Brief History:

It is commonly believed that greenhouses began with orangeries in the 17th Century and frequently greenhouses are credited to Italian or French fruit growers who created orangeries to bring their tender citrus fruit trees inside.

Whilst it was during the 17th Century that orangeries first became fashionable and widespread among the wealthy the common belief is not strictly true. Because, there is strong evidence to suggest that the idea was around long before the 17th Century with it making sense that the invention was really down to the Roman empire and born out of necessity because they wanted to enjoy favorite foods all year round and in any climate they were conquering, not just what was available and in season.

Initially, tender foods were grown on carts and wheeled around to make the most of the sun during the day and wheeled in and covered to keep warm at night. This led to the cart being covered with transparent selenite crystal to protect the plants.

By the 13th Century, Italians had created botanical gardens to protect the tender tropical plants explorers were returning with from warmer climates. In the 15th Century Angelo Barovier, a Murano glassmaker created a way to make clear glass and soon Murano glassmakers were producing clear glass sheets. Increasingly the wealthy were embracing the clear glass topped structures. They were seen as a status symbol in Europe and many castles and big country houses had an orangery built, which was often a leisure area for the ladies of the house to grow orchids and roses all year round.

These days’ greenhouses are widespread and very useful for gardeners and come in all shapes and sizes, from very small basic setups to rows of polytunnels on a farm or elaborate designs as a statement of wealth. There are many choices of material for greenhouses and there really is something to suit everyone and with the benefit of engineering and scientific developments, today’s greenhouse buyer has a wide choice to suit their requirements. For example automatic opening windows and vents, automatic watering systems and developments of materials like polycarbonate and aluminium.

Most gardeners do so with love and passion, so it is essential to choose the right greenhouse for the purpose based on amongst other things that include needs, wants, location and budget, plus of course what they want to grow. Another consideration is the type of greenhouse environment you are going to create and the greenhouse will fall into these categories:

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Cold House:

To provide protection for plants, there is no heating, so temperatures still change with the seasons and the weather. This type of set-up is used to extend the growing season by starting crops earlier in the spring and can also shelter crops in the autumn.

Cool House:

With minimal heating, the cool house protects plants that cannot cope with very cold weather and the heat is usually thermostatically controlled to maintain an even temperature above freezing.

Warm House:

These maintain a higher temperature allowing a broader range of plants to survive cold winters or to allow the continuation of growing plants out of season that does not need a lot of heat.

Hot House:

Hot houses are used for growing tropical plants in cooler climates or growing plants out of their natural season will almost certainly also need an additional light source.

What is Best:

Today we are going to look at the different materials commonly used to create greenhouses and explore which are best for differing applications:

Glass:

Think of an old traditional greenhouse and you will think of glass and it is still considered the best by many people. A glass greenhouse, is multifunctional, usually pleasing to look at and has many benefits and only a few drawbacks. Glass greenhouses can be plain and functional or elaborate and stylized.  People have been making clear glass greenhouses for centuries but that does not mean that glass is necessarily the best choice.

Advantages of Glass:

• Glass lets in most of the light at 90% or above
• Glass transmits and holds more light than alternative plastic panels
• Glass is easily sourced and is sustainable
• Glass last a long time
• With glass, there are plenty of upcycling opportunities

Remember the main thing plants need to thrive is light. Clear glass lets in more light than any other greenhouse material. So much so that at times it is necessary to coat the upper panels with whitewash or use shades to reduce the heat in the greenhouse during prolonged spells of very hot, sunny weather.

Glass intensifies the light and by transmitting the brighter light the glass intensifies the quality of light to the plants.

A consideration for many consumers is the sourcing and sustainability of the things they buy. Glass is an ancient product that is easily sourced and is sustainably produced. Glass is highly recyclable and easily recycled.

Glass is long-lasting unless damaged. Glass will last for many years and even then if the greenhouse is taken down for some reason the glass panels can still be used elsewhere in the garden or for other purposes.

Disadvantages of Glass:

• Heavy
• Fragile
• Needs a strong framework
• Skill is required to assemble
• Takes longer to assemble

Because glass is heavy it costs more to transport, it is also prone to cracking and corner damage if not packed and handled extremely carefully during transportation.

However, once it is in place, glass forms a strong reasonably windproof shelter or home for plants. In fact, it’s the worst problem during extremely windy weather is other things like branches or garden furniture being blown into it and damaging the panels by smashing them.

Due to being heavier than the alternatives, glass needs a strong and precise framework to support the weight.

