Mini greenhouse buyers guide

With many gardens being on the small side, and gardeners wanting to extend the growing season many people turn to a mini greenhouse or a small greenhouse to allow them to grow more. Mini greenhouses allow them to create a specific climate or micro climate when they can’t find room for a walk-in greenhouse.  These structures are flexible too and are generally larger than standard cold frames allowing many more plants to be grown in a small footprint.  This guide covers the main considerations when looking at a mini greenhouse.

What is a mini greenhouse?

A mini greenhouse is a standard term that covers a wide range of products and homespun designs.  Wide, Tall, deep or shallow they generally take up less than 3 square metres or about 10 square feet.  They differ from cold frames that tend to be much smaller and, whilst movable, mini greenhouses normally are sited semi-permanently. They offer the ability to control humidity and temperature that allows starting seedlings earlier in the year, better overwintering of plants and, with added knowhow they can be used to grow year-round.

Some mini greenhouses come as a complete growing system and offer the grower the ability to grow year-round with added heater elements and grow lights to boost and extend the seasons.

Why choose a mini greenhouse?

The dominant factor for many people is the space and space efficiency that a mini greenhouse offers to the grower. Whilst compact in size, it’s usual that all of the space is dedicated to growing as opposed to a conventional, larger greenhouse where space has to be provided for the gardener to get in and move around.

They are convenient too and can be put into a garden and moved out of the way according to the season if that is the preference. Often light and highly mobile, these structures are easily put in a shed or garage for the winter if no onwards growing is planned.

How to choose a mini greenhouse?

There are a wide variety of mini greenhouses on the market starting with small metal frame and simple plastic covers with zip up covers costing £50 – £75 all the way up to premium mini greenhouses costing up to nearly £1000. The latter often being built of hardwood or aluminium with glass panels. So why pay more?

Like anything, you kind of get what you are paying for.  The lower end models tend to be good for a season or possibly two before the polythene cover is beyond reasonable use and they don’t stand up to high winds or snow.  That puts your plants at risk as you might be chasing the clear tent around the garden with your plants inside ruining all your hard work.

So, the appearance, materials and quality of the product are also really important as is the thought gone into the design.  High quality hardwood frames look great and, provided the wood has been well seasoned should last for years.  Take care with all wooden greenhouses to look at the quality of the joints as those made from cheaper wood may warp over time making their use trickier. Aluminium is sturdy and rust proof and lasts a long time and, provided that the aluminium is of sufficient thickness they are really sturdy. Additionally, aluminium frames can be ‘powder coated’ which gives them a great decorative finish which will last many years.  Look carefully at all of the pictures and read the features before making a decision.

Mini greenhouse sizes

People have limited space and hence looking at mini greenhouses in the first place so make sure that you can maximise the use of any space that you allocate.  Often this means increasing the height by looking at taller models with multiple levels of staging or shelves. If you can, avoid models that have hinged doors as they reduce the maximum depth you can fit into any given space as you need to give over space to allow the doors to open.

If in doubt go for a larger size rather than smaller.  Many people complain that they don’t have enough space to grow in and so be critical about how much you grow and the amount of space that is required. It’s easier to leave a little room but not so easy to make space when the greenhouse is already full.

Polycarbonate or glass mini greenhouse?

Setting aside the cheaper plastic cover or thin acrylic sheet covered greenhouses, you have a choice between polycarbonate or glass and there are advantages and disadvantages to both:

Ventilation in a mini greenhouse

Ventilation is highly important in any greenhouse no matter the size.  Ensuring that plants don’t get too hot in the summer and too cold in the winter is of crucial importance and the reason that many buy a greenhouse in the first place. Ensuring that there is some throughput of air to prevent fungal/mould infection is key to success so making sure that there is adequate ventilation is of high importance.

