Greenhouses and polytunnels are both structures that allow for regulation of climate and enable you to grow a wider range of vegetables, fruits and plants than you would be able to outdoors. Temperature, light and shade, humidity levels, irrigation etc can be controlled to varying degrees depending on which of the structures you utilise and what modifications you may add along the way.
Each structure has distinct characteristics and advantages that may suit different goals or circumstances. Which one is the best for you? Here are some of the factors to take into account.
Polytunnels are by far the more economical choice, especially if you have a large garden or area for planting. The larger the area you intend to use in square feet, the more money you will save with a polytunnel simply because the polythene cover and simple hoop system is much cheaper than the more complicated greenhouse frame and glass. While polytunnel polythene/plastic may need to be replaced after 5-7 years, it is normal for the cover to last 10 years or more if the structure is in a sheltered location and any rips or tears are repaired with polytunnel repair tape.
If you have a warm south facing wall on your property, a less expensive greenhouse option is the lean-to greenhouse, which is constructed against either a garden wall or an outer wall of your house. There are also small 'starter' greenhouses with a modest amount of room for potted plants and seedlings.
Ease of Construction
Again Polytunnels have the advantage here. Many greenhouses will need professional installment, or at least someone who really knows what they’re doing. A polytunnel can be assembled in the space of a day - it is a two-person job though so ask for some help!
Another possibly limiting factor is that greenhouses need to be constructed on a flat, hard surface. Polytunnels, on the other hand, can easily be placed on uneven soil or slopes.
The simplicity of the polytunnel structure makes it easily adaptable whether you wish to add wider doors or extra ventilation at the gable ends. Polytunnels are also easily extendable by adding extra hoops and re-covering with new polythene if your 'grow your own' hobby turns into a bigger project.
Greenhouses are much more difficult to modify once in place, and the glass panels can be a little treacherous if doing so. However greenhouses are very adaptable as they can be easily fitted with rainwater collection systems, automatic window venting and other methods that work towards providing an ideal environment for crops and plants to grow.
This is one area where greenhouses really come into their own. Polytunnels have a very functional, utilitarian look to them - while that’s still a very pleasant environment to be in, they do not look so good from the outside. The clean lines and fully translucent glass of a greenhouse looks attractive from the outside and tends to emphasise the lush beauty of different plants and foliage within. Glass has another slight edge over polythene as many of the plants that really benefit from being grown in a greenhouse are ones that thrive on direct light and sunlight.
A greenhouse can also be a very meditative place to spend your time, almost like having a portal to another, more peaceful environment.
This one can be debated. While greenhouses are technically ‘for life’ once they’re installed, accidents can always happen, whether it’s wayward footballs or extreme weather and glass panels can be fairly expensive to replace.
Polytunnels on the other hand have become much more durable and weather-resistant, particularly if you look for the right materials like high tensile steel or heavy-gauge, insulating polythene. Polytunnels can be specified to withstand severe storms without damage to the structure, and if the worst happens the replacement/repair costs can work out relatively cheap.
Heat Retention and Light
Greenhouses have superior heat retention in the summer months - indeed almost too much if they are not adequately ventilated or shaded as there may be a risk of heat spots and scorched plants. This can be countered by shading your greenhouse with suitable shade paint or netting. In the winter greenhouses can be susceptible to drafts and may need more artificial heating, they can also experience a wide temperature range as the relatively small space heats up and cools down quickly.
Polytunnels have a more even heat retention throughout the year - they won’t match the heat retention of a greenhouse during the summer months, but they can be somewhat less susceptible to low temperatures in the winter depending on what kind of cover you have (thermic polythene can prevent heat loss during cold spells).
Another area where greenhouses come into their own is with light transmission - plants that enjoy direct sunlight will thrive in the environment of a greenhouse. Lush, exotic-looking plants are particularly suitable and popular with greenhouse enthusiasts.
Ventilation and Irrigation
Climate conditions can be more precisely controlled in a greenhouse via the use of HVAC systems, automatic venting windows and rainwater catchment systems. The HVAC systems can monitor temperature and take cooling action when the greenhouse is overheating.
Having said that, a polytunnel can be adapted or optimised in many similar ways. Ventilation can be increased by widening the entrances at either end or by raising the side. The most important thing for ventilation and air flow (which will prevent condensation and mould-growth) is to ensure that doors or vents are left open when the weather allows to allow air to flow through the structure.
Similarly, foggers/misters can be used to reduce the temperature in hot conditions. Water collection and irrigation methods (such as a gutter system) can also be added to a polytunnel but with a little more difficulty in comparison to a greenhouse due to the domed shape.
Which to choose?
Basically, a greenhouse or polytunnel both provide the same benefit - a sheltered environment with warmer temperatures than those outdoors. They can both be used for germinating seeds, producing seedling plants (for planting outside later) and for growing crops that need a lot of heat to produce a good crop.
The main difference, other than those listed above, is size. If you only need space for sowing seeds and bringing on plants and for growing warm climate crops, a greenhouse is probably sufficient for your needs. If you are more interested in becoming self sufficient and to grow as many crops as you can in a season, a polytunnel may be the best option because of its size. A larger covered growing area means you can grow multiple crops to maturity (including those normally grown outside) and start growing earlier in the season and carry on growing much later in the year than you would be able to outdoors; you will be able to extend your yearly growing window by 8 weeks or more.