Vegetable Garden

Gardening Under Cover – Where to begin with a greenhouse or polytunnel

January 29, 2013
Hot House from Polytunnels Ireland

Growing Undercover in a Hot House – picture courtesy Polytunnels Ireland

Gardening Undercover – Greenhouse or Polytunnel

Note: The following article has been amended to include more recent links. If you’re thinking of buying or making a greenhouse or polytunnel, it might help.

How many times have you read the words ‘gardening under cover’? Did you immediately know what it meant? I remember reading the sentence years ago and assumed it meant gardening in a beautiful old Victorian style greenhouse. It didn’t occur to me that gardening ‘under cover’ could also mean under a cloche, in a polytunnel, a lean-to plastic greenhouse, under an empty fizzy drink bottle, or even on a windowsill, though reading the words literally, I guess it seems obvious now.

This week a reader contacted me looking for advice on gardening undercover, or more specifically, how to grow vegetables in her brand spanking new polycarbonate greenhouse that was still in its box. Her contagious enthusiasm was delightful and one I could relate to. We’re at the time of year when we begin to feel that bubbling sense of anticipation as the new growing season approaches, Spring is almost upon us! 

I was glad the reader got in touch before she’d opened the box. It’s at this stage that many people go wrong. She’d already done her homework and made her purchase. Now for the exciting part.

If you haven’t yet made your purchase, the following might help:



Glass or greenhouses are generally more aesthetically pleasing, will last longer, let in more light and hold their heat longer but they can cost considerably more than polytunnels. If you have your heart set on a greenhouse but finances are an issue, keep an eye on the small ads as they do pop up for sale from time to time or if you’re handy, make your own.

Greenhouse or Polytunnel

A Greenhouse made from Recycled Windows


Polytunnels make gardening undercover affordable for many. You can get a much larger growing area for your money but you will have to replace the polythene every five to ten years, depending upon the gauge you buy and your location (some will be more exposed to weather conditions that others). The metal frame will make up the majority of your outlay but once you have that in place it will last you for years.

Greenhouse or Polytunnel

Geodome available from

Like greenhouses, geodomes and hot houses look good and their shapes offer quirkiness to your garden. They are also able to withstand all-weather conditions so if you live in a particularly windy area, they might be an excellent choice. Again they’re quite pricey but can be used as sunrooms, kids play areas, somewhere warm to site your giant paddling pool, or of course, somewhere very nice to grow your veggies. Do ensure there is ventilation though. Pay the extra for windows.

The Polytunnel People have an excellent guide which mentions 7 things to consider and The Greenhouse People go into a lot more detail, taking things into account such as where to site it, whether to choose glass or polycarbonate,  ventilation and size, with much of the advice applying to polytunnels too. We bought ours from local company Highbank Orchards who were very helpful.


I put the question out there on social media, “Is Cheaper Better” and received many responses that you can read here. Ultimately, we were advised to buy as big as you could afford or have space for and it’s worth shopping around. Polytunnels will usually come in basic kit form that you can put up yourself, or with additional extras such as doors, shelving etc. Most companies will have a service that erects them for you. Consider this carefully (see below). My experience is that it takes patience and a few people to erect a polytunnel well. You may find it worth paying the extra.

If you can’t afford to buy new, I wrote a post in April last year looking at the cheaper versus expensive debate where there were lots of ideas for making your own structure. You can also take a look at the Greensideup Pinterest board dedicated to Greenhouses and Polytunnels where there are some fabulous recycled and new creations.

Greenhouse or PolytunnelYou’ve researched, picked up or had delivered your new greenhouse or polytunnel, what next?

Read the Instructions

How many times are we told to read the instructions before we start to use something but skip bits because we’re in a hurry? Carefully open the packaging, find the instructions then check all the pieces are there and that they are the correct ones.

