Among the many different styles of greenhouses, the lean-to greenhouse can range from a quickly-built temporary lath-and-poly shelter for hardening off seedlings, to a solidly-built extension to the house with a foundation, heating, etc etc.

Between those extremes, many people find that a kit-built or ready made lean-to greenhouse is easy to build and extremely useful. The most important point to consider is the location – which way the greenhouse will face once it’s built. Since it only has one side and half a roof to collect sunlight instead of two sides, it’s very important that it get plenty of sun for as much of the day as possible (though in some locations, afternoon sun may be too hot).

Some lean-to greenhouses are large enough that you can walk inside through a door at the end, others are smaller and accessed through the side. With a larger lean to greenhouse attached to the house, you can have a door from the house straight into the greenhouse, which is wonderful when the winter weather outside is frightful. It also allows the house to benefit in winter from the warm damp air from the greenhouse.

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What’s in your hoophouse or polytunnel this winter?

Here’s what’s in mine (I’m on the Canadian southwest coast, USDA zone 7-8, temperate maritime climate):

Self seeded corn salad (mache). Last spring’s uneaten corn salad flowered so I waited to collect the seed – but waited a little too long so a lot of it fell before I grabbed it. For a long time it didn’t germinate and I wondered if it was no good, but come the cooler temperatures this fall it germinated like mad, so that section of the southern bed is a solid mass of corn salad.

Last spring and summer the center bed, under the trellis, contained strawberries, but they didn’t like the heat and even less being buried in bean vines. So I moved them to the northern side bed, where they will be cooler in the summer (that side rolls up) and have more space. Up to a week or so ago they were still determinedly setting flowers, but they seem to have given up now.

Along with the strawberries in the N bed I moved a load of clumps of corn salad, all of which are happy and perky. Luckily we like corn salad! Lettuce seed has been sprinkled around, too, to germinate when it feels ready during warmer spells, and grow on in the spring. There would be spinach seed too except I’ve run out.

Peas (bush shelling peas) will go in the south side bed in the next few weeks, again to germinate when they feel like it and give us an early crop next spring.

The center bed is empty now, waiting for compost and a good digging over to fill the holes where I took out the strawberries with plenty of roots, and to wait for tomatoes next April.

If you use your garden greenhouse to overwinter plants which can’t stand frost, then you’ll need some method of heating it – usually a greenhouse heater. If it’s attached to the house you may be able to hook in to the home heating system or simply leave a door open between the house and the greenhouse, but freestanding greenhouses need some kind of heat source if they are to stay above freezing when the outside temperatures drop below. There are ways of using natural heat sources like compost or small livestock, and you can reduce the extra heat needed by using water or rock as a heat sink to store daytime heat, but many people install a greenhouse heater of some kind.

Along with the greenhouse heater should come insulation: there’s no point having all your heat disappear through uninsulated walls and glazing! You’ll also want a min-max thermometer to keep track of temperatures, and possibly a thermometer that has a remote readout in the house so you can really keep a close eye on things. See below for details on these.

Greenhouse Heater Fuel Types

The most common greenhouse heaters run on electricity or propane, but depending on your location you may also be able to get kerosene (paraffin) heaters, oil heaters or natural gas heaters. Electric, oil and natural gas heaters require that you have electricity, oil or gas supplied to the greenhouse, whereas propane or kerosene heaters can be freestanding and portable (though propane heaters can also be permanently installed and piped to a large propane supply tank outside the greenhouse). Occasionally people install wood or pellet-fired stoves in greenhouses: like oil or natural gas these need a flue or chimney for safety, and are most often installed against a solid wall. Check local building regulations to find out whether you have to have venting for combustion-based heaters.

Greenhouse Heating System Features

Whatever heat source you use, there are some things you want your greenhouse heater and associated heating system to have.

• A way of distributing heat around the greenhouse (reflectors or fans)
• Temperature control (thermostat)
• Automatic turn-off feature during a malfunction or emergency
• System for handling fumes or combustion gases, if any

Electric Greenhouse Heaters

These heaters generally range from 1000 – 3000W depending on your power source (120V or 220-240V) and will put out around 5000 – 10,000 BTU’s, enough to heat up to a 275 square foot greenhouse.

There are different designs but most come with indicator lights, overheat and tip-over shutoffs. If you want to add a thermostat you’d need to wire that separately to control the heater – consult a qualified electrician.

Good points of electric heaters are that they do not give off any fumes and thus can’t damage plants that way, they are very easy to install if you already have power in the greenhouse, they are portable, clean and quiet. You need to watch where the fan is blowing hot air – it’s better if it doesn’t blow directly onto plants – and be very careful that the heater and the wiring can’t get wet. If you have a large area to heat you may be better off with several smaller heaters than one large one, so that the warm air is distributed more evenly around teh greenhouse.

Propane Greenhouse Heaters

Propane heaters come in two types – those which can be used in an enclosed area, and those which must be vented. The ‘closed area” type have a low-oxygen shutoff system which means that they will not continue to burn under conditions which might cause production of poisonous carbon monoxide.

Smaller models can run off the small 1lb camping propane cylinders but this is very uneconomical if you need to run the heater for extended periods. An alternative is to use a heater which can attach to a large cylinder like those used to run BBQs, using a hose. Commercial-size models might be better attached to an exterior propane tank such as those used to supply home heating and kitchen appliances.

