Before You Buy a Greenhouse Read This!

Is a greenhouse the best investment you can make or an expensive waste of money? It is a big commitment of money and space, so you don’t want to find that once it’s up you’ve chosen the wrong size, location or setup, as it’s not an easy fix! I’ve used greenhouses for 30 years and I’ve just replaced mine. Before you buy yours, let me share what I’ve chosen, where I’ve put it and why those decisions matter.

Get it right, and it can become the secret powerhouse to your garden where you’ll be able to raise plants so much earlier in spring and keep harvests coming right through to winter too.

Is a Greenhouse Worth It?

For me, the value of a greenhouse comes down to a handful of things that make a big difference: faster, more reliable germination; stronger young plants; much better results from warmth-lovers like tomatoes and cucumbers; and a significantly reduced risk of problems like tomato blight.

There’s something quite freeing about a brand-new greenhouse – so much opportunity! For me, that means much more in-built shelving with a wide shelf at waist height – perfect for all those seed trays and pots – and a narrower shelf above it that continues along the back wall. These make for really efficient use of space, and are going to be perfect for starting off my seedlings and young plants from late winter onwards to enjoy a jump-start on the season, with plenty of shelving space left over for overwintering container crops.

Greenhouses are super-handy for extending the season later in the year too, so I’ve already brought in my container-grown sweet potatoes so they can bask in the warmth and enjoy a final push of growth before it’s time to harvest them. A greenhouse also means you can pick other warm-season staples like tomatoes and peppers for longer too.

I’ve included a full-length in-ground bed which faces the afternoon sun, where I’ll grow sun-loving staples next season. It’s in the same position as the in-ground bed of my old greenhouse. However, because this bed now runs along the entire length of the greenhouse, it will increase how much I can grow, while the full-height glazing will maximise light transmission and growth rates.

Greenhouse Glazing Options

The type of glazing you opt for when choosing a greenhouse is perhaps the biggest decision you’ll make, and really it comes down to glass versus polycarbonate.

Twin-wall polycarbonate is tough, rigid and more budget-friendly, and it’s arguably the safest choice if you’ve got overhanging branches nearby that might come crashing down in a storm. But the big trade-off is that all-important light transmission, which isn’t nearly as good as glass. Plus, over time, polycarbonate tends to turn brittle and can start to discolour.

Glass, on the other hand lets through far more light, which matters enormously in a temperate climate like mine, especially early and late in the growing season when every scrap of light counts. And so long as it doesn’t break, it lasts indefinitely.

I plumped for toughened safety glass too, so it’s a lot more robust than you’d expect. For me, in my climate, with that lovely traditional English look I was after, glass was the clear choice!

Ventilation

Good airflow is just as important as good light transmission, and this is something that matters year-round. In summer it’s about keeping the air moving so plants remain healthy and stress-free, and preventing overheating. In winter, ventilation stops growth going soft during a sudden warm spell, which would then leave plants vulnerable when frost returns. It also helps keep disease at bay.

I purchased a feature I’ve never had before but am totally delighted with, and recommend to all prospective greenhouse buyers: automatic vent openers. These work thanks to a wax-filled piston inside the vent arm. As the temperature rises, the wax inside expands, and that pressure gradually pushes the vent open, entirely mechanically, no external power source involved. Then as temperatures cool back down, the wax shrinks back, and so it closes again on its own. It doesn’t rely on finding time to open up before work, or getting home in time to close them before a cold night sets in. Thoroughly recommended!

The layout of the vents matters too. Because hot air rises, you want vents up at the top of greenhouse so that heat can escape, as well as an intake down low so cooler air can be drawn in. I’ve got louvred vents down the sides and one at the back, as well as four double roof vents, which together create a fabulous flow of air right through the space. Cool air comes in low, warm air is pushed out high.

If you are in a very hot climate you might also consider adding a fan for extra cooling.

Flooring

Flooring is one of those details easily overlooked, but just like decent ventilation it can change how your greenhouse performs. You could go with gravel, which drains nicely and is cheap and easy to lay, or even just bare soil.

I’m using slabs for my flooring because of the thermal mass they offer. Bricks would work just as well. Throughout the day, the slabs soak up warmth from the sun then, as temperatures drop in the evening, they release that heat slowly back into the greenhouse. It’s a simple, passive way to buffer against sudden temperature drops, and it makes a real difference to frost risk at those crucial pinch points in the season, like early spring.

Another bonus is that they’re great for ‘damping down’ in the heat of summer – simply water the slabs, and the evaporation that follows can help to quickly lower the temperature.

Shading

Another tool in my cooling arsenal is shading to keep my greenhouse cool in summer – specifically roof blinds on the sun-facing side of the greenhouse, which are another feature that’s new to me. They are simply hooked with a reach pole, pulled down, and latched into place. Releasing them is just as easy. It takes seconds, and it gives me another layer of control over temperature on those really scorching days.