There’s nothing worse than finding your missing pieces or incorrect ones have been accidentally packed when you’re half way through your build. I couldn’t understand why a local polycarbonate greenhouse was in pieces within a year of being put up until I discovered that most of the window clips hadn’t been used so all the windows fell out at the first gust of wind.

Don’t start erecting it if you’re in a bad mood or if the weather’s not favourable!

Really, don’t. I speak from experience and I’m generally a good-natured soul. I’ve yet to put up a greenhouse though have had a hand in several polytunnels. Putting anything together that you’re not used to can be stressful and put a considerable strain on a relationship! Add unfavourable weather conditions to the mix and blood pressure levels may rise. You’ll also risk setting sail the most expensive kite you’ll ever fly if you’re trying to put a polytunnel up in anything but the calmest of conditions.

Greenhouse or PolytunnelClipping windows in place and erecting frames will all be much harder if it’s windy, or even too hot (tempers may fray in the heat). However much you want to start growing, it’s worth waiting that extra day or so, or consider paying experts to do it.

If you’ve bought a mini greenhouse which isn’t concreted down, fix it firmly to the floor, a wall, anything. Almost everyone I have spoken to (including me) has had one of these turn itself upside down or crash over complete with its entire contents.

Ground preparation

Whether you’re erecting a small greenhouse on a concrete base or a large polytunnel onto soil, it’s much easier to prepare the ground before you place the structure on top (and essential in the case of the concrete base). If you don’t you’ll risk breaking or putting holes in your structure before you even begin. If you’re planning to grow vegetables directly into the soil as opposed to containers, but aren’t sure when you’ll get around to erecting the structure, cover the soil with cardboard. The weeds will die and the soil will begin to break down without you having to lift a fork.

Place a layer of cardboard on the ground, spread a layer of organic matter (straw/manure/compost) over the top then add another layer of cardboard. This is known as lasagna gardening.

Different structures require different ground preparation but our standard polytunnel required a trench to be dug around the entire structure, about a metre outside the poles. to bury the plastic into and keep it taught. Bare that in mind when measuring.

What can I grow undercover?

Anything! After the long wet summer I’ve been day dreaming about growing all my vegetables undercover! For most of us however, that’s not an option so when considering what  to plant it’s worth baring a few things in mind.

  • Do you want to harvest all year? If the answer is yes then ensure you leave space for your autumn/winter veg. It’s easy to get carried away and fill your soil, containers or shelves with hot climate fruit and veg only to find you’ve no space left for carrots, Christmas potatoes or spinach.
  • Space. Bare in mind how much room the plant takes when it’s growing versus how much space you have. For instance a winter squash could take over a tiny greenhouse and only provide you with a couple of fruit. You might be better to grow it in a large container outside on a warm patio and save your greenhouse space for vine tomatoes, hot chilli peppers, mediterranean herbs such as basil and a single courgette plant.
  • Shelving is important if you plan to raise plants from seeds as a few trays can quickly fill up a small greenhouse if you don’t have any.
Greenhouse or Polytunnel

Shelving in a lovely home made greenhouse from A Thin Man’s Blog (go see how he made it)

Ventilation & Watering

I couldn’t write a post on growing undercover without mentioning ventilation. It’s vital. Without an adequate air flow you’ll risk Botrytis (grey mould) and powdery mildews as well as develop a build up of pests that love warm dry environments such as red spider mite that will thrive. Access to water also has to be taken into consideration. I wrote a post a couple of years ago on watering plants and seedlings which might be helpful and another on setting up an irrigation system in a polytunnel.

Go on a course/keep a diary

Lastly, buy a couple of good books about growing undercover and/or attend a workshop. I’ve held morning workshops about growing in a polytunnel and many other training centres offer them throughout the year.

If you have a polytunnel Joyce Russell wrote an excellent must have book called The Polytunnel Book – Fruit and Vegetables All Year Round and Klaus Laitenberger published Vegetables for the Polytunnel and Greenhouse a couple of years ago.

How long will it last?

