Alternatives to influence the length-growth of the hydrangeas

On these pages we have edited the conclusions of Andreas Pellens‘ thesis for the degree “Meister” which he wrote in the year 2001. Besides using growth retardants it is possible to influence the length-growth through the cultivation technique.

This method mainly is a question of economy and you have to ask yourself if the result bears sensible relation to the expenditure of personnel. Unfortunately, with most varieties quite often it did not. Otherwise the request of most clients for more environmental protection should be a reason to not neglect this possibility completely.

With the following measures a reduction of the length-growth has been achieved in the past. In how far to my mind they are practicable and feasible, I will explain later.

  1. The Management of Temperature
  2. The Balance of Water
  3. Timely Cultivation measures

1 The Management of Temperature

Before I explain more about the management of temperature it's to be said that especially for this measure the time of forcing plays an important role. When forcing early (beginning mid-November until end of December), in choosing the method should be taken into account that most varieties do not need a growth retardant then.

Therefore you should not choose a strategy that inhibits length-growth, otherwise the result would be a product too short which would not bring good proceeds. Furthermore is to be said that strategies like the negative "Diff" (enhancing the temperature during night) are economically and energetically unjustifiable when forcing late (beginning mid-April).

Concerning the management of temperature we have to distinguish if, to get a compact and strong structure in the plant, we want to try it simply by lowering the average of the day temperature, or by applying a so-called temperature-strategy like e.g. "Diff" or "Cool Morning".


1.1 Influence of "Cool Morning" on the length-growth of hydrangea macrophylla

Gardeners already thought about temperature-strategies for reducing the length-growth in past times. That's how the idea came for trying the effect of "Cool Morning". Many of the experiments then did not have a remarkable result, though "Cool Morning" has some advantages compared with "negative Diff"; that is, saving energy, and the possibility to use it at all seasons. That now in late forcing is needed to be able to do without inhibitors. (Röber and Bauer 1995).

The experiment Professer Röber did in Weihenstephan in 1995 based exactly on this statement. He tested the effectiveness of "Cool Morning" on the varieties "Hermann Dienemann" (red, syn. "Leuchtfeuer" ("navigational light")), "Nymphe" (white), and "Renate Steininger" (blue). All three variants seldom can do without growth retardant.

The plants were set on 2nd January 1995 and held under a temperature of 22°C till 8th January 1995. After that the temperature was reduced step by step every sixth day for another 2°C. Then it was kept constant with a days' average of 16°C.

Purchase was started on 22nd March 1995.

The "Cool Morning"-method was used from 10th January till 13th March 1995. One hour before dawn the reduction of the room temperature by 10 degrees was begun and then held for four hours. One hour after dawn already the room temperature was increased again.

The results were astounding.

Though no result worth mentioning had been gained till then, now a reduction of the shoot length of about 2.5 cm was achieved. These 2.5 cm were also achieved by the inhibitor "Alar" with a compression of 0.4%, in comparison with untreated plants.

On the other hand, "Cool Morning" leads to a delay in the growth time during the forcing-period, because the average day temperature has been reduced. This effect can be avoided by increasing the temperature during the day. (1995)

Effects of "Cool Morning" in comparison to a treatment with "Alar" 1995 (Röber)



1.2 "Diff"

When talking about „Diff“, the first question has to be about the kind of strategy you want to follow.

“Positive diff” means, the day temperature lies significantly above the night temperature. This offers high energetic advantages. During daytime, the sun’s energy is used, while at night, when heating is necessary, the temperature can stay much lower.

Unfortunately, till now we could not gain good results with “positive diff” for hydrangeas.  

Things look better with „negative diff“. Good results have already been described in the past. The one great disadvantage is high energy requirement. Until now it bears no relation to the use of growth retardants. The use of “Alar” or “Desmel” is much cheaper still and – unlike negative diff – applicable the whole year around.    

Anyway, at the Horticultural Center Münster/Wolbeck extensive tests have been carried out to investigate the influence of different temperature strategies on plant growth and shaping. The influence of negative diff  in comparison to constant temperatures has been tested with the variety “Leuchtfeuer”. The negative diff strategy with temperature difference of 3°C (17°C over the day/20°C at night) compared to the constant temperature strategy with 18,5°C has led to significantly less plant height. Sprouting and cultivation time have not been effected.

The Influence of negative diff on plant growth (Richter, 1993)



On the basis of these results additional test on positive and negative diff were carried out with the varieties „Leuchtfeuer“ and „Libelle“ at mid-december (illustration 4). The strongest retardation of length growth in comparison to a constant 19°C has been achieved with a negative diff of 8°C (15°C over the day/23°C at night). With a negative diff of 4°C (17°C over the day/ 21°C at night) a slower length growth could be achieved as well. In contrast to that, the conditions of a positive diff support the length growth. The different “diff”-strategies had no effect on cultivation time.