Avoiding greenhouses with front hinged doors that have no means of securing them to a desired position is a good place to start.  Without some sort of securing open or closed, doors may bang shut in the slightest of breeze potentially upsetting trays and pots at the same time. Also be keen to avoid any greenhouses that have an opening lid with no strong means of fixing it in place otherwise they may lift and bang in those same stronger winds.  This is especially true of glass greenhouses when you might lose the glass panes too.

Where to place a mini greenhouse?

The first question here is; On what?  Mini greenhouses are highly flexible and can be sited on a wide variety of surfaces from soil, to raised beds, on decking, balconies or straight onto a hard surface like a patio. The choice is really yours. Just make sure that whatever you choose to site yours on, that you have some means of securing it down or to something firmly if it is more than around 2.5ft tall.

Next the aspect in relation to the sun is really important versus what you grow.  What we mean here is which way does the front of your mini greenhouse face.

South facing; This is usually the best choice but be aware that the greenhouse will get very hot during late spring and on into the summer therefore some shading and adequate ventilation may be required.

East or West facing; This is also a good choice for many as it strikes the balance between getting direct sunlight year-round and also avoiding full sun all day in the late spring and the summer.

North This is a good position for young plants, especially later in the season where they can be out of that direct sunlight. It is, however, not suitable for the sun loving plants such as peppers and tomatoes. For those, they can be starting in a north facing mini greenhouse or grow system and then transferred to a sunny spot once all chances of frost have gone and they can then grow on outdoors.

How do I keep a mini greenhouse cool?

As discussed before, ventilation is key to a good success with any greenhouse.  Having adequate ventilation keeps your plants at the right temperature and avoids the nasty diseases such as mould and fungal infections.

Towards the upper end of the price bracket, some mini greenhouses are fitted with temperature activated louvers or lifting lids.  You are able to easily set the temperature at which these activate and they then vent the hot air out at the top, or let cooler air come in at the base.

Most mini greenhouses are also fitted with some form of door system. On hotter days you can then simply open the doors remembering to ensure that it stays secured open and can’t move in the wind which may knock your greenhouse over.

How to heat a mini greenhouse in winter

Whilst on sunnier days in winter the sun can warm your greenhouse, there are several ways that the temperature can be helped.  If you have the space, you can help matters by putting some kind of small, thermal store inside the greenhouse in the winter.  Something like a small, usually black, tank of water works well that heats up in the sun during the day and releases back ground heat at night.  Have a look here for other ideas like this. http://greenhousegarden.com/thermal-massheat-storage

Conventional greenhouse heaters, if small enough also might be an option, these come normally as paraffin or electric heaters designed for the purpose.  They are widely advertised.  Be sure to use a heater that is safe for use in a greenhouse and not a standard house heater, particularly if it’s electric.  Also be sure that it gives out the right amount of heat for the space that you are planning on using it in else you may end up cooking your plants inadvertently.

Some greenhouses also come with specific heating systems inbuilt including soil heaters and heated seed trays.

Do mini greenhouses protect from frost?

Any protection that you give a plant is going to give a degree of protection from frost. However, even in the UK the temperatures can go down really low.  According to the Met Office; the lowest temperature recorded in the UK is -27.2°C on 30 December 1995, at Altnaharra; and on 10 January 1982, at Braemar.

Clearly at these types of temperatures any plant is going to struggle in any unheated structure.  There are also specific categorisations of frost ranging from slight [-0.1C to -3.5C] to very severe [below 11.5C] and the type of plant grown will determine its frost hardiness.  There are good guides to plant frost hardiness by the RHS and how to garden dealing with frost by Garden Organic.

That all said, even an unheated mini greenhouse will give some front protection for your plants especially if it is on or adjacent to a wall or patio.  It’s worth looking at the type of plant that you are trying to overwinter, where the location of your mini greenhouse is going to be and you can always use some limited heating to keep the plants just above that frosty temperature.

Where to find good growing information for a mini greenhouse?

There are a huge number of resources out there to help you from web forums, facebook groups and of course regular books. Our recommendation is to look at Dr Hessayon book on greenhouse growing.