A You can sow seed in early January, but growth will be slow in midwinter, even with plenty of heat, because of low light levels. And however early they're sown, pelargoniums won't be safe outdoors before mid to late May.
If you leave sowing until late January or early February, seedlings will develop more rapidly as days grow noticeably longer. They'll also be more stocky than early-sown plants and less likely to suffer from botrytis or pelargonium rust.
To achieve good-sized young plants, grow seedlings on in individual containers, potting them up each time you see roots appearing at the pot bases.
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Feed regularly and keep plants in full light. In future, if you want more substantial plants for moving outdoors, take cuttings in August and grow these on through winter.