Grow Your Own Marigolds from Seed! Instructions: 1. Prepare Soil: Use well-draining soil enriched with compost or organic matter. Loosen the soil to a depth of about 6 inches. 2. Start Indoors (Optional): Marigold seeds can be started indoors 4-6 weeks before the last frost date in your area. Use seed trays or pots filled with seed-starting mix. 3. Plant Seeds: Plant marigold seeds about inch deep in the soil or seed trays. Water gently after planting.