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VHB and ReduHeat

ReduHeat helps to buff er the temperature, so we don’t have to open the vents so wide and humidity can be maintained

ReduHeat helps to buff er the temperature, so we don’t have to open the vents so wide and humidity can be maintained

Good light levels are vital to pot herb production, both for speed of throughput and crop quality, but plants need to be protected, too, from excessive summer temperatures.


VHB’s West End Nursery at Angmering, West Sussex, on the south coast of the UK, has been using the temporary glasshouse coating ReduHeat as a way to shade plants between the end of March and August for the last four years.

Five hectares of the 6ha of glass at West End are dedicated to pot herb production – 11 million pots are grown each year for several of the UK’s leading supermarkets. That means general manager Simon Smith and his 18-strong nursery production team work to demanding production schedules and quality specifi cations.

“All our production planning is based on knowing how many days are needed from sowing to the fi nished crop for each type of herb,” says Mr Smith. “Some are particularly light sensitive; for example, a crop of basil takes 36 days in June but 55 in the duller conditions of winter. “And for all the herbs, crop quality is very sensitive to light in terms of the plants’ appearance and the infl uence of diff erent wavelengths on fl avour compounds.”

Clearly, any reduction in light levels in summer would have a serious impact on production schedules, which is why the nursery opts for ReduHeat, because it blocks out light at the wavelengths that cause heat to build up without losing too much of the light that the crop needs for growth. “Being able to reduce heat without losing light transmission is especially important in spring and autumn,” says Mr Smith. “Glasshouse screens supplement its eff ect in the height of summer.”

Pot herbs need a ‘buoyant’ atmosphere so the glasshouse is vented throughout the summer. “The properties of ReduHeat mean we can keep minimum vent in dull weather without losing too much heat because there is still enough solar gain,” says Mr Smith. “The coating also helps prevent the stress the crop can experience when we get extremes of temperature, for example, when we move from dull to bright weather.”

Basil, one of VHB’s biggest lines, tends to scorch under very bright conditions – but if the vents have to be fully opened to cool the glasshouse, the humidity can drop too low for the crop, to around 40% RH rather than the 70% RH Mr Smith aims for. “ReduHeat helps to buff er the temperature, so we don’t have to open the vents so wide and humidity can be maintained,” he says.

Coriander is one crop that really benefi ts from summer shade. “Too much light makes plants too ‘soft’,” says Mr Smith. “We currently use screens to provide additional shade but I would prefer not to because they can restrict air movement. I understand there may be scope in future to develop coatings that act as spectral fi lters too.

Ideally, we would like to be able to shield light at the wavelengths that induce the plants to stretch, as well as those which cause heat to build up. In the meantime, we are considering using a higher concentration of ReduHeat on the areas of the nursery used to grow coriander as it would mean we could keep the screens open for longer.”

Mr Smith says he’s been really pleased with the consistency ReduHeat off ers over the four to fi ve months he uses it. “Compared with coatings I’ve used in the past it breaks down less readily and is less prone to washing off in a rainy summer – yet it’s very easy to remove, with ReduClean, when you want to,” he says.

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One type of screen at all locations makes managing the cultivation uncomplicated

Tholen – Make the right choice once for a screen cloth and then ‘roll out’ this installation in phases over all your greenhouses.

Tholen – Make the right choice once for a screen cloth and then ‘roll out’ this installation in phases over all your greenhouses. That is what Rick Tesselaar has chosen. In 2017, Rick started thinking about his screen choice. He knew exactly what he wanted: one canvas. And that cloth had to be energy-saving and serve as a sunscreen on hot days and meet the guidelines for light emission during the period of exposure.

Contrary to the trend of installing two or three canvases one above the other, Rick has found his canvas that meets these three requirements. The first phase was completed in 2018. In the meantime, almost completely screened with PhormiTex 77 from Phormium in the 10 hectares of glass. This year the last meters will be taken care of. Rick Tesselaar: “Of course we measured and assessed the performance of the screen after the first phase was installed. We had no complaints about that. There was no reason to go back to the screen choice for the following phases.”

Fortunately, it went well, because Rick does not want to make the work for his cultivation manager unnecessarily complicated. “There are already so many variables. One screen for all three locations is easy to control.”

Sheep with five legs

Rick: “This fabric is the golden mean for us. It can be used for many purposes. This is the optimum for us; the result and costs have been weighed up. We use PhormiTex 77 in the summer as a sunshade, at night as a light screen and in winter as an energy screen.” On hot days, it closes the canvas for 60%. We meet the light emission requirements; we can close it well at night.” And the aluminum strips of this woven fabric counteract radiance. That was important for Rick. Because if the radiance is too high, the heads can get too cold and they can pop off. That’s what the grower wants, of course. not: “Those aluminum tracks are a godsend for this.”

