Building a Small Greenhouse: How-To Guide and What to Grow

One captivating way to enhance a garden’s allure is by constructing a small greenhouse. And perhaps you find yourself hesitating, unsure if investing is the right thing to do. Such uncertainties can be a stumbling block for many aspiring greenhouse enthusiasts.

So we’re here to help! In this guide, we will guide you through the process of building a small unit. Keep reading to get started!

lean-to glass or polycarbonate greenhouse against a wall past foliage
Credit: @vorn101

What Is a Greenhouse?

A greenhouse is a garden structure made with transparent walls, windows, and ceilings. It acts as a growing house for plants, flowers, and vegetables. It works by converting sunlight into heat to create optimum growing conditions.

Greenhouses are also where your parents always were when you were younger! But there are a lot of different types of greenhouses, and one might suit you better than another, such as:

  • Lean-to greenhouse: Also known as a ‘wall’ greenhouse. This type features a design that utilises an existing structure to stand.
  • Detached greenhouse: A standalone building, as the name suggests, popular for larger gardens.
  • Ridge and furrow greenhouses: Also known as gutter-connected. These are the rows of greenhouses most often used in commercial growing.

Moreover, there are differences in what those greenhouses are made from. For example, there are a lot of reasons to buy a metal greenhouse. While some tend to opt for polycarbonate or wooden greenhouses.

Polycarbonate is a plastic polymer. It’s thin and lightweight and can be stretched over the frame of a greenhouse. Often, it’s used to replace glass used in traditional greenhouses. Still, there’s always the option of:

Are usually made with an aluminium frame. They’re lightweight and strong and let in a lot of light. Although, metal will conduct heat away from your plants.

Strong and rustic-looking. They’re durable and robust. But remember, wood will need regular treatment.

Usually made from shatterproof safety glass. Good for gardens with children and won’t degrade in UV light.

Strong and lighter than glass. Polycarbonate is also better at insulating your greenhouse. It is expensive, though. Check out this post for more on polycarbonate greenhouses.

Then there are options depending on what materials you’ve chosen for your greenhouse. For example, if you went for glass – do you want floor-to-ceiling panels, and do you want them double-glazed or not?

Do you want to maximise the trapped heat inside and protect it from things like footballs? If so, you might want to think about building a dwarf wall around the base.

On top of that, you might see greenhouses with either gabled designs or curved arches. Polycarbonate suits curved arches as it’s lightweight. Gabled designs offer a more classic look to glass greenhouses. 

So if you think you’ve just about got your head around all that, you might be ready to call it a day. At this point, you might already be off to buy a polycarbonate greenhouse.

Why Build a Greenhouse?

Whether you’re already an avid gardener or not, having a greenhouse can:

  • Extend your growing season
  • Allow you to harvest some crops earlier than usual
  • Help you tend to delicate plants that would be hard to grow otherwise

A greenhouse is also great for people who might be put off by things like being out in the cold. Or by having to kneel and bend down for long periods of time. 

So are you a gardener who wants to have control over the growing climate and environment? If so, a greenhouse is the answer to all your prayers.

Why bother building one, then? Well, to be honest, we probably wouldn’t. Unless you’ve got tools, time, and energy, nowadays, it’s cheaper to buy one. Especially if you opt for a greenhouse with easy-to-assemble tongue-and-groove panels. 

But if you do want to have your cake and eat it; we’ve got the solution. You can still own or buy a greenhouse and build one. And by that, we mean – learn how to build a mini greenhouse.

vegetable bulbs growing in rows in soil in a greenhouse
Credit: @everything_louise_loves/Instagram

Mini Greenhouses

Another name you might hear for a small greenhouse or a mini greenhouse is a cold frame.

A cold frame is a small outdoor structure like a planter box. It’s got a glazed lid or window on a slope that can be opened and closed. They’re great fun and easy to build, but they won’t replace a proper greenhouse. This is because their conditions can be hard to manage.

Cold frames work on a pretty rudimentary system of simply propping up the lid and then closing it again. This allows you to keep your plants warm at night but sun them during the day.

