Protected cultivation


This sub-theme covers the subject of how plants respond to a number of abiotic and biotic stresses and how this can be efficiently used in protected (greenhouse) cultivation and in the field. Essentially Abiotic Stresses result from climatic factors: soil contamination, salinization, light, ozone levels, temperature extremes, water and minerals, relative humidity, wind, hail and frost, overuse of fertilizers and plant protection chemicals. Biotic stress derives essentially from pests, diseases and parasitism. In most cases plants already under significant abiotic stress are more vulnerable to biotic stress.

RDI – REGULATED DEFICICT IRRIGATION: is a cultivation protocol that replaces less water than plants loose through transpiration. It has been found to induce the production of stress hormone abscisic acid (ABA) that at the growing tips limits water loss through evaporation and slows down stem elongation. In some crops this has enabled growers to significantly reduce the application of plant growh regulating chemicals and reduced the water consumption by up to 20-30%.

DIP – DEFICIT IRRIGATION PRACTICE used especially in growing grapevines in hot dry climates. Irrigation applied to alternate sides of a barrier that separates the root system into two parts. The resulting water stresses causes the plants to produce stress homones that positively affect fruit quality and control successive vegetative growth.


Through temperature control, some techniques limit plant growth without applying growth regulators. For example, the DIF (difference between average daytime temperature (TD) and average night temperature (TN). If the value is negative (cold day – warm night) on some plants – among ornamentals, the poinsettia and the chrysanthemum – limits the lengthening of internodes. The technique is more commonly used in Northern Europe, because in the most southern countries the daytime temperatures are always too high.

Instead, the technique used in southerly climates is the ‘COOL MORNING’ (cold morning). It can be applied if the temperature difference between the inside and outside of the greenhouse is between 3 and 5 ° C. In the coldest time of the day (about 2 hours before dawn) the greenhouse in opened and the heating switched off for about 4 hours. Since the early hours of the morning are those with the most active growth for the plant, the low temperatures slow it down and the plant is naturally dwarfed. The average daytime temperature is not lowered because this would lengthen production time.

Naturally occurring plant stress dynamics are exploited in the cultivation of plants in RETRACTABLE ROOF GREENHOUSES.


Plant stress dynamics are exploited naturally in the cultivation of plants in RRG’s (sometimes called Open Roof Greenhouses). This technology  is having an increasing impact on the way growers cultivate plants.  A typical example comes from Australia On the market over the last 15 years, there are numerous models with various methods of opening and closing the roof. Retractable styles include the roll-up (see Rovero  -PT) and the curtain type (PT) (; while the open-roof types (which most often have polycarbonate or glass glazing) include peak-hinged DeForche and gutter-hinged styles from Van Wingerden. See also manufacturer Idrotermserre.

One trend in greenhouse construction aims at a perfect environment for plant growth and development reducing plant stress to almost zero. In contrast, the philosophy behind retractable roof greenhouses (RRG’s) is that plants respond positively to some stress thereby growing better and more naturally. I was privileged to meet Richard Vollebregt, CEO of the Canadian Company Cravo Equipment Ltd, one of the pioneers and leading exponents of retractable roof greenhouses (RRG). See Cravo Corporate Video. The measuring of leaf temperature in strawberry plants grown in tunnels and in a retractable roof greenhouse perfectly illustrates this. See: Video with audio summary by Richard.

Sustainable Strawberry Project 2014/2015. The sustainable strawberry project co-funded by Marks and Spencer, Total WorldFresh and Cravo Equipment Ltd. has completed it’s first year results. Crop yields in the automated retractable roof house produced 96% first class fruit and an increase of 22% overall, in comparison to tunnel production. High average Brix factors were recorded successively as the season advanced. One of the most promising results, demonstrated so far, concerns the reduction of chemical fungicides and pesticides. Using the Cravo control system as a combative tool, facilitated an observation based attitude to disease control. Consequently this led to a 94% reduction in chemical applications and a 0% residue test result. According to Andy Mitchell, agronomist at Marks & Spencers, the Cravo System has the potential to change the industry as we know it. For M&S it delivers: pesticide reduction, quality improvements, water capture and products the result of reduced environmental stresses. For our growers it offers a true solution on sustainability in the very competitive area of fruit production. See: Innovation Leadership Award  See also: Cravo – strawberries and  overview trials 2015