Arjan van der Veer of Phormium adds with a laugh: “Rick puts it well. One thing is worth mentioning: the aluminum strips in the PhormiTex line retain the heat better in winter, but in summer they are they are also cooler due to better reflectivity than the white straps.”

As a grower, Rick was advised by colleagues and by his permanent screen installer. Huisman Scherming is their cloth specialist. Rick was in contact with Dave Boer about screen selection and installation. “The great thing about Dave is that he gives clear and good advice. He sincerely helps you find the best solution. That’s nice.” 

Tesselaar Alstroemeria

Tesselaar Alstroemeria annually produces more than 36 million stems under 10 hectares of glass. And therefore belongs to the larger alstroemeria growers. The different types of alstroemerias are grown at three locations in Luttelgeest. They deliver to wholesalers, retailers, supermarkets and have a cash & carry. They agree the method of delivery with the customer.

The company is led by Rick and Karolien Tesselaar. They are a bit quirky, as they write on their website, and like to chart their own course. Rick: “There is a large market for alstroemerias. We consciously build our organization with well-qualified personnel. Growth should not be limited by people. That is why the qualifications of our personnel are absolutely leading. Our company must continue to develop. system must be stable.” 

Thermal and shading screens inside a greenhouse in northern Europe

An ideal choice for sun-shading in wide span greenhouses

A rolling roof screen gives you extensive control possibilities. It permits both two and one-sided screening. For example, just on the sun side, so that burning is prevented. Or just on the cold wind side, so that you save on heating costs during the day while your crops keep on growing. The screen sections can be controlled separately for aeration purposes, so that your greenhouse is always ventilated. Our advisors can help you choose the right fabric foryour crop as well as the optimal installation possibilities.

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Indoor climates

Venting naturally in the summer

Sometimes the simplest forms of venting make the most sense.

Sometimes the simplest forms of venting make the most sense.

Temperature and humidity – maintaining this balance can be a constant source of struggle for greenhouse producers. As summer time rolls around, sometimes the simplest forms of ventilation can make the most sense in time and expense.

“Natural venting is by far the most popular form of ventilation for modern greenhouse growers in the Northern Hemisphere,” says Leigh Coulter, president of GGS Structures, based in Vineland, Ont. As she explains, natural venting is equally popular for gutter-connected and ground-to-ground freestanding greenhouses, where this type of ventilation can excel. To understand why, Coulter draws a literal picture shown here.

“Roof vents operate on the principal of the chimney effect. Hot air inside the greenhouse rises naturally. By placing a venting on the leeward side, wind travelling over top the vent creates suction pressure that pulls the hot air out of the greenhouse. Vents closer to the ground on the windward side let cooler outside air come in to replace the hot air that is being pulled out.”

Gutter Connect chimney effect 1
With vents on the leeward side, wind travelling over top creates suction pressure that pulls hot air out of the greenhouse. Vents closer to the ground on the windward side let the cooler outside air into the greenhouse

Built from taller posts, the height of gutter-connected structures allows for a greater chimney effect. Freestanding greenhouses can make use of both roof vents and rollup sides to increase the amount of natural ventilation. In a correctly designed greenhouse, the results would be even cooling within the crop, for a more uniform product.

That being said, there are situations where natural venting may not be as suitable.

“If you need your greenhouse temperature to be below ambient temperature and you are in a dry climate zone, then fan and pad cooling is better than natural ventilation,” Coulter explains. “But fan and pad do not work as well in humid climates, so natural ventilation is better in those situations.”

Before the summer heat peaks, Coulter recommends checking airflow from vents, having racks greased and conducting a proper maintenance check. “To visualize airflow you can use coloured smoke bombs as a low tech solution to see how the air flows through the greenhouse. This technique can also be used to see where you have air leaks that you may want to eliminate or reduce before winter.”

She emphasizes that ventilation is only one part of environmental control. “Humidity and temperature and light levels also play a roll in determining how the greenhouse systems operate. A good environmental computer will take inputs from both the outside environment and the growing environment and determine what needs to be adjusted to maintain the ideal environment for the crop.”

For those looking to retrofit older greenhouses, Coulter says they’ve been able to add natural venting to many older greenhouses over the years.

“All that is required is for us to know the structural details of the existing greenhouse, the geographic location for climate data, and the crops you are growing. From there we can build a plan with the grower.” 

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Venting naturally in the summer 96

In January’s issue of Greenhouse Canada, readers were introduced to the concept of ‘Growing by Plant Empowerment’ (GPE). Combining grower experience and knowledge of plant physiology, the goal of GPE is to optimize the behaviour of plants in the greenhouse environment by maintaining critical balances involving energy, water, CO2 and assimilates within the plant.

These balances can be monitored by sensors, combined with crop measurements, then interpreted in the context of plant physiology and physics to help finetune and improve the crop.


