All You Need To Know Before Buying a Greenhouse

There’s nothing quite like a greenhouse to help you grow more of what you love over a longer season. But buying one is a big decision, with lots of things to get right, so I’m going to share everything you need to consider when picking the perfect greenhouse for your unique garden.

What Size Greenhouse is Best?

If you’re a keen gardener, you’ll appreciate that feeling of never quite having enough space to grow everything you’d like! And that certainly translates to a greenhouse. Few gardeners regret having more greenhouse space, so my advice is to go all out for the biggest one you can afford!

But make sure you have enough space left around the greenhouse to look after it. Greenhouse glass gets dirty through the year, limiting the light available for plants inside. The path around mine makes it so much easier to clean the glass and change any broken panes. There’s room for a water barrel or two, and plenty of space to make it easy to harden off young plants for a week or two as I acclimatise them from the cosseted environment inside the greenhouse ready for planting outside.

Of course, a bigger greenhouse will be more costly, but a footprint that gives space for both raising seedlings and overwintering tender plants, then growing summer staples like tomatoes is worth the investment. For many people, a greenhouse of about 6 x 8ft (1.8 x 2.4m) gives plenty of room for seedlings in spring before switching to summer crops.

Freestanding greenhouses are the most popular type, but lean-to greenhouses that make use of an existing wall can be fantastic because that wall will absorb and reflect back heat onto plants grown inside. This makes a powerful difference in winter or early on in the growing season – it’s like having a free source of heat to keep plants warmer overnight.

My greenhouse, with its wooden frame, is really showing its age. Moss readily grows in many of the areas where water collects, and parts of the frame are starting to rot. A more durable alternative would be an aluminium frame. Many greenhouse suppliers offer the option of powder-coating the frame to a color of your choice, turning what was a purely utilitarian object into a thing of great beauty that could well become the proud centerpiece to your garden.

Glass vs Polycarbonate Greenhouses

After size and style, the next major consideration is whether to opt for glass or clear polycarbonate glazing. Polycarbonate is strong and a lot safer as it won’t shatter, which offers peace of mind if you’ve got kids running about or are in a windy area. It’s lightweight and flexible, but has poorer light transmission than glass. Polycarbonate is also prone to discoloring and scuffing, and over the years can become quite brittle, making it likely to need replacing sooner.

Glass on the other hand is clearer, so more light can reach the precious plants inside – and it looks the business too! But traditional glass can shatter into dangerous shards which are difficult to remove from soil, while overlapping panes are notoriously tricky to clean.

The solution is modern tempered glass. Tempered glass is more expensive, but it’s much stronger than traditional glass and usually comes supplied as bigger panes, making it dramatically easier to keep clean because there are fewer fiddlier joins. And if it does break, the glass typically shatters into many smaller pieces, which will pose less of a danger than great big sharp shards.

Air Flow and Insulation

You might be tempted to think that greenhouses are all about trapping warm air – and that’s true, up to a point. But without good airflow, on hot sunny days you could end up literally cooking your plants.

Stagnant air also increases the risk of diseases and pests. Most greenhouses will come with at least one roof vent, but my top tip is to make sure it also has one or more side vents. This will create a chimney effect, drawing in air from the low-down vents and expelling it from the roof vents, keeping the air inside moving.

Even in late winter a sunny day can cause temperatures inside the greenhouse to soar. Seedlings may be just fine with that, but with it comes a risk of encouraging lots of fresh, sappy growth too early on in the season – which is then susceptible to a sudden drop in temperature or disease. Plentiful ventilation should encourage steadier, stockier, and ultimately stronger growth in your plants. The more openings the better!

Install automatic roof vent openers if you can. These contain a wax-like substance that expands when it gets warm, pushing open the vent without the need for you to be involved – handy if you’re a little forgetful like me!

Heating a greenhouse can be a costly affair, so very few gardeners do that any more. I prefer to start tender seedlings in my house, then move them out to the greenhouse once it’s warm enough. I can always bring them back indoors if an especially chilly night is forecast, or throw some horticultural fleece over seedlings to give a little extra insulation. You can insulate your greenhouse by fixing bubblewrap to the inside of the greenhouse for the colder months.

Greenhouse Flooring and Foundations

Any greenhouse needs a firm, level base to sit on. Bare soil can shift and settle, so opt for a skirt of concrete, brick or paving slabs that the frame can sit on, or purchase the base kit that’s usually offered with an aluminum greenhouse.

Gravel or stone chips are a cheap flooring option inside, and allows water to quickly drain away, but keeping it clean is tricky because it’s hard to sweep up potting mix or plant matter that drops down. Before long you’ll have weeds growing between the chips.

A more solid flooring like slabs or bricks is preferable because it will act a bit like a storage heater, soaking up heat during the day to release it at night, keeping the air space above that little bit warmer on chilly nights. It can also be wetted or ‘damped down’ on hot days to help knock temperatures back a little and raise the humidity, which will protect against common greenhouse pests like red spider mite.

Must-Have Greenhouse Accessories

Any greenhouse will benefit from some sort of bench (often known as ‘staging’) on which you can put your plants and seedlings. By raising plants off the ground, you’ll also have space underneath to store pots, tools and potting mixes. Extra shelving offers additional space for various bits and pieces.

If there’s enough space for a separate potting bench, this is well worth including. I like the flexibility of my setup of a simple table and chair, which is easy to move so I can lift them outside to work on warm days. My staging easily comes apart for speedy removal in the summer when more of the footprint is taken over by tomatoes and cucumbers growing in the greenhouse beds.

I like to keep a close eye on the temperature – important for tender seedlings early in the season, and for monitoring heat levels in summer. A simple maximum-minimum thermometer does the job. You can choose a simple analog one, or opt for models that can be viewed remotely on an internal display or even your phone if you want to keep a really close eye on things.

Along with adequate ventilation, greenhouse shading helps keep temperatures from soaring too high on hot days. Internal blinds add extra cost but are undoubtedly convenient. An inexpensive options is shade paint, applied before the start of each summer and washed off in autumn.

Rainwater is best for plants, so it pays to use as much of it as possible. Check whether the greenhouse you’re interested in comes with guttering, and connect up one or more water barrels to make it really quick and easy to keep your crops quenched.

Let me know if you’re considering a greenhouse in the comments below – I’d love to know what you’re going to grow in there!