Planting from seed
Planting a flat of marigolds from the garden centre is easy, but growing them from seed is almost as simple. "As long as they have the warmth they need [20°C/70°F] to germinate, most marigolds are pretty much guaranteed," Dam says.
It's possible to sow marigold seed directly in the garden, although plants won't bloom until late summer. Wait until after the final frost and choose a sunny location. Cover the seeds with a thin dusting of soil and keep the bed moist. While Dam remembers his grandfather seeding a row of the old-fashioned 'Sparky' directly into the garden every year, he doesn't recommend outside sowing. Not only does direct sowing result in later flowers, because marigolds are planted so close to the surface, weather or animals may disturb the seeds.
For earlier blooming flowers, start seeds indoors about six weeks before your last expected frost. Marigolds can be transplanted into the garden when they're quite young, so you may want to start them in pots rather than flats. That way you won't have to transplant them indoors and then again in the garden. Press the seeds down into the soil, then lightly dust with soil to barely cover and put them in a warm, bright location. Cover with plastic to keep the seedbed moist (remember to remove as soon as seeds germinate) or mist the soil surface twice a day until seeds germinate, usually five to 10 days. Once the plants are up and running, keep the soil moist but not waterlogged; thin the seedlings to encourage the plants to fill out.
Transplant marigolds when all danger of frost is past and the soil is warm. Marigold seedlings don't mind being moved when they're only two or three inches (five to seven centimetres) tall with only four to six leaves. In fact, plants that are already in bloom resent transplanting, and will sulk for awhile.
Choose a sunny spot and plant them in well-drained, average garden soil with plenty of organic matter. But don't overfertilize or you may end up with more leaves than flowers. Dam recommends applying a natural fish or seaweed fertilizer once or twice during the season for flower production.
For a mass of colour, plant small marigolds about eight inches (20 centimetres) apart. Plant the larger American varieties 16 to 24 inches (40 to 60 centimetres) apart; any closer and mould may develop on the plants.