The range of temperatures that seeds require to grow varies considerably betweenThermometer in plants different varieties and you should always check the seed packets which will clearly show you the best time of year to sow them and if they need to be sown indoors / under glass of can be sown outside. If temperatures are too cold, the seeds may rot and if they are too hot, they may not grow (they remain dormant).

See Also:

Watering

Suitable Compost / Soil

Correct Sowing Depth

Light