Small propane heaters are quiet, portable, lightweight and odor-free (unless, like me, you hate the smell of the propane itself!). Some can be wall-mounted as well as floor-standing.

Other Greenhouse Heaters

If you have a large home greenhouse or a commercial-sized one, then you might want a fixed, permanently installed heater which runs on oil, gas, propane, pellets or wood. These need a chimney or flue, and are often installed against a solid end wall of the greenhouse with overhead ducting to carry the warm air all the way along to the other end.

Greenhouse Thermometers

Your basic minimum and maximum thermometer will tell you the current temperature and the minimum and maximum temperature reached since you last reset it. They come in old-fashioned glass/mercury or glass/alcohol styles (which are prone to breaking) or as digital thermometers, which are easier to read and less breakable but need batteries.

An upgrade is to get one which has a remote sensor in the greenhouse and an indoor display. These have now come down to a very reasonable price, at least if you don’t need a long transmission range, and I think are worthwhile.

Some even have an alarm which will wake you from your sleep when frost threatens, so you can leap from your bed, run outside and spread blankets over your beloved plants. A delightful prospect, but possibly better than losing a lot of hard work!

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Mini greenhouses are smaller than full-size greenhouses, but big in usefulness! The term mini greenhouse can refer to several styles of greenhouse:

• A set of shelves for plants, with a plastic cover fitted over the top to protect them
• A very small conventional-shaped greenhouse, with a plastic sheet cover or rigid plastic glazing
• A lean to or closet-like greenhouse where access is “reach-in” but there’s much more space than the shelf-style units
• A cold-frame like structure
• Covered planter trays for starting plants from seed or cuttings

We’ll take a closer look at all these kinds of mini greenhouses in this article.

Shelf-Style Mini Greenhouses

This is a four-shelf 4 Tier Green House with a plastic cover which zips up on both sides to open the front completely. Plenty of ventilation like this is important with a small covered area as it can heat up very quickly when the sun gets on it.

Other mini greenhouses of this style come with 2 or 3 shelves instead of 4 – the 2 shelf versions are small enough to stand on a bench or table. The size you need depends on how many plants you plan to grow in them, whether seedlings, pot plants, tropicals, cuttings or overwintering herbs.

While these units are intended to be used outdoors on your deck or patio, they also work indoors to keep the atmosphere around your plants nice and moist.

I find a mini greenhouse like this most useful for hardening off the seedlings I start in my basement. Placed on the deck, right by the back door, mine is very convenient for opening and closing the cover, moving plants around to make sure they get even light, watering, and generally keeping a close eye on them. It allows me to expose the young plants gradually to the outside world, while taking up very little floor space, and in the summer I can take the cover off completely and use it for potted herbs and flowering plants.

Small Greenhouses

This tiny Walk-In Greenhouse is based on the same principle as the shelf units described above – a frame with a plastic cover which zips open – but it’s big enough to walk into and to have plants growing on both sides, either in the ground or in pots on benches or racks. Many other similar models are available.

Advantages include cheapness, and being portable – so you can use it on a deck or patio, or over a different garden bed at different times. This certainly beats moving all the soil out of a fixed greenhouse bed and replacing it, as some books advise you to do! It’s important to fix the greenhouse down well – as you can see, the model in the picture uses guy-lines and stakes, though on a patio or deck, weights would do the same job. You can also pack the whole thing away over the winter if needed, so no worries about it being damaged or blown away by winter storms.

One of my neighbors uses a small greenhouse like this for his tomatoes. Every year he plops the greenhouse over a different spot in the garden in the early spring and allows the soil to warm, plants his tomatoes a month earlier than he could if they were completely unprotected, gradually leaves the door open for longer as the weather warms, spends the summer picking great tomatoes while the cover keeps the rain and the blight off the plants, picks for a long time into the fall, then puts the greenhouse away for the winter until he chooses a new spot the next year. This is a great way to rotate your tomato crop to keep down diseases and pests.

Lean-To or Closet-Style Mini Greenhouse

This example is the Juliana Mini Pro 3 Greenhouse. Mini-greenhouses like this are often placed against a wall or fence, with access from one long side. Like the shelf-style greenhouses they can have shelves in, but they are several feet deep and so have much more growing space inside.

They can be placed over plants growing in the ground to provide an excellent warm micro-climate for something like a dwarf peach tree trained against a wall, but they also work very well for tall pot plants like standard fuchsias or other tender but large plants. An important feature you can see in the picture is a vent at the very top, where the heated air collects. You can make this an automatic vent which will help to keep the temperature under control even if you can’t open the main door.

My mother has had a greenhouse like this for about 20 years on her backyard patio and it’s always full, summer and winter, even though she has a large sunroom as well!

Cold-Frame Type Mini-Greenhouse

The Guarden 2′ x 4′ Mini Greenhouse-Extra Deep is a cold frame style mini greenhouse. In many ways these are more useful over winter than the shelf style. Being closer to the ground, they often stay warmer, and are much less prone to being blown over by windstorms. It’s also easier to install an automatic opener to save having to run out and open the cover every time the sun comes out, though that does depend on the model you choose.
Bear in mind that once you have a cold frame, you’ll find a million more uses for it that you would never have thought of, so although a small one is cheaper it may be better in the long run to buy as large a model as you can fit into your space and your budget.

Covered Plant Trays as Mini-Greenhouses