A more natural way to grow plants. Cravo has worked with new customers in Asia, Africa, Europe, South America and North America helping them to improve their breeding programs, growing of young plants, as well as production of cherries, vegetables, trees, and flowers. See: cultivating cherries under retractable roof greenhouses with compariusons. Hasel Tarim, Altalya, Turkey, is a leading supplier of high quality vegetable seeds (mainly tomatoes and cucumbers). They have  installed a 7,200m2 Cravo retractable roof greenhouse, the first such structure in Turkey. Hasel Tarim pdf Video Hesel Tarim retractable roof greenhouse

Q. 1 How to appreciate the role of Retractable Roof Greenhouses?

Above all, it is necessary to think like a plant. The logic behind the positive effects of using retractable roof greenhoses (RRG’s) requires an open mind and willingness to go back to the drawing board, placing oneself in the position of ‘thinking like a plant.’ A grower needs a series of instruments to measure the incident light arriving at the greenhouse roof and that arriving at the surfaces of the plant leaves, the temperature and relative humidity in the greenhouse environment, the moisture levels in the soil, the temperature of the leaf surfaces. Trials with RRG’s have demonstrated that crops can be grown with a reduction of 10% – 100% in the application rate of plant chemicals, growth regulators and fertilizers. Ways that retractable roof greenhouses can increase profitability pdf

Q.2 For what crops is the Retractable Roof Greenhouse most suited?

Retractable structures are suited to most cut and potted ornamental crops and many high value (or slightly delicate) vegetable and fruit crops. Trials in the US have demonstrated the highly effective use of RRG’s in the production of aromatic herbs (either for the fresh cut market) or as potted plants. Data demonstrates that the polyethylene roof coverings of RRG’s (that provide about 25% shading) is more effective at preventing heat build up in plant containers when compared to no roof or 50% black shade cloth.

Video presentation, tomato trial Cravo retractable roof greenhouse, Capgen Seeds, Culiacan, Mexico. Spanish language, English subtitles. Carlos Ruiz Mapula shares his insights from his experience at the Cravo Demonstration Unit at Culican, Mexico, in this video, May 2016. Take a look at a practical and informative series of videos on the Cravo website, including: Welcome to the Cravo Demonstration & Education Greenhouse (CDEG) Cravo Demonstration & Education Greenhouse; Tomato production in Demonstration & Education greenhouse 2016

RRG’s can also be used to advantage for some crops traditionally grown in the open field. A Mexican company has been growing 42 ha of  tomatoes (Roma type) in Cravo RRG’s. A 35% increase in profit per kg and a 165% increase in profit per hecatre is claimed when compared to using low cost insect net or conventional plastic greenhouses. Under RRG’s, plants are field grown when outdoor conditions are optimal and are protected when otherwise. See Video In a Cravo demonstration unit in Mexico, almost 100% pollination took place in cucumbers, tomatoes and peppers without the use of bees. There were no signs of heat stress despite air temperatures in excess of 35°C.

Growers of outdoor crops often find a significant reduction (up to 50%) in the length of crop production cycles when growing herbaceous perennials and woody plants under RRG’s.  Young plants (tree and shrub liners) potted in the spring are ready for transplanting in the autumn. The resulting liners are also taller with a greater calliper and root mass and have a high survival rate on transplanting to the field. If plants are placed under a mild temperature and nutrient stress, finished plants will also hardier with better shelf-life, although the production cycle may be lengthened. The State University of Ohio now recommends growing liners in containers in RRG’s rather than in the field. A useful overall view of RRG’s was published by Glasshouse Product News June 2002 in article by Sven E Svenson of Oregon State University.

Q.3 Do retractable roof greenhouses have other uses?

A garden centre in Portland, Oregan, is using an RRG to provide ideal conditions for plants and  customers, whether in greenhouse, shade-house or the outdoor sales area, regardless of the weather conditions outside. A notable increase in sales was obtained plus a reduced crop shrinkage by over 85%.  In Italy, Cravo RRG structures are under consideration by dairy farmers to provide improved conditions for cattle.

Q.4 Incursions of Cravo’s RRG’s in Europe? 