But as they’re small, the environment they create is more susceptible to temperature change. These are all things you wouldn’t want in a full-blown greenhouse. But cold frames are great for:

  • When you can’t afford or fit a greenhouse
  • When you need to move your growing spot
  • For growing veg in the sun or cuttings in the shade
  • Protecting plants from rain and rot, or frost

A mini greenhouse is similar to a cold frame but looks more like a greenhouse shrunk in size. It uses vertical growing on shelves and usually doesn’t have any soil contact. So there are definitely a lot of reasons for building a mini greenhouse or cold frame.

polcarbonate or glass greenhouse at the end of an allotment or garden
Credit: @growingalotti/Instagram

Building a Cold Frame

To build a cold frame, you need to:

  • Cut an old pallet or window frame to size
  • Cut out the angles of your top and bottom board on your lid
  • Stretch plastic or plexiglass over your lid (if you’re not using one with a glass window)
  • Attach hinges to your lid and top plate
  • Install a handle and cut props (or use sticks)
  • Prepare your vegetable patch
  • Place the cold frame on raised, level pavers at your proposed site

Top tip: To keep your cold frame warm at night – throw a blanket over it or leave jugs of warm water in with plants overnight. You could even line your cold frame with insulating bubble wrap!

Building mini greenhouses

Although they might all sound the same, a mini greenhouse and a cold frame are slightly different. A mini greenhouse will:

  • Usually have a plastic, sheet, or glass covering (all over)
  • Be taller than a cold frame
  • Utilise shelving

But mini-greenhouses come with problems too. For example, unlike a cold frame, they usually don’t have an easy open-and-close way of ventilating them. Depending on the material, their covers can also rip or tear easily and be fiddly to replace.

Plus, placing them again a wall (like a lean-to greenhouse) can create uneven heat exchange. This can lead to unbalanced growing conditions inside. But, like a cold frame, a mini-greenhouse is still a great little alternative.

And if you want more small greenhouse ideas, you can always try and:

  • Turn a plastic storage box upside down. Pierce holes in the ‘base’ and place plants on the lid.
  • Get some old PVC pipe together. Construct a frame and slide a soil tray underneath.
  • Wrap greenhouse plastic around a wooden frame.

Hopefully, now you’ve got some small greenhouse ideas. You could even on your way to building a mini greenhouse. And as to whether mini-greenhouses work?

They’re great for cloning plants and cuttings. You can use them to harden off young plants and protect plants from frost and rain. Use your cold frame or mini-greenhouse in:

  • Spring and early summer for extra warmth
  • Autumn for protection from the first frosts (for cuttings and young plants)
  • Winter to protect from rain and damp, and to overwinter young annuals

Planning a Greenhouse

There are a few things you need to bear in mind with greenhouses. And that goes for whether you’re building a swanky clear wall wooden greenhouse or a little cold frame. 

So before you go constructing your mini greenhouse, think about the following.

Before you start building even a mini-greenhouse, choose a flat surface. You want an area with no flooding or excess groundwater and runoff. Make sure the soil of your proposed site isn’t marshy.


Then, you’ll need to level the site. Make sure to leave a gap around your greenhouse base for accessibility. But try to keep it away from areas of heavy foot traffic, or where children play.


Also, think about how convenient your greenhouse is to get to. What if it’s raining? Can you run electricity to your greenhouse if need be?

You need to position your greenhouse or even your cold frame to maximise on sunlight.


For example, your greenhouse needs to get around six hours of sun in the winter. And you’ll need to take into account the lower solar angle during these months.


If you’re just going to use your greenhouse in spring and summer, it’s better if the ridge runs north-south. You’ll still need to pick the sunniest part of your garden, though. If you can, also try to offer your mini-greenhouse some shelter to protect it from the wind.

Before placing your mini-greenhouse you might want to do some soil tests. This is especially important if you’re planting straight into the soil inside your greenhouse. Soil tests can help you analyse your soil’s nutrients to provide the best growing conditions.

Once you’ve positioned your greenhouse, you might still want to think about:


Ventilation - Although you want to lock heat in, you still need to ventilate your greenhouse. Does your greenhouse have vents? Or will you simply be opening it like you would a cold frame?


You might also think about use thermometers to track your greenhouse’s growing conditions.


Staging - Also known as shelving, can help you maximise the space in your greenhouse. It’ll also allow for adequate drainage and you can even stage plants in a mini-greenhouse. (Not so much in a cold frame, we’re afraid!).


Guttering - You could install a water butt to collect excess rainwater. You can also think about making your greenhouse experience more enjoyable. If so, why not install a little potting bench?

So whatever size and type of greenhouse you decide to add to your garden, make sure to bear those steps in mind.

And once you’re happy with that, we can move on to part 2 of this guide – what to grow in a small greenhouse.