You can expect a decent gauge polytunnel to last around ten years, particularly if it’s not too exposed and you repair any tears with repair tape straight away. A greenhouse will last you many more but the initial costs are greater. In the end, the decision is likely to be down to your budget.

More polytunnel advice and tips can be found on the blog.


  • Reply irelandarewethereyetnn January 29, 2013 at 4:31 pm

    This is fantastic thank you sooooo much – I will take your advice and wait for a fine day to take my lovely greenhouse out of the packaging! Can’t wait to get started – will send photos and hopefully fruit soon!

    • Reply greensideupveg January 29, 2013 at 4:49 pm

      Delighted you found it helpful Ann. Best of luck and let me know if you have any more questions 🙂

  • Reply Naomi Lavelle January 29, 2013 at 5:03 pm

    Thanks a really great blog with good advice and lovely pics… I really like the geodome. It’s a wish of ours to have some kind of green house or polytunnel I am sure my creative hubby will come up with something suitable soon on our tiny budget… I’ll show him the post ;0)

    Naomi (Science Wows)

    • Reply greensideupveg January 29, 2013 at 5:17 pm

      Thanks Naomi. I totally agree about the Geodome and having spent some time in one at Bloom last year sheltering from the rain can speak from experience about it being a lovely space to be in. Best of luck!

  • Reply limmster February 1, 2013 at 12:33 pm

    Brilliant post and quite appropriate as I’m doing my protected cropping assignment on Polytunnels 🙂 Thanks!

    • Reply greensideupveg February 1, 2013 at 3:04 pm

      Thanks Belle, that’s a lovely thing to say and I’m delighted it can be of help 🙂

  • Reply Ann Marie February 5, 2013 at 2:14 pm

    The idea of a polytunnel cover as a very expensive kite on a windy day made me laugh! One other thing I’d mention is that when putting the plastic cover on a polytunnel, it’s a good idea to wait for a warm, sunny day. The plastic expands a little in the heat, so can be stretched to its fullest extent and then is much less likely to sag later. Great blog!

    • Reply greensideupveg February 5, 2013 at 2:44 pm

      Thanks! Yes, we had to postpone our own covering daily for about a week because of windy days and that’s a really good point about the warm day, thanks 🙂

  • Reply cathsveggies1 February 26, 2013 at 1:34 pm

    When I was made redundant 2 years ago, I had 2 choices… go on a once in a lifetime cruise holiday, or refurbish our garden and buy a poly tunnel. Weighing up the pro’s and con’s I decided that the cruise would only be a once off and the garden would provide us with food for years to come. So I bought a 14′ x 20′ tunnel from Polytunnels Ireland and got it erected professionally by the manufacturer. It was a lovely sunny hot day so the plastic is now as tight as a drum still. One of the best decisions I ever made 🙂 I also have one of those cheaper, green hobby tunnels where I do all my seeding trays and put some growbags there. There is also room for a seating area at the end of every fruitful day in the garden.
    I grow most of the year round and love it in the poly tunnel even if it is raining or windy outside.Thanks for this blog, always open to more advise, as gardening is an ongoing learning process.

    • Reply greensideupveg February 26, 2013 at 1:42 pm

      Cathy thanks so much for sharing that story… what a tough decision though I know I would make the same choice you did. I can spend hours in my small tunnel and sometimes wish I had more given our weather but then the sun and the wind on the skin is a lovely feeling too.

  • Reply Ena Ronayne January 2, 2014 at 1:40 pm

    For some reason this post past me by and I’m only getting to see and read it now. Ok so it’s only a year later haha, better late than never I guess.

    Thanks to Nora Stack for sharing to where I saw it. Yet another post I shall be sharing on my return to college on Monday with my horticultural students. Thanks Dee. As always I love your posts 🙂

    • Reply greensideupveg January 2, 2014 at 5:14 pm

      You’re welcome Ena and glad to hear the posts are being shared with more gardening enthusiasts!

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