The Company has an agency in Spain. More information from the company’s Canadian headquarters. Some growers of perennials in Germany and Poland are successfully using RRG’s and a special model called ‘Modello Piccirilli’ has been designed for cut flower production on the sloping hillsides of Liguria in Italy (see image below). Constructed for the Az. Agr. Piccirilli, S. Stefano Al Mare. The roof structure is a modified A-frame with a 10% slope (about 18 degrees). The roof material is waterproof, so when closed water will run off due to the slope. It will also support a snow load. The roof of the 4,000 m2 structure can be opened or closed in 2.5 minutes, its position being automatically controlled by sensors linked to a climate control computer and weather station. Sidewalls can also be opened to enable cross flow of air to reduce insect populations and help keep plants more compact. Insect screens can also be installed. This also helps reduce temperature during hot days when the roof is closed 85% to provide shading and cooling.

Q.5 What are some of the physiological details behind the success of Retractable Roof Greenhouses?

RRG’s expose plants to the best of both the natural outdoor growing environment and a controlled greenhouse environment while avoiding most of the negative aspects of each. The speed and quality of growth is greatly improved when plants are exposed to optimal outdoor growing conditions because these conditions create natural levels of plant stress which influence how plants develop. This can easily be achieved if the greenhouse roof is closed when the temperature is below 17°C,  to trap the heat, and opened when the outside temperature is between 18°C and 28°C  to allow plants to increase both speed of growth and plant quality.

RRG’s permit increased air movement, reducing relative humidity. They also permit higher exposure to UV light and greater infrared radiation. These contribute to increasing natural plant stress, allowing plants to transpire normal amounts of water. In general, plants lose too much water outside and not enough inside a greenhouse. If a plant looses a lot of water it will take steps to ensure a more constant water supply by growing more roots and a more active root system. Under such conditions, leaves tend to be smaller with a thicker cuticle, more resistant to insects and diseases, also helping the plant protect itself against excessive transpiration. Risk of foliar diseases such as Botrytis is lessened since leaf surfaces will dry-off quickly. By making it easier for plants to take up and transport minerals to the growing points, important deficiencies (such as calcium) are avoided.

Under increased PAR light, CO2 availability, leaf and soil temperatures, plant growth and quality will improve through increased photosynthesis. Internodes will be shorter and more regular, and you can expect a natural balance between roots and shoots with an increased level of generative growth. This leads to an increase in the number of flowers and the yield of fruit.

The roof is closed 85% when the outside temperature is greater than 29°C to provide some shade and cooling. This reduces the level of infrared radiation and as a result it prevents negative effects such as excessive leaf, flower and fruit temperatures and water stress. Excessive temperatures lead to the bleaching of leaves or flowers and excessive transpiration. By preventing excessive transpiration, the stomata will remain open, or their closing will be delayed, and photosynthesis can continue even around midday. Plants will also retain their turgidity, making them less attractive to insects; it is easier for insects to attack droopy, flaccid leaves. Furthermore, insects do not like being in direct sun in the daytime or high wind.

Q.6 To what extent can Retractable Roof Greenhouses resist strong winds? 

To date Cravo structures have resisted hurricane-force winds in the US when conventional greenhouses suffered severe damage.The main reason is that the covering material is held underneath the stainless steel guide wires of the roof. The roof material is connected to these wires by hooks. Retractable roofs with an A-frame structure have now been designed by Cravo to withstand winds up to 200 kph  and their flat-roof structures can with stand up to 180kph. The roof material is made of woven reinforced polyethylene film that resists tearing and has a lifespan of five to eight years. The roof material of the flat roof structures is porous so that rain will pass through it, but it will still hold in heat.

Retractable roofs are not for every grower in every climate. But they have gained favour with many growers and in the cultivation of a wide range of crops. Container rose production pdf Cherry cultivation pdf

Q.7  How does all this fit into greenhouse design and crop management?  

We know no one better than Richard Vollebregt to explain plant growth and development. In 2010 he produced a practical manual in English “Re-writing the guidelines for greenhouse design and crop management”. It is for growers, horticultural students, plant physiologists,  greenhouse technologists and extension workers interested to better understand how plants interact with the internal greenhouse and outdoor environment and what to do about it. Through a better understanding of these interactions growers can increase the quality and productivity of their plants and make savings in terms of natural and man-made resources employed, including energy. It has taken him 8 years to write and has lead to him creating the tradmark ‘Retractable Roof Production System RRPS™’. Requests to translate the work into other languages have been received by the Company but no plans are yet in the pipeline.

Further details of irrigation and temperature control and innovative agro-textiles and ideas for their application in all segments of agriculture will be considered.

©2013/2019 | HORTCOM


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