If you don’t have a heated greenhouse and want to start your seeds off early to get a head start on spring, it’s time to invest in a heated propagator.

You can sow slow-growing tender perennials such as geraniums now, so they start flowering in time for summer, along with seeds of houseplants, tuberous begonias and exotics such as birds of paradise (Strelitzia reginae), and even perennials 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 out a week or two later.

There’s a wealth of heated propagators of different sizes and prices, so do some research into exactly what conditions the seeds you want to sow need.

“Check whether the heated propagator has a thermostatic control which will control the temperature more constantly and click off if the room temperature becomes too warm,” says Richard Gianfrancesco, head of research at Which? Gardening. “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 the propagators have to be monitored more carefully to ensure temperatures inside them don’t become too high.

Think about the seeds you are going to sow in a heated propagator. 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 you want to plant tender veg outside, think about timing.

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

But seeds which take a long time to germinate, such as chillies and sweet peppers, are ideal to start off in a heated propagator. Be aware that tender plants will need to be kept indoors for weeks after being potted on, 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), which switched off the heating element at higher room temperatures to stop the compost becoming too hot.

The Garland Triple Top Electric Windowsill Propagator (£26) also scored highly thanks to its ease of use. Three separate units sit neatly in the heated base and offer great versatility.