Permaculture Garden Greenhouse

Learn how building a greenhouse can combat adverse weather conditions in any permaculture garden.


A high/low thermometer is indispensable in a greenhouse.

Photo courtesy New Society Publishers

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Not every aspiring homesteader has access to 5 gently sloping acres of rich, loamy soil. Author Jenni Blackmore presents a highly personal, entertaining account of how permaculture, "permanant agriculture", can be practiced in adverse conditions. Permaculture for the Rest of Us (New Society Publishers, 2015) describes how to retrofit even the smallest homestead, illustrating the fundamental principles of this emerging gardening technique in a humorous, reader-friendly way.

You can purchase this book from the GRIT store: Permaculture for the Rest of Us

I used to view a greenhouse as a luxury of the first degree. Now I realize they are actually an integral part of a successful garden. In essence, a greenhouse must allow in as much natural light as possible, while protecting plants from extremes of temperature and precipitation. In the spring and late fall it’s necessary to keep plants warm but in summer it becomes equally important to keep temperatures from rising too high. A thermometer that records both highs and lows is essential for doing this. Such a thermometer has a round dial face and three hands, very much like a typical clock. One hand registers the lowest temperature it has dropped to, one registers the highest temperature it has risen to, and the third hand marks the actual temperature in the greenhouse at any given moment. They are not terribly expensive and are so useful in monitoring the temperature fluctuations within the greenhouse that I would class them as indispensable.

A greenhouse has a special feeling all its own, whether on a cool spring day with rain pattering on the roof or a late summer day when tomato vines are threatening to poke their way through the roof. One of my earliest childhood memories is being shown my uncle’s greenhouse. I was not allowed to enter but the spicy, exotic smell of tomato vines was enough to hold me wide-eye and enthralled as I peeked around the rickety old door. Even to this day that smell transports me back through time and across the ocean to the grimy industrial north of England and my uncle’s greenhouse. Perhaps that’s where some of the seeds for my own life’s journey were planted. Who knows?

Our greenhouse is a simple frame affair covered with a heavy duty, semi-transparent plastic. It measures sixteen feet by nine feet and didn’t cost much over a hundred dollars to construct. I was fortunate enough to be in a large hardware store when several rolls of translucent plastic tarp were being dragged out from some dusty corner of the storeroom. As they were not “in their system” no one knew what price to put on these rolls of poly sheeting, so they were priced ridiculously low for a quick sale. For once I was in the right place at the right time and I bought all that was available, knowing it would come in handy for any number of projects for years to come. Without this lucky purchase we probably would have used a heavy gauge vapor barrier, which I’m sure would serve the purpose equally well, or special purpose greenhouse film which is also available — at a price.

There are many really good designs for greenhouses available through university extension offices and it’s best to see what’s available before deciding which is the best fit for the space and requirements it is expected to fill. I would encourage starting small and moving slowly, not just with greenhouses but with all aspects of development. This will help to keep the learning curve from feeling like a roller-coaster ride!

We recently attended a workshop on “hoop houses,” which are large, commercial (market-garden sized) greenhouses and which are very economical to purchase and construct. They have steel ribs which support a “skin” of specially treated plastic that has a UV protective coating on the inside, giving it the magical ability to bounce sunlight back onto the plants for prolonged benefit. Certainly worth checking out for anyone who wants to jump in with both feet but really, it’s better to start small so that mistakes and crop failures are similarly sized. From this perspective, our smaller, less advanced, basic greenhouse presents itself as an infinitely more user-friendly space. Here on the coast we get phenomenal wind pressure at times but we’ve only had to repair the plastic cover once. I believe the key to success here is to have the plastic stretched as tight as possible and to reinforce all contact points.

