)

The trial conducted by Professor Röber in Weihenstephan in 1995 was based precisely on this statement. He tested the effectiveness of “Cool Morning” on the varieties “Hermann Dienemann” (red, syn. “Leuchtfeuer”), “Nymphe” (white) and “Renate Steiniger” (blue). All three varieties rarely manage without inhibitors in forcing.

The installation date was 2 January 1995 with a temperature of 22°C from 2 January to 8 January 1995. After that, the temperature was gradually lowered by a further 2°C every 6 days until 23 January. After that, the daily average temperature remained constant at 16°C.

Sales started on 22 March 1995.

The “Cool Morning” method was carried out from 10 January to 13 March 1995. One hour before sunrise, the room temperature was lowered by 10°C for a period of four hours. Just three hours after sunrise, the original room temperature was restored.

The results were astounding:

Although no significant success had been achieved with “Cool Morning” to date, a reduction in shoot length of approx. 2.5 cm was achieved. This 2.5cm was also achieved by the inhibitor variant with a compression of 0.4% “Alar” compared to the untreated control.

On the other hand, “Cool Morning” leads to a delay in the cultivation period during the forcing phase because the daily average temperature has been reduced. This effect can be avoided by raising the temperature during the day period. (Strauch 1995)

Effects of “Cool Morning” compared to an Alar treatment in 1995 (Röber)


1.2 „Diff“

When thinking about “diff”, you should first ask yourself what strategy you are pursuing.

“Positive diff” means that the daytime temperature is significantly higher than the night-time temperature. This in turn has major energy benefits. During the day, any solar energy is utilised, while during the heating-intensive night, the temperature can be set well below the normal daytime average temperature.

These are all advantages of the “positive diff”, but unfortunately it has not yet been possible to achieve good results with hydrangeas.

The situation is different with the “negative diff”. Good results have already been achieved with this in the past. The major disadvantage of the “negative diff” is the high energy expenditure it entails. Up to now, it has had no relation to the known inhibitors. The use of “Alar”, or more recently “Desmel”, is still far more favourable in terms of costs and, in contrast to “negative diff”, can even be used all year round.

Only now are people thinking about using the more expensive options for production.

Extensive trials were carried out at the Westphalia-Lippe Horticultural Centre, “Wolbeck”, to investigate the influence of different temperature strategies on growth and plant structure.

In trials, the effects of “negative diff” were tested with the “Leuchtfeuer” variety in comparison to constant temperature control.

The “negative diff” strategy with a set temperature difference of 3°C (17°C during the day/20°C at night) led to a noticeably lower plant height compared to constant temperature control at 18.5°C. The sprouting and the cultivation period were not affected.

Influence of “negative diff” on plant growth 1993 (Richter)


Based on this result, further tests were carried out with the varieties “Leuchtfeuer” and “Libelle” at a forcing date in mid-December, in which the effects of different “positive and negative DIFF” temperature settings were compared with those of a constant temperature setting. (Figure 4).

At a “negative diff” of 8°C (15°C during the day/23°C at night), the greatest inhibition of shoot length growth was achieved compared to the constant temperature setting of 19°C.

With a “negative diff” of 4°C (17°C during the day/ 21°C at night), lower shoot length growth was also achieved.

In contrast, the “positive diff” conditions promoted shoot length growth in the corresponding plants. The different “diff” strategies had no influence on the duration of cultivation!

The “diff” strategy showed no effect on the number of shoots and cones, although a decrease in cone diameter was observed at the “negative diff” setting of 8°C.

Due to this effect on the cone diameter and thus on the quality of the plants, the “diff” setting of 17°C during the day and 21°C at night is considered optimal for cultivation.

The effect of the ?diff? strategy on the shoot length growth of hydrangeas has been demonstrated.

Parallel oxygen release and ? consumption measurements on hydrangeas of the “Leuchtfeuer” variety led to a better understanding of the mechanism of action. The measurements show that a negative oxygen balance is achieved for the daytime cycle due to high night-time temperatures in combination with low daytime temperatures.

The negative oxygen balance indicates that high night-time temperatures accelerate the respiration of those assimilates that are built up to a lesser extent during the day at low temperatures.