The „diff“-strategy showed no impact on the numbers of sprouts and umbels, though a reduction of the umbel diameter was reported under a negative diff of 8°C. Due to this effect on the umbel diameter and therefore on the quality of the plants, a temperature of  17°C during the day und 21°C at night is being seen as the optimum for this culture. An effect of the „diff“-strategy on the length growth of hydrangeas could not be proved in these tests.

A parallel measurement of the oxygen release and usage of hydrangeas with the variety “Leuchtfeuer” led to a better understanding of the mechanism of influence. The measurements reveal that high nighttime temperatures in combination with low daytime temperatures lead to a negative oxygen balance. This indicates that high nighttime temperatures speed up the respiration of those assimilates which are in small amounts build up under low temperatures during the day.

Additional effects hint at a changed ratio of the phytohormones in sprout and root. This can be directly put down to the higher consuming rates during the night.

Usability and effect of the „diff“-strategy on a large scale depends on climate conditions.

It is highly important to keep an eye on the actual temperatures in the greenhouses to keep day temperatures level. The settings of the ventilation should only be 1 to 2°C above those of the heating. For temperature control the kind of sensors used has to be taken into account. Only ventilated and shaded sensors mirror the actual air temperature more or less reliably. This is of importance especially because the temperature givings from the tests are in most cases based on this kind of measurement. Therefore, a premise for the application of modern temperature strategies for growth control is a modernization of the sensors.

The effects of the „diff“-strategy can differ from company to company, dependent on the varieties and cultivation conditions. Under unfavourable circumstances it should be used as “positive diff” to enhance the length growth, under favourable circumstances as “negative diff” to reduce the length growth.  

The effect of a negative diff is, in most cases, not sufficient to retard the length growth alone. To ascertain the production of high quality hydrangeas a combination with other kinds of growth control, like the dry cultivation, is sensible. 

Coordination of all growth factors towards a high quality production determines the success of the “diff”-strategy!

Influence of different „diff“-strategies on the length growth (Richter, 1993)



1.3 Reduction of Daily Mean Temperature

If you do not have to or want to be on the market with flowering hydrangeas at a very early date (February) there are the following possibilities to influence length growth as well:  

By a forcing start at the beginning of January and the faster reduction of the daily mean temperature you can retard lenth growth. Crucial for a compact and high quality plant is the choice of the variety. You should choose varieties that achieve a good quality with relatively small amounts of growth retardants under normal cultural conditions (daily mean temperature of about 19°C). Feel free to ask us for support in your choice.

Normaly hydrangeas are cultivated at 20° to 21°C in the beginning for about 2 weeks until all buds have sprouted, and then reduce the temperature to about 18°C. To achieve a more compact merchandise one can also start at about 18°C and after two weeks lower the daily mean temperature to 15° or 16°C. You obtain a more compact plant with harder shoots, stronger umbels and a more intense colour. The reduction by this method is not very large, though. Additionally, you have to calculate with a 1 to 2 weeks longer cultivation time.  The method is no magic formula, but by using them in combination with other cultivation techniques you can enhance the retardation.


2. Water Metabolism

A reduction of shoot growth has also been watched when plants were being stressed by drought around the roots throughout the growth period. This has already been tested in 1994/95 at the „Staatliche Versuchsanstalt für Gartenbau“ (State Research Institute for Horticulture) in Weihenstephan. The question has been if hydrangeas react to draught stress and if all varieties react in the same way. Three strong growing varieties have been tested under three different treatments each.

One part of the plants was watered needs-oriented. A second part was given a surplus of water and a third was exposed to draught stress. The later got 50 ml water a day for 25 days, then 75 ml for 25 days and at last 100 mal for 25 days. At the time of umbel development the plants were watered needs-oriented. The results were quite impressing (see figure 5).

But one should not forget that such undersupply is not without risks. Especially with blue hydrangeas the version of draught stress is problematic, because due to the administering of aluminum a salinization can arise quickly. The stock has to be checked more often, as well, as it may dry up easily.  

Length growth during forcing for the variety „Leuchtfeuer“ (Haas, 1997)



3. Timely Cultivation Measures

Most important for an alternative cultivation are timely measures. In case of hydrangeas that means an early spacing and maybe an interim spacing. If you have enough room at your disposal it is advisable to place the plants at their final distance from the start.  

Early spacing is linked to only slightly higher costs. The only cost factor actually may derive from the possible necessity to heat up the greenhouse 5 days earlier. When these early cultivation measures are used in combination with “Cool Morning” or “negative diff” you can manage completely without retardants.






This post is also available in: Deutsch