Tips for gardeners growing indoors

Author and polytunnel gardening expert, Klaus Leitenberger extols the benefits of growing under cover and gives good reasons why avid food gardeners should consider investing in a polytunnel for year-round crops. 

I have always been passionate about growing food — all kinds of it and I never liked buying food that can be grown in my own garden. Unfortunately in our climate there is a whole range of common vegetables that will do poorly outdoors and we need the extra heat and shelter of a greenhouse or polytunnel to get a good crop.

Only in the most favourable areas could you grow tomatoes, peppers and cucumbers outdoors and even there, the yield will only be a fraction of what can be achieved indoors.

For example my favourite tomato variety ‘Sungold F1’ can produce about 500 most delicious cherry tomatoes per plant in a greenhouse or polytunnel. Outdoors you’d be lucky to get more than 50 tomatoes per plant.

Our growing season is also relatively short but with the help of an indoor space we can prolong the production period for most vegetables. You can harvest your first early potatoes in April, your carrots and beetroot in May and you can harvest salads all year round. I promise you: getting a polytunnel or a greenhouse will revolutionise your gardening life. It will open up new avenues, you’ll experiment with new vegetables and you’ll be guaranteed a good food supply throughout the year.

You can also do some gardening work even if it rains outside. Even if we get a poor summer you can get the illusion that you are in a different country and enjoy the warmth at home.

Why protected cropping?

Polytunnels are an excellent way of increasing productivity at a low cost. There are many important benefits.

For growing tender vegetables which require higher temperatures than normally exist in an Irish summer, eg. tomatoes, peppers, chillies, sweet potatoes, aubergines, cucumbers, basil, sweetcorn. Even grapes, peaches and nectarines can grow successfully in Ireland provided your polytunnel is large enough.

* For extending the growing season in the spring and autumn. Examples: carrots, potatoes, calabrese, celery, courgette, beetroot, scallions, spinach, radish and annual herbs such as dill and coriander.

* For winter cropping. In a polytunnel fresh salad leaves can be harvested all through the winter. Most of these wonderful crops are available thanks to Joy Larkcom (a famous vegetable garden writer) who travelled all over Asia in search of these most amazing winter hardy salads. These crops actually grow much better in the colder months.

If you try to grow them in spring and summer they tend to bolt more quickly. My favourite types are: Oriental mustards ‘Red Frills’ and ‘Green Frills’, tatsoi, salad rocket ‘Victoria’, wild rocket, mizuna and mibuna. All of these have a delicious mustardy taste. To add some milder leaves into the mixture you need to grow winter purslane (also known as claytonia or miner’s lettuce) and corn salad (also known as lamb’s lettuce).

For plant propagation.

Polytunnels and greenhouses can successfully be used for raising plants. If you have electricity nearby you can install a heated bench or propagator. If you don’t have a propagator you just need to start sowing a little later. In some years I sow my leeks into a seedbed in the tunnel around February/March and then transplant them out (bareroot) a couple of months later.

Choosing a site for your polytunnel or greenhouse

The following list of considerations indicates optimal conditions for siting your polytunnel or greenhouse. Obviously you may have to compromise to adjust to your own circumstances. The two most important considerations are good light and shelter from strong winds.

* Shelter. It is absolutely essential to choose a relatively sheltered spot — you don’t want to worry during every storm or gale if your tunnel blows away or your glass panes break. A windbreak will provide adequate protection. The best windbreak is a natural hedge. Artificial windbreak netting also works well.

* Good light. Good light levels are essential for healthy plant growth especially from autumn until spring. Position your greenhouse or polytunnel in order to get maximum sunshine throughout the year.

* Aspect. Ideally align your greenhouse or polytunnel in a north-south direction so you won’t get any shading from tall crops that grow in the centre bed. Your polytunnel or greenhouse shouldn’t be sited next to buildings and trees as they would shade it and the roots of trees may grow into them.

* Access. The closer the greenhouse or polytunnel is to your house, the handier it is for you. They both require daily maintenance especially ventilation and watering. Obviously if you have a very pretty wooden or Victorian greenhouse you may make it a central feature in your garden. If you have bought a polytunnel and are not too pleased with the aesthetics of it you may want to screen it a little bit.

* Frost pockets. If possible avoid low-lying areas in your garden where cold air accumulates otherwise there will be a danger of frost damage.

* Access to water. A reliable source of water is absolutely crucial for the success of your greenhouse. Rainwater collection from the greenhouse roof is a great addition but rarely enough unless you have plenty of barrels to fill. A good source of mains water nearby may prove essential. In a polytunnel it is either very difficult or impossible to collect the rainwater so mains water is absolutely crucial.

* Electricity. A safe electricity power point inside the greenhouse or polytunnel is highly beneficial. It is necessary for heated propagators and you may also choose to have a light bulb in case you want to go on a slug hunt. You can also use it during early winter evenings. But other than that it is not absolutely essential.

* Planning permission. If you are worried about planning regulations you can enquire from the local council whether you need planning permission for a polytunnel or greenhouse.

Greenhouse or polytunnel?

The choice between a greenhouse and a polytunnel is quite simple. A greenhouse (also referred to as a glasshouse), is a lot more expensive but more attractive and has better heat retention and light transmission compared to a polytunnel. Unless you can build your own greenhouse using some recycled materials, your choice may naturally be a polytunnel as they have become very affordable in recent years.

Polytunnels come in all sizes and you can pick one that suits your garden. They are fairly easily assembled, but it would be better to get some help and advice from somebody that has done it before.

There is no right or wrong time of year to put one up. You can sow or plant some vegetables nearly all year round. Just make sure that you put up the plastic on a sunny day as it expands when warm and shrinks a bit when it’s cold. This helps to get it really tight and stops it from flapping afterwards.

Soil fertility

There is a simple rule: The healthier your soil is, the healthier your crops will be. In a polytunnel or greenhouse we produce a lot more food compared to an outdoor plot — that means we need to feed it a lot more and the best feed for the soil is compost or composted manure. They enliven and improve the soil

Klaus Leitenberger has two gardening books and heritage seeds available to buy on: 


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