Probably a greenhouse constructed with recycled windows would be more in keeping with permaculture ideals and perhaps it would be more appealing to look at, if some thought were given to its design. Traditionally the main requirement for any greenhouse was that the structure ran east to west, giving maximum southern exposure, and that light should be able to enter from the roof and from the south and east and west facing walls. There is some debate as to whether this can cause overshadowing of plants on the north side, especially in winter when the angle of the sun is low. A row of taller plants or vines along the southern interior wall will to a certain degree shade plants to the north of them, even in a greenhouse with a clear roof. Also, the shade from exterior sunblocks, such as trees, will change seasonally. I only mention this to suggest that the traditional east-west axis of a greenhouse might need to be adjusted to accord with the main purpose, which is to allow for as much natural light as possible. This is where detailed zone and sector maps are so useful. A greenhouse can cover workable ground with plants growing directly in the soil or it can house pots and seed trays, raised beds or a combination of all three. Presently all our greenhouse plants are grown in pots or seed trays.

Our choice of where to build the greenhouse was limited by the availability of flat ground. Planting directly in the ground was not an option because of poor soil quality in this area. In time we will probably construct a raised bed down one side but presently we rely on large pots. This allows for a certain flexibility of movement in the early growth stages but the downside is that pots dry out quickly in the heat.

Various irrigation methods are available for greenhouses. I was fortunate enough to notice some sizeable fiberglass troughs in the “bone yard” of a local business depot. Overgrown with weeds, they’d obviously been sitting there for a while and when I asked if they might be for sale at a reasonable price the proprietor was delighted. He had a massive cleanup underway and was wondering what on earth he was going to do with this unclaimed custom order. It never hurts to ask! These troughs make perfect baths for the growing pots to sit in and, while making watering so much easier and efficient, they also help to guard against things drying out. On the downside, standing water can contribute to mildew and rot (a major problem in any enclosed growing space), not to mention mosquitos, so while very useful the troughs need to be monitored carefully. It’s always good to be on the lookout for the less obvious alternatives and these troughs are a good example of that. Remember, in permaculture there are no problems, only creative solutions.

Once the greenhouse is established, what to put in it? Broadly speaking there are three main purposes for a greenhouse: for starting seeds; for growing plants that require a warmer, more protected environment than outside; and for prolonging seasonal growth. Some seeds are just not meant to go in a greenhouse as they are hardy, not fond of too much heat and prefer to go directly in the ground — beets, carrots, parsnips, spinach or chard for instance. On the other hand I wouldn’t consider seeding cabbage, Brussels sprouts or leeks directly to ground. They need the extra time that pre-seeding indoors gives them. Other plants such as tomatoes, peppers, aubergines and cucumbers will complete their whole cycle in the greenhouse because in coastal Nova Scotia temperatures fluctuate too much, even in mid-summer, to ensure ripening.

One difficulty is to decide exactly when to start pre-seeding. I usually leave it perhaps later than I should because in the past I’ve ended up with trays of gangly seedlings, trapped in too small seed cells, that can’t be put outside because there’s still snow on the ground. Certainly not before the end of February, I’ve promised myself, while secretly deciding that mid-March is a safer bet. Another thing I’ve promised myself is not to overplant. This is easy to do when a scant tablespoon of seeds might plant more than five or six seed flats. How much cabbage are you likely to eat? Of course, by the same token, how many plants are the slugs going to eat? It never hurts to have a few extras on hand and if all the plants prosper they can easily be gifted or sold.

Brussels sprouts are members of the Brassica family, along with cabbages and broccoli. They are all best started indoors because they’re fairly slow to mature. Once established, they are very hardy but unfortunately cabbage white butterflies find them irresistible. They lay eggs on the plants which hatch into little green caterpillars. These feed on the plants before continuing their cycle, becoming cabbage whites and laying their eggs . . . Without a doubt the best way to avoid this problem is to cover the transplants as soon as they are set out with some kind of filmy row cover which will allow light and air to penetrate while deterring any flying predators. Commercial row cover is available but discarded window sheers will work equally well.

Brussels sprouts are quite fun to grow as the miniature cabbages form where the stems of large unwieldy leaves attach to the plant’s tough woody stem. In the fall the leaves wither and begin to fall off, leaving a pyramid of Brussels sprouts. They are very hardy and the flavour is improved by exposure to frost. It surprises me that more people don’t grow Brussels sprouts but when I voice this opinion it’s usually met with rolling eyes and condescending sighs.

