Choosing a greenhouse or tunnel for a small garden

Harvest vegetables months earlier thanks to glass or plastic

A greenhouse or tunnel extends your growing season by six weeks on both sides. Tomatoes, peppers and cucumbers ripen in June instead of August. Herbs grow year round. This is not a luxury for large gardens: even on five square metres you can fit a sturdy greenhouse and grow vegetables you would never get outdoors.

The choice usually comes down to this: glass stays beautiful and lasts longer, plastic is cheap and quick to install. The right size depends on your garden and what you want to grow. This guide helps you make that choice.

Tunnels versus greenhouses: the difference

A tunnel is a plastic half cylinder on a steel frame. It costs around 400 to 800 euros for 4 by 2 metres, goes up quickly, but is usually worn out after about five years. A good choice if you want to start carefully.

A greenhouse is a sturdy structure with four walls and a roof, built in glass or polycarbonate. Glass costs more, roughly 1,500 to 4,000 euros, but stays attractive for thirty years. Polycarbonate sits in between: about 800 to 2,000 euros and clear for some fifteen years. Tunnels and greenhouses give a similar growing result, but a tunnel needs more upkeep, such as replacing the plastic cover and checking the frame.

Glass or polycarbonate: what to look for

Glass stays crystal clear and loses very little light, which gives the best growth. The downside: hail can break it, and a broken pane is expensive to replace. In busy residential areas or near playgrounds, polycarbonate is the safer choice.

Polycarbonate is slightly less clear, you lose about 5 to 10 percent of the light, but it is unbreakable and builds up less heat. Glass can reach 40 to 45 degrees Celsius on a sunny day, polycarbonate usually stays closer to 35 to 38 degrees Celsius. Glass works well for tomatoes, polycarbonate is enough for peppers.

Size and location determine your choice

How big should it be? For two people growing vegetables for six months, plan at least 4 by 2 metres, so 8 square metres. That still fits comfortably in a small garden. For four people, go for 6 by 2 metres, since a smaller greenhouse fills up fast.

Location matters just as much as size. A greenhouse needs at least six hours of sun, preferably eight. Place it running east to west, with the door facing south and the back wall facing north. In partial shade vegetables still grow, but peppers and melons never reach their full potential.

Also think about your neighbours. A greenhouse stays in place for years, so agree with them first whether you can place it around 30 centimetres from the boundary without any disputes later. For a larger, permanent greenhouse, check with your local council too: permit rules for garden structures differ between municipalities in Belgium and the Netherlands. Wondering whether a greenhouse actually fits your plot, and how much space it really needs? On gardenworld.app you can see that in advance, in a design based on your own garden.

Ventilation determines temperature and mould

This is where many beginner growers go wrong: a closed greenhouse can reach 50 degrees Celsius on a warm day. Tomatoes behave as if they were in the Sahara, flowers drop and fruit fails to set. Ventilation is therefore essential.

You have two options. Automatic ventilation uses two roof vents with auto openers on opposite sides of the roof. They open around 22 degrees and close again at 18. Each one costs roughly 50 to 100 euros and is easy to install. The second option is opening and closing the door and windows by hand yourself: it works, but only if you are actually there at the right moment.

Water and drainage: the moisture problem

Rainwater dripping inside a greenhouse quickly becomes a moisture problem, not mist, but real water droplets sitting on the leaves, which invites mould. Good drainage under the greenhouse floor is essential: gravel, shells or geotextile fabric on a sand base, so the water drains away properly.

Water a greenhouse from below where possible, using a drip system with a timer. Watering from above leaves the leaves wet for too long. In summer, give roughly four litres per day per square metre, about half that in spring. In winter, water only when the soil is completely dry.

Cost table for four popular sizes

A 4 by 2 metre tunnel costs around 400 to 600 euros fully fitted, including frame, foil and tools, and lasts about four to five years. A 4 by 2 metre polycarbonate greenhouse costs roughly 1,200 to 1,800 euros and lasts around twelve years. A glass greenhouse of the same size costs about 2,500 to 3,500 euros and lasts around thirty years.

Extras add up quickly: vents around 100 to 200 euros, drip irrigation about 200 euros, a thermostat around 80 euros, internal shelving about 150 euros. Add those to a polycarbonate base model and you quickly reach an investment of around three thousand euros. So start with a tunnel until you are sure you want to commit.

What you can grow inside and when

Tomatoes: sow in February, plant out in April, harvest from May to October. Grow three stems per plant under glass, two in a tunnel. Peppers: sow in March, plant in May, harvest from July to November. A greenhouse full of peppers can give around two hundred peppers per season from four metres of growing space.

Melon and cucumber: sow in May, about six weeks to grow on, harvest from July to September. Melons want full sun, cucumbers prefer partial shade. Herbs grow all year round: basil, oregano, parsley, dill. Grow lettuce and spinach in spring and autumn. In winter, force chicory or pre-sow strawberries.

First year: preparation

Do not start with vegetables straight away. First test the soil, install drainage, hang a thermometer and prepare the water connection. Buy a simple greenhouse, a tunnel or a 4 by 2 polycarbonate model, and do not pay for everything at once: ventilation, shelving and irrigation can all come later.

In the first season, plant about ten tomato plants, five pepper plants and two cucumber plants. That already gives you roughly thirty kilos of food. Double that in the second year. This way you learn what works, and what does not, without making big mistakes.

Step by step

Step 1: Measure your spot

Do you have a space of four by two metres with six to eight hours of sun? If so, go ahead. If not, choose a tunnel, since it is more compact.

Step 2: Choose glass or plastic

Glass lasts around twenty years, plastic around five. Pick glass for a permanent setup, plastic if you are still testing the idea.

Step 3: Prepare the ground

Level the area, add drainage with about 10 centimetres of gravel, and lay a foundation of pavers or concrete.

Step 4: Assemble and test the ventilation

Put the greenhouse up, check that the door and windows open easily, and hang a thermometer inside.

Step 5: Prepare drip irrigation

Set up the water outlet and lay the line for the drip system.

Step 6: First sowing

February: tomatoes. March: peppers and cucumbers. April: leafy greens and herbs.

Frequently asked questions

Can I build a greenhouse myself? Yes, a polycarbonate greenhouse with a wooden frame is a realistic DIY project. Cutting glass yourself is risky and best left to a professional. Building it yourself takes time but costs much less.

How warm does a greenhouse get in winter? At night, the temperature inside stays only two to three degrees above the outside temperature. During the day, sunlight can add five to ten degrees. So on a four degree day outside, the greenhouse can reach around seven degrees during the day. Growth slows right down in winter, so pre-sow in February under grow lights instead.

How long does a tunnel last? About five years, after which the plastic cover becomes brittle. The frame usually lasts another ten years, so you only need to replace the cover once, at a cost of around two hundred euros.

What if my greenhouse ends up in the shade? Vegetables still grow, just more slowly. Tomatoes need full sun, while lettuce and herbs tolerate as little as four hours. Move cucumbers to a sunnier spot, ideally facing south.

See it in your own garden first

Not sure whether a greenhouse fits your garden, or what you could grow in it? On gardenworld.app you can design a free preview of your garden with a greenhouse and planting of your choice, so you see exactly where it would stand and what would grow there. The detailed design with a plant list costs 7.99 euros, a one time payment with no subscription.