Glass greenhouses need care and skill to assemble, in most cases, the panes of glass are slid into place and supported by clips. Care is needed to assemble carefully and in the correct order. It is essential that the framework is correctly assembled to ensure all edges and angles are correct, otherwise, the strain will crack the glass. For this reason, glass greenhouses also need a firm and level footing.

Because glass greenhouses are made with lots of smaller panes of glass that need fitting carefully into place the glass greenhouse takes longer to assemble. This could be an additional cost implication if you are paying someone else to assemble it for you.

Acrylic:

An alternative to glass is acrylic and it is easy to cut to size and work with and although more expensive than polycarbonate it is considerably cheaper than glass, probably due to ease of transportation. Acrylic is a useful, clear plastic and because it resembles glass it is used in places where a safer alternative is required such as shower doors and shielded booths. Acrylic is clearer than polycarbonate, so a greenhouse made with it would more closely resemble glass.

Advantages of Acrylic

• Lower price
• Lightweight and easy to transport
• Durable with good impact strength
• Acrylic insulates better than glass
• High light transmittance at around 85% when new and can be manufactured to stay clear for longer
• Easy to install

Price-wise, Acrylic versions are not one of the cheapest available, so this may be applicable if you are on a tight budget or need to maximize profit. With acrylic, you can have a lower initial outlay and a higher proportion of short term profit.

Because Acrylic is tough and light it is cheap and easy to transport and this too helps in, keeping the cost down.

Acrylic is strong and durable with high strength properties; it does not shatter like glass and will survive most impacts. Acrylic is used for things like bulletproof windows, so should be strong enough for most greenhouse applications.

The light transmittance of greenhouse quality acrylic is very good. By paying a bit more it is possible to buy versions that prolong clarity and can remain clear for up to twenty years.

Disadvantages of Acrylic:

• Energy use in the manufacture
• Short lifespan
• Not readily biodegradable and lack of recycling uses

The manufacturing process is intensive and high energy, so this is not a “green” choice.

Short lifespan, cheapest versions go brittle and cloudy after just a few years as they react to the sunlight. Spend a bit more and look for a clarity length guarantee.

Acrylic is fast being superseded by polycarbonate as a low price alternative to glass.

In common with many other plastics, acrylic takes a long time to breakdown in landfill and there is a strong anti-plastic movement at the time of writing.

Polycarbonate:

Increasingly, polycarbonate greenhouses are becoming more popular and more readily available in stylish designs. Whilst traditionalists will stick to glass come what may, many gardeners are turning to the advantages of polycarbonate.

Polycarbonate panels are increasingly being used in a variety of applications, including greenhouses. Polycarbonate is a thermoplastic and it is tough, resistant to ultraviolet rays and longer-lasting than a lot of plastics. Different grades are produced right up to optical grade for glasses and sunglasses.

Advantages of Polycarbonate:

• Low price
• Lightweight and easy to transport
• Easy to install
• Good light transfer around 83%
• Shatterproof
• Holds heat well
• Resistant to ultraviolet rays
• Available in single or multilayers
• Easily recycled

Price, where the price is a consideration it is likely that the money-savvy buyer will opt for a polycarbonate greenhouse as a much cheaper point of purchase option to glass or acrylic and polycarbonate greenhouses are widely available.

Because polycarbonate is shatterproof and light it is cheap and easy to transport when compared to heavier and more fragile glass. Like acrylic, this helps in keeping the point of sale costs down.

Installing the polycarbonate greenhouse is significantly quicker and easier. It is also a much less skilled job to put the panels and frames together when compared to glass. The sheets are often larger and they can be flexed to create an attractive curved surface.

Polycarbonate does not shatter when hit by flying objects or large hailstones, whilst a tree falling on it is likely to cause damage, the risk of branches and other objects breaking it in high winds is far less than for glass, in fact, polycarbonate is used to make police shields and multi-layered versions are quite strong.  Multi-layered versions cost more but retain heat better and some expert says diffuse more light as well.

Although polycarbonate takes many years to break down it is readily recyclable and there are centers that specialize in the process to shred, wash, grade and granulate for re-use.

Disadvantages of Polycarbonate :

• Scratches easily
• Manufacturing process
• Lightweight means not suitable for high wind locations
• Attracts dust
• Condensation

Polycarbonate is a type of plastic that scratches easily, so consider carefully if you plan to landscape planting or paths around the greenhouse, shrubs and the constant passage of people with equipment could cause unsightly scratches. If these are low down the light and heat properties are not affected much, but the appearance will be.