What to plant in also requires some thought. I remember seeing a nifty little gizmo in a tool catalogue that was designed to make seed cells out of newspaper. This is probably the greenest option but I must admit I’ve never done it. We don’t get the newspaper is my default reasoning, but secretly I’m not sure that I’d have the patience, or the time, to sit and fold newspaper strips around a wooden cone night after night.

We tend to favour compressed peat pellets that look like miniature hockey pucks until they’re introduced to water, which causes them to swell into adequately sized seed cells. The nice thing about these is that they’re made of compressed sphagnum moss. This allows the tiny roots to spread with ease, while absorbing sufficient water. They can be planted directly into the ground without disturbing the sensitive root growth which is a great plus and should be kept in mind when considering the options: plastic but re-useable versus organic and way more plant/planet friendly. Whatever your choice it’s always good to shop around as the prices vary considerably from source to source. Suppliers know that a dash of spring fever plus a severe case of itchy green thumb can cause any parsimonious budgetary concerns to float away on the first mild breeze.

Once the seeds are well sprouted they might need to be put (still in the seed pellet) into bigger (three/four inch) pots if a huge amount of root growth is sticking out the bottom and the soil outside is still too cool to plant in. Such a scenario is unlikely to happen with leeks, for instance, because of their slow germination time but it could easily happen with squash. Leeks therefore would be seeded several weeks before squash. There’s always some trial and error but a good gardening book or two will help to eliminate some of it. Most importantly it’s best not to expect one hundred per cent success from every single seed that’s planted. Mistakes will of course be made and conditions will vary, so what might not grow well one year may thrive the next.

Two things that might not initially come to mind along with those first bright greenhouse plans, are the need for adequate ventilation and airflow, and access for insects, both beneficial and not so. If there is not enough airflow the plants will become prone to mildew, and once any sign of mildew is noted the plants must be removed and destroyed. It’s a common problem and can become virulent. We have a door at the east and west ends of the greenhouse and there is a vent above each door that stays open even when the door is closed. In damp or overcast weather this is not enough, and we need to install a fan and rethink the vents. We definitely underestimated the need for consistent air flow, especially in the extra moist climate that prevails here.

Insect access is another need that mustn’t be underestimated because if flowers aren’t pollinated they won’t produce any fruit. The scarcity of bees has been noticeable for a couple of years now, which touches on the topic of installing a beehive. More on that another time, but not in this book. On several occasions Calum has spent considerable time dressed in a fuzzy yellow and black striped bee costume, fertilizing the greenhouse plants with a fine sable brush.* This method does work if you have more patience than I do.

Of course, open access to insects allows for less welcome visitors. Aphids have been our main problem for the past couple of years. They go crazy for aubergine plants. When researching what to do about aphid infestations I came across one site that recommended installing a couple of aubergine plants to distract the little pests from the rest of the plants. Hmmm! While I’m sure this is a perfect solution for anyone who doesn’t particularly care about aubergines, it really wasn’t much help to us. Ladybugs work better. Aphids to ladybugs are like chocolate to a chocoholic. Good garden suppliers actually sell ladybugs by the hundred. Of course, there’s no guarantee the ladybugs will do the honourable thing and stick around once you’ve purchased their freedom. In truth, I expect they pigged out so much that they couldn’t fly off even if they’d wanted to, but we’ll just say they were being loyal.

Maintaining the equilibrium of a greenhouse truly brings into focus the importance of natural balance. Certainly, the aphid-ladybug connection underlines the value of keeping part of any property in its natural state, to allow the proliferation of natural predator-prey relationships. No matter how small the area being worked, any successful permaculture plan requires a natural zone, usually labelled as zone five, the furthest perimeter of the bullseye which has as its center the heart (or house) zone, called zone one. Zone one can be viewed on an inspirational level as the intent to live in harmony with nature, rather than to attempt to dominate it. Zones two and three are also planned around human activity: which garden beds will be visited the most regularly, what’s the most convenient placement of the chicken house, the compost bins? These are the considerations for areas closest to home whereas zones four (food forest) and five (wild zone) are where the vision or intent of zone one is honoured in the naturally occurring systems left to flourish untouched.

Reprinted with permission from by Jenni Blackmore and published by New Society Publishers, 2015. You can buy this book from our store: Permaculture for the Rest of Us