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Solar Energy During Winter For Summer Crops

Alternative systems for heating greenhouses for growing vegetables and fresh herbs in the winter

growing vegetables & fresh herbs winter
 Alternative systems for heating greenhouses for growing vegetables and fresh herbs in the winter.

As a result of increasing fossil fuel prices and restrictions on emissions of CO2 and other pollutants, there is a need to find alternative systems for heating greenhouses for growing vegetables and fresh herbs in the winter. One possible alternative is the use of solar energy. Heating greenhouses during the night with solar energy requires a combination of several components based on the following principles: 1. absorption of the energy from the sun (during the day), 2. storing the energy and avoiding losses of energy to the surroundings and 3. using the energy at night.

Absorption of solar energy is increased by covering the soil with a transparent plastic sheet. The wetted soil has a capacity to store a large amount of heat energy. An additional method of absorbing and storing solar energy is by placing horizontal and vertical transparent PE (polyethylene) water tubes in the greenhouse tunnels (figs. 3 and 4). Using black PE to construct the water tubes allows for greater heat absorption than the commonly used transparent tubes.

21.03wetted soil
The orientation of the greenhouse can also increase the absorption of solar energy. A walk-in tunnel orientated east-west will absorb substantially more energy than a similar structure orientated north-south. Placement of the self-supported vertical water tubes on the north side as a “wall of water” increases energy absorption and storage therefore raising greenhouse night temperatures up to 16 C0 in Israel, depending on local radiation and climatic conditions.

Losses of energy to the surrounding is reduced by covering the greenhouse with a double layer of plastic sheets with IR blockage, with a gap of air between the sheets or by the use of thermal screens or insulation blankets. From sunset when there is no more accumulation of heat, the water tubes and the soil release heat energy in to the volume of the greenhouse.

Similar practical applications of these principles are used elsewhere in the world. In parts of China solar energy is stored in a thick wall made from mud or clay bricks. In Israel we have applied these principles in walk-in tunnels for the growing of basil. The combination of these components have allowed us to grow basil (a summer crop) in the winter, while increasing production, preventing plant diseases, and improving quality while reducing costs and emission of greenhouses gases.

Using horizontal water tubes is not a new method for it was tried years ago. Even though the horizontal tubes have some contribution to warming the greenhouse, they are located in the worst place in the greenhouse, the coldest and most shaded place, and they are exposed to mechanical damage. A unique solution of arranging the water tubes vertically  is a good and effective solution to the problems of the horizontal water tubes. They stand independently they are not supported by the greenhouse structure. They have a rigid frame made out of a metal sheet sleeve, or are supported by using a sleeve made from a metal wire mesh.

The vertical tubes are exposed to the sun and they are not in the way of the workers in the greenhouse. It is possible to store a much greater volume of water in the greenhouse to enable greater storing of heat energy. Positioning the vertical tubes correctly reduces shading problems winter and is suitable for multi-span greenhouses and north-south orientations in walk-in tunnels. 

The orientation of the walk- in tunnel has a major influence on the absorption of solar energy. The east- west orientation permits much more sun light to enter the greenhouse. In winter, when the sun is low, the sun beams hit the plastic covering of the north south orientated walk-in tunnel in a sharp angle. As a result part of the beam is reflected. The sun beams hitting the east- west orientation are almost at a right angle, which results in much more light entering into the tunnel, leading to higher yields.21.03figure 9

This knowledge gave way for the development and construction of the “Eden” greenhouse. The “Eden” greenhouse is oriented east – west and the vertical tubes are located on the north side. They form a “wall of water”. This location has an advantage that there is no shading on the crop and there is no physical disturbance to the workers. A relatively large amount of water can be kept in the walk in tunnel (8 m3 water in a 30 m length walk-in tunnel). The “wall of water” absorbs the solar energy during the relatively hot days and releases the heat during night, creating optimal temperatures for crop production.

The reason for using water in the tubes is the high specific heat of water compared to other materials. Water is available and will not contaminate the soil if the tubes are damaged. A thermal picture that was taken at night shows the heat stored in wall of water and the influence on plant temperature. The yield of the basil crop was winter significantly higher in walk-in tunnels equipped with a wall of water .

In conclusion, we developed a simple, sustainable, nonpolluting, no emissions system for growing summer crops in winter by raising the temperature using only solar energy. It is possible to grow basil crops in the winter in Israel by using: PE mulching, water tubes, thermal screens and double layers of PE covering material. The best results were achieved by using a wall of black PE water tubes standing on the north side of the east-west orientated tunnel. 21.03figure 10

Orientating the tunnel in an east-west direction has a considerable advantage over north-south orientated tunnels. This method made it possible to grow basil free of winter diseases without needing chemical spraying of fungicides. Depending on the climatic conditions there is a possibility of using only some of the aforementioned methods to produce high yields of excellent quality.

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Winter For Summer Crops



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