Plastic manufacturing processes tend to be high energy and there are increasing concerns surrounding the manufacturing process of polycarbonate being detrimental to public health.

A major advantage of polycarbonate is its lightweight, but this is also a disadvantage when it is used with aluminum to make a greenhouse if you live in an area subject to windy weather. The greenhouse could literally be picked up by the wind and blown away. The gardener will need to consider robust systems to anchor the greenhouse to protect from this scenario.

The chemical make-up of polycarbonate makes it an attractant to dust and dirt, so the polycarbonate greenhouse will need regular washing down on the outside to maintain good light transfer to the plants inside.

Polycarbonate greenhouses are prone to condensation build-up between the layers and this can lead to algae build up in impossible areas to clean leading to light reduction. This uncontrolled condensation in the greenhouse can alter the atmosphere and heat as well.

When buying polycarbonate it is worth considering that it is available in several different thicknesses made up with layers of the polycarbonate and whilst a single layer may be cheaper it will have less strength and heat retention than multi-layered versions.

Polyethylene Film:

Polyethylene Film is widely used in large polytunnels to grow commercial crops, commercial growers are well set up with equipment to manage the system utilizing large frame hoops covered in polyethylene. For the garden grower, it is unlikely that this is a serious contender. Although small lightweight grow-houses can be bought very reasonably to overwinter a few tender plants or to use as a partway house when moving plants out of a heated greenhouse.

Advantages of Polyethylene Film:

• Low price
• Lightweight and easy to transport
• Good light transfer
• Can be used in single or multiple layers
• Coated to protect from ultraviolet rays

The low price of polyethylene film needs to be offset against its lifespan of around three to five years. When utilized as a commercial system the polyethylene is easy to fit, replace and manage.

Because it is supplied in large sheets of what is basically UV treated plastic the film is lightweight and very easy to store and transport.

Clear polyethylene has good light transfer and there are opaque options available where controlled conditions are needed but less intense light.

A major advantage of this system is that the film can be used in single or multiple layers. Single layers will let more light in, but allow more heat to escape. However, many commercial growers utilize a system whereby warm air is blown between layers to create an insulated area the heat retention is far more efficient and this is a cost-effective method of increasing heat when required.

Proper nursery standard film retains the low price but is treated against ultraviolet breakdown to prolong the life of the film.

Disadvantages of Polyethylene Film:

• Not visually attractive
• Can tear and split
• Short lifespan
• Biodegradability

For the garden user, the polytunnel is probably the least attractive option. For commercial growers with the equipment and systems in place, they are proven to be highly effective. But when balancing appearance, initial installation, and the short lifespan they are not a realistic choice for many gardeners.

The polyethylene is prone to tearing and splitting and if a split is caught by the wind it will soon increase in size rapidly.

The average life of a polytunnel is around three years, some growers manage to gain longer, but three years is a sensible assumption.

Whilst plastics are biodegradable they take many years to break down and there is a lot of bad press about plastics. As the sheeting used is actually treated to slow the breakdown, this may need to be a consideration when making a purchase.

A Mention of Frames:

Generally, the framework of greenhouses is either aluminium or wood.

Frame Materials:

There are many options for frame material when buying a greenhouse these days. Many of the mass-produced versions have aluminium frames or anodized aluminium frames. Frames are also made using steel, plastic or wood with Cedarwood being the most expensive and considered the best.

Aluminium Frames:

Aluminium is the most used frame these days and that is probably because it is lightweight, easy to store and move as well as being the least initial outlay and needing little to no maintenance. Unpainted aluminium framework will spot and fade as the season’s pass and will last many years with just cleaning as part of the greenhouse cleaning routine all that is required.

Painted, anodized and plastic coated options are available and these cost more and require periodic painting and possibly repair or replacement to the plastic coating if it gets damaged. The aluminium framework tends to be very thin, so does not block out much sunlight and also only has a very slight shadow, which is great for optimizing light. A major advantage of an aluminium frame is that it will not rust or deteriorate in use.

Widely used due to its ease of manufacture, lightweight and low maintenance. Aluminium is long-lasting and a well-engineered greenhouse frame will give many years of trouble and maintenance-free service.

Steel Frames:

Steel tends to be found in commercial situations and many of the older greenhouses and orangeries have steel frames. Steel has largely been replaced by aluminium.

