Answer: French Vanilla marigolds will reach a height of about 24" and a width of 18" or more in a good garden soil with proper fertilization and watering.
I would space the plants 18" to get a good, solid cover early that will continue on through the season. Slightly wider spacings of 24" are fine, but will take much longer to give the solid cover look.
Fertilize them well at planting and repeat fertilizations every 4 to 6 weeks to keep plants healthy and vigorous. Watch out for spider mites as they are really fond of marigolds in the heat of summer. For a spectacular fall display start some more seed for transplanting into the garden in late August. Mite problems are greatly reduced in fall and the marigolds will put on quite a show until a frost shuts them down.
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