Part 2: What to Grow in Your Greenhouse

So you’ve settled on a greenhouse. Or you’re at least thinking about building a cold frame or mini-greenhouse with the steps we showed you earlier. Great

That means you can start putting it to use in no time. But a greenhouse is a lonely place without things growing in it. So for part 2, we’ve picked some of our favourites from our list of best plants to grow in a greenhouse.

And as we’ve been talking about mini-greenhouses, we’ve picked ones that are best suited to small structures and cold frames. Check them out, below.


Sow spinach, early spring-mid June. Spinach grows close to the ground and won’t be restricted by a cold frame.

Sow beetroot and carrots to keep soil soft and dry. This can prevent rot and make them easy to dig up.
MINI-GREENHOUSEKeep young plants ready to transplant into the garden in your mini-greenhouse.

You can even grow plug plants and salads from seed.

Plant cucumber seeds mid-spring in an unheated mini-greenhouse.


COLD FRAMEEarly summer - plants like tomatoes, aubergines, and peppers can be moved to a cold frame before growing outside in summer.

The increased warmth from a cold frame will speed up growth for chillies.

Use your cold frame to propagate plants. Start from seeds or semi-ripe cuttings.
MINI-GREENHOUSEProtect crops like cucumbers, peppers, aubergines, and tomatoes.
Think home-grown Mediterranean veg!


COLD FRAMESalads like winter lettuce and lamb’s lettuce can be sown in late summer/early autumn.

Protect young plants and cuttings from early frost, snow, and damp.

This way you can extend your harvest by around 4 weeks.
MINI-GREENHOUSETake cuttings from fuchsias and geraniums to use as plant material for next year.

Like your cold frame, salad crops such as lettuce can be sown for a steady supply over winter.

Veggies like spring cabbage are almost ready for planting out.


Have you ever wondered – ‘what can you grow in a cold frame in winter?’. Well, have a look below.

Sow radishes at the very end of Winter. Get your cold frame in place about 2 weeks prior to warm up soil.

Start hardening off plants about a week before the final frost date for your area.

Overwinter fresh annuals until spring.
MINI-GREENHOUSEOverwinter delicate plants like fuchsias and geraniums. Bring on winter bulbs in your mini-greenhouse.

And there you have it, a quick guide to some great crops for beginners to grow in a mini-greenhouse!

Cold Frames and Mini-Greenhouses

So now you know that a cold frame isn’t going to replace a gutter-connected greenhouse. And you actually know what both of those things mean!

Now, cold frames and mini-greenhouses do have their drawbacks. But they’re serious workhorses considering their size! So if you want a little extra space to grow salads and bulbs even in poor growing conditions, we’d recommend them.

But, as we said, you can’t do much in the way of a nice cup of tea on a potting bench with either option. You won’t be able to do a walkthrough of all your staging with wonderful winter bulbs.

This is why we always think of cold frames and mini-greenhouses as cherries on the cake. The real main event is a full-sized greenhouse where you can grow year-round. So why not take a look at our range of customisable tongue-and-groove wooden and polycarbonate greenhouses?

And whilst you make up your mind which one to get, you may as well consider investing in a metal greenhouse.

Shop Greenhouses


In spring, in a small greenhouse, you can grow plug plants and plant cucumber seeds. In summer, grow Mediterranean crops like aubergines, tomatoes, and peppers. In autumn and winter, use your small greenhouse to overwinter delicate plants. You can continue growing leafy salads.

Where energy from the sun is re-absorbed and trapped after reflecting off the Earth’s surface. This, in turn, increases the temperature of Earth, much like the inside of a greenhouse.

We tend to think a wood-polycarbonate hybrid gives you the best structure, insulation, and sunlight. If you want, check out our list of the best greenhouses. 

As we’ve shown in this guide, you can build a cold frame or mini-greenhouse out of things lying around the garden. But if you want something you can move around and grow in year-round, you’ll need a full-sized greenhouse.

For example, our BillyOh Lincoln polycarbonate greenhouse starts at £425.

A gable greenhouse has an apex roof that connects to the upright walls. This is in contrast to a curved greenhouse design.

It depends on what you intend to use it for. If it’s for a hobby, then one of the biggest disadvantages - that it needs a lot of care, might turn out to be an advantage!

Further costs may be incurred depending on whether you want to heat your greenhouse or not. But this is all personal preference. The benefits of gardening and harvesting crops, we believe, far outweigh any negatives.

We suggest starting with low maintenance plants like salads, lettuces, and leafy greens. Or, you could look at easy-to-grow and hardy plants. Think courgettes, sugar snap peas, and even squashes.