Smart Garden Wall Thermometer: Easy-Read °C and °F Thermometer for Greenhouses, Sheds, Polytunnels and Indoors

Know what it's really like in there

A greenhouse on a sunny March afternoon can feel like the tropics. The same greenhouse at four in the morning can be barely above freezing. Without a thermometer, you're guessing at both, and plants pay for the wrong guesses. Seedlings scorch when the vents stay shut. Tender plants blacken when a cold night slips past unnoticed. A thermometer on the wall turns those guesses into numbers, and once you know the numbers, you know when to open up, shade, wrap up or move things around. The Smart Garden Wall Thermometer is a simple, slim, no-fuss way to do exactly that. It needs no batteries and no app, and there’s nothing to set up. You hang it where your plants are and glance at it whenever you’re passing.

What is the Smart Garden Wall Thermometer?

It's a wall-mounted thermometer with a robust plastic housing and large scales marked in both Celsius and Fahrenheit. It measures 19.3cm tall, 5.1cm wide and just 4mm deep, and weighs about 40g. That makes it easy to fit on a greenhouse frame, shed wall, potting bench upright or windowsill surround without getting in the way. Smart Garden designs it for use indoors or under cover outdoors. That covers greenhouses, polytunnels, sheds, garages, porches, conservatories and garden rooms.

Who makes it?

Smart Garden Products is a British garden products company based in Abingdon, Oxfordshire. It supplies a wide range of garden essentials, from plant supports and growhouses to meters, lighting and wildlife care. The wall thermometer sits in its range of growing meters, alongside a wooden version.

Why do I need a thermometer in my greenhouse?

Because temperature drives almost everything plants do under glass, and it changes much faster than you'd expect. A small greenhouse can rise 10 to 20°C above the outside temperature within a couple of hours of sunshine, even in early spring. On clear nights, it can lose that heat just as quickly. A thermometer tells you: When to open vents and doors, so seedlings and young plants don’t cook on a bright day. When to shade the glass, usually from late spring onwards, to keep summer temperatures in check. When frost protection is needed, such as fleece, bubble wrap insulation or a greenhouse heater. Whether conditions suit what you’re growing, so you can move plants to warmer or cooler spots. How effective your insulation or heating actually is, rather than how effective you hope it is.

What temperature should a greenhouse be?

It depends on what you're growing and the time of year. As a broad guide: Most summer greenhouse crops, such as tomatoes, cucumbers, peppers and chillies, grow happily in daytime temperatures of roughly 18 to 27°C. Once temperatures climb much above 27 to 30°C, growth slows and some crops struggle. Tomato flowers may drop, and plants wilt even with damp roots. That’s the signal to ventilate, damp down the floor and shade the glass. Overnight, most of these crops do well staying above about 10 to 13°C. Cucumbers, peppers and aubergines are the most sensitive to cold nights. Over winter, a frost-free greenhouse is one that stays above 0°C. Many gardeners aim for a minimum of around 5°C to overwinter tender perennials such as pelargoniums, fuchsias and salvias. Citrus and more tender plants generally prefer it warmer. For germinating seeds, many crops need warmer conditions than the greenhouse air offers in early spring. Tomatoes, chillies and peppers often germinate best at around 18 to 25°C, which is why many gardeners start them on a heated propagator or a warm windowsill. A thermometer on the wall lets you compare what your plants want with what they’re actually getting.

Where should I put a greenhouse thermometer?

Placement matters more than people think. A thermometer in direct sun will read far higher than the air your plants are sitting in, because the housing and liquid absorb heat from sunlight. For readings you can trust: Hang it out of direct sunlight, ideally in a shaded spot or on the north-facing side of the structure. Mount it at roughly plant height, since air at the ridge can be much warmer than air at bench or floor level. Keep it away from heaters, heating pipes and the door, where draughts and warm air give misleading readings. Avoid fixing it directly to glass, which heats and cools faster than the air around it. If your greenhouse is large or has distinct warm and cool areas, two thermometers in different spots will show you the difference. That’s useful when deciding where to place tender and hardier plants.

