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The choice between a heated and an unheated propagator comes down to three things: the seeds you are sowing, the time of year, and the temperature of the place where the propagator will sit. A heated propagator supplies warmth, with temperature control depending on the model; an unheated one only traps humidity and a little solar heat. Neither type will make up for old seed, incorrect sowing depth, or poor light after germination.
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What each propagator actually does
An unheated propagator is a lightweight, usually plastic tray with a clear lid. The lid holds in moisture and creates a slightly warmer, more humid atmosphere than the open air, but it has no heat source. Without sunshine it tends towards the surrounding temperature; direct sun can make the enclosed space much hotter. A lid is not frost protection or a cooling system.
A heated propagator adds a heat source, usually in the base; only some models have an adjustable thermostat. The RHS describes it as a portable heated structure with a vented lid and adjustable, constant, thermostatic temperature control. On a suitable thermostatic model, this helps regulate the compost temperature, which is useful when the surrounding air is too cool for the seeds you want to sow.
When a heated propagator is the better choice
Heated propagators earn their place when you are sowing tender or half-hardy crops early in the year. Slow-growing chillies and some other tender crops may be started in late winter, provided you can supply suitable light and warmth afterwards. Many other crops are better sown later in spring: courgettes, for example, quickly outgrow an early sowing space. Follow the crop-specific sowing months as well as its germination temperature; an unheated greenhouse can still be too cold during the recommended window. The RHS specifically recommends a heated propagator to help ensure germination for these crops, and notes that in cold weather an unheated greenhouse propagator may struggle to maintain warm temperatures.
Some seeds have higher temperature requirements. The RHS germination guide for harvested seed lists many genera that germinate best between 18°C and 24°C, including Nicotiana (21–24°C), Ipomoea (18–24°C) and Salvia (18–24°C). If your growing space cannot reliably stay in that range, a heated propagator with a thermostat removes much of the guesswork.
A heated propagator is also useful when you need predictable, even warmth over several weeks. A suitable thermostat regulates heat around its sensor. Check the actual compost temperature and the maker’s operating range, because cold surroundings, sensor position and sunlight still affect performance.
When an unheated propagator is enough
Many hardy annuals and vegetables germinate well at 10–16°C, which is often achievable on a bright windowsill or in a greenhouse from mid-spring onwards. The RHS germination guide gives examples such as Calendula (13–18°C), Centaurea (10–16°C) and Helianthus (10–16°C). For these, an unheated propagator with a clear lid can provide the humidity and slight warmth needed without any electricity.
An unheated propagator is also a sensible choice if your home or greenhouse is already consistently around 18°C. The RHS advice on sowing seeds indoors says most seeds need a temperature of around 18°C to germinate, but check the seed packet. If your room meets that, the lid alone may be all you need to keep moisture in and speed germination.
Some seeds germinate better with a day–night temperature swing, which an unheated propagator on a windowsill naturally provides. A constantly heated propagator may be less suitable for those, so check whether the seed packet mentions fluctuating temperatures.
Finally, unheated propagators are cheaper, use no electricity and have no thermostat to set. If you are sowing a small batch of hardy seed in spring, they are often the practical choice.
Decisive checks before you choose
- Read the seed packet. It should state a germination temperature or a recommended sowing time. Compare the stated range with measurements at the proposed position. Added heat is useful only when the growing medium would otherwise be too cool.
- Measure your actual growing spot. Use a minimum/maximum thermometer in the room or greenhouse where the propagator will sit. Compare both night and daytime readings with the requirements of your particular seeds; there is no single minimum that suits all crops.
- Consider the time of year. Sowing in late winter usually means cooler indoor temperatures, so a heated propagator extends your sowing window. Sowing in mid-spring may make an unheated propagator perfectly adequate.
- Plan for after germination. Both types need attention once seedlings appear. Remove the lid as soon as seedlings emerge, and move them to a bright, cooler spot. If you use a heated propagator, move emerged seedlings to suitable growing-on conditions. If slower seeds remain in the unit, keep their required heat rather than switching off every tray together.
Practical limitations to keep in mind
A heated propagator is not a mini greenhouse. It warms only the seed tray area, not the surrounding air. Seedlings still need good light and a frost-free place to grow on after germination. In a very cold unheated greenhouse, even a heated propagator may struggle to hold its set temperature if the ambient temperature is far below the thermostat range; the RHS warns about this. Check the manufacturer’s stated operating conditions.
An unheated propagator has no active heat source, although sunshine can warm it above the surrounding air. In a consistently cool position the compost may be too cold for tender seeds, while direct sunshine can overheat it. It also cannot protect against a sudden cold snap unless you move it somewhere warmer.
Neither type overcomes poor seed viability, old seed, incorrect sowing depth, or insufficient light after germination. Heating cannot restore dead seed, although seed viability does not end automatically at the printed date, and an unheated propagator will not speed up a seed that needs cold stratification before it will germinate.
If you buy a heated propagator, check whether it has an adjustable thermostat. Some basic models have a fixed heat output, which can overheat a small tray or dry out the compost quickly. A thermostat lets you match the heat to the seed packet’s recommendation.
Product types to look for
You do not need a specific brand to make a good choice, but these product types are worth comparing.
- Heated propagator with adjustable thermostat. Look for a clear vented lid, a thermostat range that covers the temperatures on your seed packets, and a base that fits standard seed trays or modules. Check the power consumption and whether it needs an RCD-protected supply, especially in a greenhouse.
- Unheated propagator with adjustable vents. Look for a sturdy clear lid with vents you can open as seedlings grow, and a base deep enough for your pots or modules. Check that the lid seals well enough to hold humidity but can be propped open for ventilation.
- Clear plastic lid or recycled food container. For small batches, a piece of glass, a propagator lid or a clear plastic fruit tray over a pot or tray can act as an unheated propagator. The RHS suggests this as a low-cost option. Make sure it is clean and lets light through.
Useful products for this job
The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.
- Heated propagator with adjustable thermostat: Provides controlled warmth for tender seeds that need 18°C or more, especially when sowing early in the year. Check the thermostat range covers the temperatures on your seed packets, the base fits your seed trays, and the unit is suitable for the location (e.g., RCD protection for greenhouse use). Search Amazon for Heated propagator with adjustable thermostat.
- Unheated propagator with adjustable vents: A low-cost way to hold humidity and slight warmth for hardy seeds when the room or greenhouse is already warm enough. Check the lid has adjustable vents, the base is deep enough for your pots or modules, and the lid seals well enough to retain moisture but can be propped open. Search Amazon for Unheated propagator with adjustable vents.
- Reusable clear propagator lid: A replacement lid can let you reuse an existing seed tray; a suitable clean container already at home may avoid a purchase. Make sure the lid or container is clean, lets light through, and fits over your pot or tray without touching the compost surface. Search Amazon for Reusable clear propagator lid.
Related garden guides
- Greenhouse thermometers: tracking minimum and maximum temperatures
- Growing winter salad in a cold frame
- More greenhouse & growing guides
Sources and further guidance
Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.