Nasturtium always germinates. It is a foolproof annual.
Nasturtium is a half-hardy annual. It will tolerate cold, but not a heavy frost and young plants are best planted out when the risk of serious frost has passed. You can sow indoors or direct into the border outside.
For indoor sowings any time from late Feb/March onwards, into ordinary compost which is pre misted, so it's a little damp. Keep warm in a conservatory, porch or window sill and make sure the tray does not dry out and they will sprout. They always do.
For outdoor sowings, leave until late April/May and sow where you want them to grow. Do not enrich the soil for Nasturtium, otherwise they grow lots of leaves and not enough flowers. Plant in a spot which gets sun for at least half the day.
Once you have purchased your first packets of Nasturtium seed, you can easily collect seeds as at the end of the growing season. You will find dozens of seeds on the Nasturtiums, pick them off, dry them well in a warm place and then into a paper bag to store over winter in a cool dark place, a fridge is ideal.