Rhubarb Chard
Swiss Chard in the US

Right, for sowing Swiss Chard, the timing really depends on where you are, you know, your USDA zone. My GrowMore CookMore app gives you the exact dates for zones 5-7, but you'll want to adjust for your local last frost. Generally, you can sow seeds indoors a few weeks before your last frost, or directly outdoors after all danger has passed. Plant them about half an inch (1 cm) deep, and if you're sowing direct, aim for about 4-6 inches (10-15 cm) apart, thinning them out later. You can do succession sowings every few weeks until mid-summer for a continuous harvest, or later in zones with a long growing season.

If you started your chard indoors, once the seedlings have two proper leaves, you'll need to prick them out gently and pot them up into bigger pots. They're not keen on root disturbance, so be careful. Before planting out, harden them off for a week or two, gradually getting them used to the outdoors. Plant them out about 8-12 inches (20-30 cm) apart, or a bit closer if you want baby leaves. They'll grow well in containers too, as long as they get enough space and good compost, like I say.