Why use a Greenhouse at all?
Most parts of the planet have seasonal weather which varies considerably over the course of the year. Many of us live in places where the winter months are cold, excessively wet and inhospitable for growing plants outdoors. The appeal of growing in a sheltered environment – with a roof – is clear to anyone who has experienced a north-European winter.
Early attempts to create modified growing areas for tender or warmth-loving plants can be traced to the Romans who used a variety of tricks to grow Cucumbers under sheets of oiled cloth or possibly the mineral Selenite.
Many improvements to these basic methods were attempted over the course of the following thousand years or so, but the modern-type Glasshouse came into being sometime in the 1660’s in the Netherlands, where the first glass-based greenhouses were constructed.
The first recorded heated greenhouse (or Stove House) was constructed in the Chelsea Physic Garden in 1681. By the 1800’s greenhouse technology was already greatly improved, and modern greenhouses do not differ a great deal from them in principle of operation, although modern specimens tend to be of a more simple design.
The Chief Advantages
Greenhouses work by trapping heat from solar radiation inside a transparent structure of (usually) plastic or glass, occasionally using supplemental heating and lighting in the winter months, and being cooled in the Summer months by shading, ventilation or air-cooling systems.
The advantage of this highly controllable growing environment is the ability to create the precise growing conditions needed for a particular plant, and to do so when the prevailing natural climate would otherwise make this impossible if growing outdoors. By using a greenhouse environment it is possible to grow crops that would otherwise not be possible or productive outdoors (such as Melons), or by bringing forward the date of the first harvest (as in the notable case of Strawberries and Tomatoes, which can be harvested months earlier or year-round with supplemental lighting and heating).
For the home gardener, the benefits may be the ability to start off seedlings early, to house frost-tender specimen plants, or to give an edge to your vegetable gardening.
Drawbacks to Greenhouse growing
The advantages of the greenhouse are at their maximum during the cooler months of the year (ie, not the Summer) – and in the height of the summer months the Greenhouse can become more of a liability, requiring extra care to make sure that excessive heat or pests do not become a problem.
Watering always requires some extra attention, as the greenhouse roof prevents natural rainfall from irrigating the crops, and this is an acute issue in the warmest months when watering has to be carefully timed to avoid temperature shocks etc. In addition, the entire growing environment needs to be carefully monitored to ensure that the correct humidity, ventilation, lighting and temperature levels are provided for your plants.
Thankfully, there are many devices and gadgets now available that allow a higher degree of autonomous climate-control, or control from a distance using an app or similar – however all of these come with an up-front financial cost.
Key Uses of a Greenhouse
For most home-gardeners, the greenhouse will be used in three major ways:
- Starting seeds and seedlings early in the season for planting outdoors as soon as it becomes warm enough (or forcing early crops)
- Providing a warmer climate for warmth-loving plants during the summer months especially Fruit and Vegetable plants such as Tomatoes, Peppers, Aubergines, Melons and Cucumbers.
- Overwintering frost-tender plants which require sheltered winter climate.
How to get a Greenhouse
There are many different sizes and styles to choose from, ranging in size from the tiny and compact to the huge and expansive. Provided there is the ability to adequately ventilate the greenhouse you will have to decide which design will work best for you.
There is no clear advantage of plastic versus glass. Although glass is always prone to accidental damage, it can theoretically last for a long time before needing to be replaced, although when it does break it is a hazard to deal with.
Plastic, on the other hand will typically last for up to ten years if it is specially designed for greenhouse use – otherwise many plastics degrade very quickly in sunlight. You may have seen pictures of greenhouses made of drinks bottles – nice idea maybe, but in reality the plastic will degrade in the sunlight over a year or two and become unusable and un-recyclable.
Polytunnels, used commonly in commercial horticulture, use a special grade of polythene which has been specially treated to make it selectively infrared reflective, and resistant to UV degradation, being perfectly recyclable in most cases even after the full nominal use period.
Greenhouses can be ordered as a kit for home assembly, or a full greenhouse-building service can be used.
Features every greenhouse should have:
Doors – yes the doors need to be both present and wide enough to actually be useful. If you intend on taking a wheelbarrow inside the greenhouse then the door should be comfortably wide enough to accommodate it.