It’s absolutely worth considering in-built shading like this as it makes protecting plants from hot sun a doddle, and unlike shade paint, these can be pulled down or released in a moment in response to changing weather, so there’s no having to second-guess what the summer’s likely to be like.

Roof Pitch

Consider the pitch of your greenhouse’s roof too. It should be steep enough that should you get lots of snow it easily slides off rather than just sitting there and putting weight and stress on the structure. This isn’t something I especially worry about in my mild climate, but if you are somewhere with proper winter snowfall, it’s absolutely worth considering a good snow-sloughing pitch before you buy.

Gutters

In my last greenhouse I didn’t take nearly enough advantage of all the rainwater shed from the roof. Broken and leaky gutters, and a barrel on one side only, meant lots of this precious resource was simply wasted. But not any more!

Gutters are something I insist you include in your new greenhouse! Mine came with integral gutters on both sides of the roof, complete with preinstalled downpipes ready to feed into barrels to capture every drop that falls on the roof, ensuring a steady supply to water the plants growing inside.

Size

I’m truly blessed to have a good-sized greenhouse, which means plenty of space to try growing all sorts of things.

It’s tempting to go as big as your space and budget allows of course, but there is a case to be made for not going too large, as a smaller greenhouse will be cheaper to heat, if you choose to do that. But if like me, you’re not planning on heating your greenhouse then going as large as you can does kind of make sense – this is a permanent structure after all, and planning for future growing ambitions is a wise move. A keen gardener never regretted having more space to grow!

Position

Accessibility really matters. You want to be able to walk straight in and use the space without any faff, otherwise it’s all too easy for a greenhouse to become somewhere you visit once a week rather than once a day.

And, of course, sunlight matters too. Don’t underestimate how much shade a nearby tree, fence or building will cast at different times of year. It may be worth watching where shade falls over the course of a year before committing to a spot.

Mine was an easy decision in one sense, because my garden slopes quite a bit and so had to choose one of the few flat spots available. Plus, my new greenhouse sits on exactly the same footprint as my old one.

You’ll often hear that a greenhouse needs full sun, and that is generally good advice if your garden is blessed with a position like that. But my spot is a little different. It gets lovely winter sun, while being lightly shaded in places in the peak of summer. And that has worked in my favour; it takes the edge off the most intense summer heat, while my sun-loving crops like tomatoes still do brilliantly. So don’t be afraid of a spot that isn’t textbook-perfect if the trade-offs make sense for your climate.

The Base

If you’re starting from nothing, budget proper time and money to get the ground level and firm. As I said, this is a permanent structure, so we want a base that’ll last the test of time. And if you have any pipes, cables or irrigation lines running underneath your chosen spot, check before anyone starts digging!

The base to my previous greenhouse had slumped over time, causing it to crack and giving a less-than-level foundation, so I had the base rebuilt to get both a level and really solid finish – perfect for a greenhouse that should be here for decades to come.

Regular viewers will have been very familiar with my old greenhouse that featured in so many of my videos. I was sad to see it go but, honestly, its time had come. Much of the wood was rotting and the glazing was beginning to pop out in a number of places. I suspect this may have been down to the old foundations gradually slumping and distorting things. The aluminium frame of my new greenhouse will last for a lot longer anyhow and – bonus – it won’t rust or rot!

The good news is a keen local gardener responded to my call out to come and take it away, and the old greenhouse will be getting a second life not too far from here – so it’s gone to a good home. Fabulous!

Essential Greenhouse Accessories

Now let’s talk accessories, starting with probably the single most useful bit of kit: a simple min-max thermometer. It tracks the highest and lowest temperatures since you last reset it, so you always know exactly what range you’re working with. It can be surprising how much warmer it gets in a greenhouse than outside, even on a fairly normal day. A thermometer like this is especially important for checking overnight lows in spring, when you need to be certain those tender seedlings aren’t about to be caught out by a cold snap.

For watering, I use my trusty hand-held sprayer for freshly sown trays, and a watering can for everything else.

You might also want to include a potting bench and perhaps a chair for comfort while working and to give somewhere to sit back and relax – I don’t think there’s anywhere quite as delightful to unwind with a cup of tea on a rainy day than a greenhouse. Just magic!

The ultimate accessory is of course our Garden Planner, which includes several greenhouse size options – plus a scalable one in case your greenhouse isn’t a standard size – that can simply be selected, dropped onto your plan, then expanded to the correct size. Once in position you can lock it into place to become a seamless part of your plan and then add in your greenhouse crops accordingly.

It’s sometimes difficult to know exactly how much a greenhouse can extend the growing season by. But the Garden Planner can help with this too, so you can get those timings just right. Add plants to your greenhouse then check the accompanying Plant List and scroll down to find your greenhouse plants. You’ll see that the sowing, planting and harvest dates have all jumped forward compared to the plants growing outside, while you could potentially be picking a lot later in the year too.

You can try the Garden Planner for free by taking advantage of our 7 day trial.