Plant Hydrangea Seeds in spring for vibrant, long-lasting blooms. Hydrangeas thrive in well-drained, slightly acidic soil with partial to full sunlight. Start indoors in seed trays, about 6-8 weeks before the last frost. Transplant seedlings once they reach 2-3 inches tall. Keep soil moist but not waterlogged. Hydrangeas are perennials that bloom in late spring to summer and can grow for years when cared for properly.