Benching – in greenhouse terms this means the shelf-like tables that allow plants to be grown at a height comfortable to reach without bending. Multiple levels of benching and shelving can be incorporated, so that plants can be grown on the floor under the bench, on top of the bench, and on shelves also.
Water-saving devices – Trays that collect runoff or excess water, capillary matting, and other devices and tools can be a necessity in the summer months, otherwise you may find yourself spending a lot of time watering.
Temperature control – This works both ways – firstly in the Summer your plants will need a lot of ventilation on a hot day. This should be at the top of the greenhouse, and at the bottom to allow a through-draught to pull the cool air in as the hot air escapes.
In the winter months some supplemental heating may be necessary, depending on which plants you plan on keeping in the greenhouse, and how warm they need it to be. Even tropical plants still like to be relatively cool and dry during their dormant period, and this usually occurs in the winter for most species. For this reason it is not usually necessary for the greenhouse to be hot in the winter, but rather in the temperate-cool range of around 10-15C at the coldest, and most plants can take temperatures close to freezing (5 to 0C) at night during their rest period – but check to see what your plants need! The best heating methods would be controlled by a thermostat – either an electric heating cable (or mat), an infrared (carbon) heater, or a heat-pump. Gas heaters can also be used, but it is tricky to control the temperature precisely and there will be condensation build-up as well as the risk of carbon monoxide if there is a ventilation problem. Humidity should also be kept to a minimum in the winter – and this is hard if you are burning gas or paraffin in the greenhouse, as this produces significant quantities of water vapour.
Irrigation/Watering – Keeping your plants alive with the right amount of water is tricky at the best of times, and this can be a challenge in the summer months when watering may be needed a few times a day. The use of automated watering devices is fantastic – especially for larger greenhouses. It doesn’t mean that the greenhouse can be left entirely unattended – a leak can cause big problems if left to itself for a while. So you will still need to check on the greenhouse on a daily basis at least – but this does still allow you to get on with everyday life while the sun shines. The best method for most gardeners is to use a watering timer attached to an outdoor mains tap, which allows the water to pass through at certain points during the day. This needs to be a good quality product, and not one that will leak due to poor design or manufacturing. The water can then pass through irrigation hose to be allowed to reach its destination via a series of mini-sprinklers which can be placed at regular intervals to ensure even coverage. Larger plants can be attached to dripper-irrigation, which allows constant but careful watering.
Shading – Greenhouses get HOT in the sunshine, and this will kill plants by cooking them. Temperatures above 38C will cause critical heat stress in most plants, and shading from the full midday sun can be necessary. Reflective insulation foil, cloth, and special white shading paint can all be very effective at reducing the amount of stress the plants are under.
Pest Control – Unfortunately, dealing with pests is always a task for the gardener. It is always best to be proactive at dealing with pest problems, ideally when they are still POTENTIAL pest problems. So make sure that there are no places for Rats to nest, or burrow through, that rodents cannot climb or jump easily onto the benching, (or get in the greenhouse at all) and that you set appropriate traps. Mice, Rats and Voles can jump a considerable height when they want to – a metre (3 feet) is quite possible for a mouse, and rats can jump even further. I have previously dealt with Rats that exceeded 60cm (2 feet) in length, including the tail, and they can jump distressingly far. Rodents can eat their way through a lot of precious greenhouse plants in very little time, tending to strike in the cold months of autumn and spring. Slugs and Snails will also make an effort, although their jumping skills are not so well-renowned. Most pest problems during the summer will be the creepy-crawly kind, and the best approach is a proactive one which uses predatory bugs to keep the balance for you, introduced early enough in the season to prevent outbreaks from occurring.
In addition to the initial outlay for the purchase and build of a greenhouse, there will be a cost for the maintenance as well. This may include the cost of repairing or replacing a part of it (such as a pane of glass), or the costs of heating and watering. A winter storm may cause considerable damage, so consider this possibility and perhaps a larger greenhouse may benefit from an appropriate insurance plan. Even a small greenhouse can cost several hundred euros, and larger ones will cost several thousands. The payoff however is hard to measure with money, and a greenhouse can become a truly special extension of the home and garden.