Do I need a heated propagator?

The honest position is that a heated propagator solves a germination problem, and most people's actual problem is a light problem. Heat gets a seed up; light is what turns it into a plant. Buying heat without solving light usually produces the same failures a fortnight earlier.

Step by step

1. Work out whether you have a germination problem at all.If your seed comes up reliably and then goes long and pale, heat is not your issue. That is light, and a heated propagator will make it worse.
2. Know which seeds actually need it.Chillies and aubergines want 20 to 25°C and sulk below it. Peppers, tomatoes sown in February, basil, and tender bedding such as begonias and cosmos benefit. Brassicas, lettuce, peas, beans and most hardy annuals do not.
3. Consider the free alternatives first.The top of a fridge, an airing cupboard, a shelf above a radiator or a sunny windowsill through the day all reach 18 to 22°C, which covers most of what people sow. The catch is that a dark airing cupboard needs checking daily so seedlings are moved to light the moment they appear.
4. Choose a thermostatic model if you buy one.Cheap unthermostatted propagators run at a fixed offset above room temperature, so they overheat in a warm room and underperform in a cold greenhouse. A thermostat is the difference between a tool and a gamble.
5. Set the temperature by the seed, not to maximum.Most seed germinates best between 18 and 22°C. Chillies and aubergines want 25°C. Above about 25°C many seeds germinate worse, and lettuce refuses altogether above roughly 25°C.
6. Take the seedlings off the heat as soon as they germinate.Bottom heat after germination drives soft, stretched growth and dries the compost unevenly. Move them to the brightest, coolest place you have.
7. Solve the light at the same time.If you are sowing in January or February, a simple LED grow light for fourteen to sixteen hours a day is what actually determines the quality of the plant. Heat plus a dim windowsill is the recipe for leggy seedlings.
8. Consider a heat mat instead.A soil-warming mat with a separate thermostat is more flexible than a moulded propagator: it takes whatever trays you have, and it can be turned off while the trays stay put.
9. Do not run it all winter.It is a germination tool used for a few weeks a year. Left running under seedlings for months it costs money and produces poorer plants.

Tips

• If you sow only in March and April, a heated propagator is very unlikely to earn its place.
• An airing cupboard costs nothing and works, provided you look in it every day.
• The order of value for most people is: better light, then a cold frame, then a heated propagator.

Common mistakes

• Buying heat to fix leggy seedlings — Legginess is a light problem. Adding warmth to poor light accelerates exactly the growth you are trying to avoid.
• Leaving trays on the heat after germination — Warm roots and dim light produce soft stretched plants and uneven drying. The moment they are up, they come off.
• Running an unthermostatted propagator in a warm room — It adds a fixed number of degrees to whatever the room is, which can push it well past the point where germination falls off.

Alternatives

• Airing cupboard or top of the fridge: Free, warm and effective. Must be checked daily so seedlings go to the light immediately, because these places are dark.
• Soil-warming mat plus separate thermostat: Flexible and accurate, takes any tray, and can be switched off without moving anything.
• LED grow light: Solves the problem most people actually have. Fourteen to sixteen hours a day, close above the seedlings, from January to March.
• Sowing later: Costs nothing and often produces better plants. An April sowing in natural light routinely overtakes a February one on a dim sill.

Questions people ask