Plastic Frames:

I have not known a plastic frame that has the necessary rigidity and longevity required for a greenhouse. Plastic works with polyethylene because both will have a short lifespan.

Wood:

Wood is a highly attractive and tactile choice and perfect, where a beautiful structure is needed.  However, wood does require maintenance, and the thicker framework will cast shadows.  Softwood is cheaper, but will not last as long and require regular wood treatment or painting.  Cedarwood is more durable than treated softwood but less durable than aluminium, but is beautiful and because wood retains heat well it is a good choice. 

Some greenhouses come with partial wood sides at the bottom, which retains more heat, but blocks out some light and this option is a consideration depending on the requirements of the gardener. For example, a grower wishing to grow tall, long-stemmed flowers might choose this option because the flowers will grow up to the light coming from above the lower wooden area.  Equally, a gardener growing plants on a raised area will benefit from Cedarwood for the bottom of the sides to keep the greenhouse warmer when the external temperature dips.

So What is Best Glazing Material for Your Needs?

All these materials are extremely good at letting light and heat in. However, they are also extremely good at letting heat out as well. Because they have such a large area designed to let light and heat in, all, greenhouses will usually overheat during the day in very hot weather unless some form of temperature control is utilized.  Then, because they provide very little insulation, if the temperature drops at night they lose a lot of the accrued heat.  This can be lessened by using heat-retaining slabs to walk on in the greenhouse and adopting raised bed systems that are above the ground frost level.

All of the materials above work for the following uses:

• Bringing on seedlings
• Extending the growing season
• Starting outdoor plants
• Growing plants that would not thrive outside in the area you live
• Healthy fruit and vegetable plants that produce more fruit and vegetables
• Fresh cut flowers all year round
• Cold, cool and warm house systems

Glass, acrylic, and polycarbonate are also effective for:

• Overwintering plants
• A hothouse system

Glass is the best for

• Growing seedlings, cuttings and plants that require high light levels.

So whilst it is easy to see that a glass greenhouse with an aluminium frame is the most versatile it is also possible to utilize other materials depending on your requirements. A beautiful Cedarwood and polycarbonate house could have a gently sloping roof and be beautiful and functional at the same time.

Weather Considerations:

The lightweight structures created with non-glass systems may not be suitable for windy areas or careful consideration is needed to ensure they are well anchored down in place utilizing large ground spikes that will need checking from time to time and in the case of polyethylene sheeting additional strapping over the structure may be needed in high wind to prevent tearing and losing the house.

Glass really is not the best solution if you have a high occurrence of hailstones, the stones could shatter and even strengthened versions may crack and break and dealing with broken glass is never pleasant.

Finally:

Using a greenhouse extends a gardener’s growing season and widens the array of plants you are able to grow. While the structure itself is relatively simple to build, there are several other elements needed to create the proper temperature and environment that will make your greenhouse a successful place for starting and raising your favorite flowers, vegetables, and plants.  Things like the positioning of the greenhouse and the plants within it, ventilation, heat, shade, irrigation and/or location of water supply are all considerations that will influence the choice of the greenhouse.

Because light is so crucial to enable the plants to photosynthesis a gardener using a greenhouse may need to arrange the plants to make the most of the light and position them according to their individual needs. A partially wood structure may be the best option if your plan is to grow long-stemmed plants, but too much light would be lost for things like salad vegetables if growing at ground level.  It is possible to shade the lower clear section with sacking or even use sheets of non-clear polycarbonate or acrylic to encourage taller plants to grow up to seek the light. Many greenhouse structures can be adapted with the use of benches, raised beds, shading, and choice of ground cover to cater for more than one group at once.   Although if you wish to operate more than one type of house i.e. you need a cold system to protect plants and desire a hothouse system to grow orchids you will need separate greenhouses for each purpose.

The choice of material may well, in the end, come down to aesthetics, that is choosing your preferred appearance to suit the rest of your garden.  A methodical ordered type of person is likely to choose a tidy, uniform row of rectangular greenhouses, whilst an artistic person may choose an elaborate beautiful structure and some gardeners would choose different structures for different parts of the garden.

When it comes down to it unless you live in a very windy area or have a high risk of large hailstones, there is actually such a small difference between the choice of material for the hobby gardener where margins of light and heat are not so critical when not chasing profit.  It may, in the end, come down to what is available in your area and the initial outlay cost.  What is certain is that glass is the only covering that excels at all levels and a glass and aluminium greenhouse will last the longest with minimal care.