IF YOU don’t have a heated greenhouse and want to start your seeds off early to give yourself a head start on spring, it’s time to invest in a heated propagator.

Not only can you sow slowgrowing tender perennials such as geraniums now, so they start flowering in time for summer, but you can also sow seeds of houseplants, tuberous begonias and exotics such as birds of paradise (Strelitzia reginae), and even perennials which you want to flower this year.

First-year flowering perennials, which can be sown early in heated propagators and then grown on in a greenhouse or cool room, include achillea, helenium, echinacea, salvia and coreopsis.

They can be hardened off in a sheltered spot in early May and planted in their final position a couple of weeks later.

Garden centres, mail order and online retailers offer a wealth of heated propagators of different sizes. Prices can vary enormously, so it’s worthwhile doing some research into exactly what conditions the seeds you want to sow need.

“Check whether the heated propagator has a thermostatic control which will check the temperature more constantly and click off if the room temperature becomes too warm,” said Richard Gianfrancesco, head of research at Which? Gardening, the Consumers’ Association magazine.

“This is important because windowsill temperatures tend to be unstable, in that they might have sun for part of the day and be cooler at other times.”

Cheaper propagators tend to have one or two settings, such as cool and warm, irrespective of indoor temperatures, which means that the propagators have to be monitored more carefully to ensure temperatures inside them don’t become too high.

Think carefully about the seeds you are going to sow in a heated propagator, Mr Gianfrancesco advised. If you want to grow tomatoes, cucumbers and courgettes which you are expecting to grow on in the greenhouse in summer, you can sow them early in heated propagators.

If, however, you want to plant tender veg outside, think about timing.

Courgettes, melons and sunflowers, for example, are traditionally sown late and can’t be planted out in the garden until the end of May or beginning of June, so they won’t benefit from being germinated in a heated propagator early because you’ll still have to keep them indoors.

However, seeds which take a long time to germinate, such as chillies and sweet peppers, are ideal to start off in a heated propagator. But be aware that tender plants will need to be kept indoors for weeks after being potted, so you’ll need enough space to keep them in your house or in a frost-free greenhouse or conservatory until the weather is warmer.

A recent heated propagator test by Which? Gardening found its best buy to be a Stewart Heat & Grow Thermostatic Propagator (£30 - stockists 0208-603-5700, stewartcompany.co.uk), which switched off the heating element at higher room temperatures to stop the compost becoming too hot.

The Garland Triple Top Electric Windowsill Propagator, at £26 (01384-278256, garlandproducts.

com), also scored highly thanks to its ease of use. Three separate units sit neatly in the heated base and offer great versatility. For example, one was fully closed for germination, another had the vents open for hardening off seedlings and one had its lid off for growing on. At just 18cm wide, it’s also ideal for a narrow windowsill.

As well as heated propagators, you can also buy warming mats which are placed underneath regular propagators and are suited to a narrow windowsill.

The trial concluded: “If your windowsill is likely to get warmer than 20C during the daytime, a propagator with a thermostat, such as the Stewart, is a good investment.

“Bear in mind that the temperature inside the propagator is about ten degrees higher than the room temperature. It would therefore be best suited to a cool room that doesn’t go above 10-15C during the day.”

● Which? Gardening is a subscription- only magazine. For details on how to buy three issues for £3, telephone 01992-822800 or visit which.co.uk.