In addition, there are effects that indicate an altered ratio of phytohormones in the shoot and root. This is directly attributable to the higher turnover rates during the night.

The use and effect of the “Diff” strategy is particularly dependent on the external climatic conditions!

The higher the radiation from outside, the more problematic it is to maintain the low daytime temperatures! It is therefore extremely important not only to regulate temperature values, but also to check the actual temperatures in the greenhouse. The ventilation values should only be 1 to 2 °C above those of the heating. When checking the temperature, the type of sensor must be taken into account. Only ventilated and shaded temperature sensors approximately reflect the actual air temperature.

This is particularly important, as the temperature data from trials at the research institutes can usually be traced back to this type of temperature measurement. A prerequisite for the application of modern temperature strategies for growth control is therefore the modernisation of the sensors.

The effect of the “Diff” strategy can vary from farm to farm depending on the variety and the prevailing growing conditions.

The application should be used under unfavourable external growth conditions as a “positive diff” to promote line growth and as a “negative diff” to regulate growth under favourable growth conditions.

The effect of the “negative diff” is usually sufficient to sufficiently inhibit shoot length growth.

To ensure the production of high-quality hydrangeas, a combination with other alternatives for growth control, such as dry cultivation, makes sense.

The success of the “diff strategy” depends on the harmonisation of all growth factors with the goal of quality production!

Influence of different diff strategies on the length growth of plants 1993 (Richter)


1.3 Lowering the daily average temperature

If you do not necessarily want or need to be on the market with flowering hydrangeas at a very early date (February), you also have the following option of influencing shoot length growth:

Length growth can also be inhibited by starting forcing at the beginning of January and lowering the average daily temperature more quickly. The choice of variety is decisive for a compact and high-quality plant under the following cultivation conditions.

Varieties should be selected that achieve good quality under normal cultivation conditions (average daily temperature of approx. 19°C) with comparably low use of inhibitors.

In order to make the right choice of variety, you should consult the respective raw material or young plant supplier.

Normally, hydrangeas are cultivated at the beginning of the forcing period for approx. 2 weeks at 20° to 21°C until all buds have sprouted and then the temperature is lowered to approx. 18°C. To achieve a more compact plant, you can also start with approx. 18°C and lower the average daily temperature to approx. 15° to 16°C after 2 weeks. The result is a more compact plant with harder shoots, firmer cones and a more intense colour. The reduction that this method entails is not very great. In addition, you have to reckon with a longer cultivation time of about 1 to 2 weeks.

This variant is not a panacea either, but if it is used in conjunction with the timely cultivation measures, for example, the compression effect is also increased.

2. water balance of the plant

A reduction in shoot growth was also observed when plants were stressed by drought in the root area during the forcing period.

The whole thing was tested in trials at the State Research Centre for Horticulture in Weihenstephan back in 1994/95. The question was whether hydrangeas react to drought stress and whether all varieties react in the same way. Three strongly growing varieties were tested and three different strategies were pursued in each case.

Some of the plants were watered as needed. Another part was given an excessive amount of water and a third part was subjected to drought stress.

Plants that were cultivated very dry were given 50ml per day for the first 25 days, 75ml per day for the next 25 days and 100ml per day for a further 25 days. At the time of cone development, the plants were fed as required. The results were impressive. (Figure 5)

However, it should not be forgotten that such a deficiency supply also entails major risks. For example, the ‘drought stress’ variant is a particular problem for blue hydrangeas, as the application of aluminium can quickly lead to salinisation.

In addition, the crop needs to be checked more frequently as there is a constant risk of drying out.

Length increase during forcing of the variety ‘Leuchtfeuer? 1997 (Haas)


3. timely cultivation measures

The most important point in alternative cultivation is timely cultivation measures. In the specific case of hydrangeas, this means early backing of the plants and any intermediate backing. If you have enough space available, it is even advisable to place the plants directly on their final spacing at the start of forcing.

Timely backing is associated with low additional costs. The only cost factor that comes into play here is the slightly higher energy consumption, which is characterised by the fact that the entire area of a set may have to be heated 5 days earlier.

If you use the timely cultivation measures together with “Cool Morning” or “negative diff”, you can get by without any inhibitors at all.