Can I use it outdoors?

Smart Garden recommends using it indoors or under cover outdoors. That means anywhere sheltered from direct rain, such as a greenhouse, polytunnel, cold frame lid area, shed, garage, open-fronted log store or covered porch. It isn't intended to be fixed to a fence in the open garden, exposed to the weather.

Ways to use it around the garden and home

Greenhouses and polytunnels are the obvious home for a wall thermometer, but it's useful wherever temperature matters: In the shed or garage, to check that stored dahlia tubers, potatoes, apples and onions stay cool but frost-free over winter. In a porch or conservatory used for overwintering tender plants, to make sure it doesn’t drop too low on cold nights. In a potting shed where you start seeds, to judge whether it’s warm enough to sow yet. On a windowsill or indoor growing area, to see whether that bright spot is actually too hot in summer or too cold at night for seedlings and houseplants. In a garden office or summerhouse, so you know whether you need the heater on before you settle in. In a utility room or larder, where stored produce keeps better in cooler conditions.

How do I read a liquid thermometer accurately?

Read the top of the column at eye level rather than from above or below, which can make it look higher or lower than it is. Read it in the scale you prefer. The °C scale is on one side and °F is on the other, so you can follow whichever system your seed packets, books or habits use. Give it a few minutes to settle if you’ve just moved it, or if the door has been open and the air has changed suddenly.

Does it record the highest and lowest temperatures?

No. This is a straightforward current-temperature thermometer, showing the temperature at the moment you look. It won't record overnight lows or daytime highs while you're away. If you want to know how cold it got overnight, or how hot it got on a sunny afternoon while you were at work, a max-min thermometer is the tool for that job. Many growers use both. A simple wall thermometer by the door gives a quick reading as you walk in, and a max-min thermometer tracks the extremes.

When should I check my greenhouse temperature?

As often as you can, especially during the changeable months. In spring and autumn, check first thing in the morning to see how cold it got overnight. Check again around midday on sunny days, when temperatures can climb fast. In summer, the midday check matters most, as overheating is the main risk. In winter, the morning check is more useful for judging frost protection. After a while, you’ll start to learn your greenhouse’s habits, such as how quickly it warms after sunrise and how much warmer it stays than outside overnight. That knowledge is worth having. It helps you decide when to sow, when to plant out and when to move tender plants under cover in autumn.

How do I use temperature to decide when to harden off?

Hardening off is the process of gradually getting plants raised under cover used to outdoor conditions before planting them out. A thermometer in the greenhouse or cold frame helps you judge the gap between inside and outside. When nights inside a cold frame or unheated greenhouse are consistently mild and the risk of frost has passed for your area, plants are usually ready to move outside for longer spells. Then they can go out overnight, and finally into their permanent spots. Frost-tender crops such as courgettes, squash, sweetcorn, beans and tomatoes need particular care, as even one cold night can set them back badly.

Looking after your wall thermometer

It needs very little care. Wipe the housing with a soft, damp cloth if it becomes dusty or grubby, which helps keep the scale easy to read. Avoid solvents and abrasive cleaners, which can mark plastic. Keep it out of direct sun, both for accurate readings and to help the housing last longer. Handle it gently, since knocks and drops can damage the glass tube inside.

Frequently asked questions

Does the Smart Garden Wall Thermometer show Celsius and Fahrenheit? Yes, it has large, easy-to-read scales in both °C and °F. How big is it? It measures 19.3cm high, 5.1cm wide and 4mm deep, and weighs about 40g. Can I use it outside? It’s designed for indoor use or outdoors under cover, such as in a greenhouse, polytunnel, shed or porch. Does it need batteries? No. It’s a liquid-in-tube thermometer, so there’s nothing to power or charge. Will it record the overnight minimum? No. It shows the current temperature only. For overnight lows and daytime highs, use a max-min thermometer. What is it made from? The housing is made from robust plastic (polystyrene).

1 x Smart Garden Wall Thermometer

Key Features:

Dimensions:

Height: 19.3cm
Width: 5.1cm
Depth: 0.4cm
Weight: 40g