When to Plant Seeds Indoors in Denver?

To plant seeds indoors in Denver, you should generally start them 4-8 weeks before May 5th, which is consistently considered the last spring frost date for the region. This timing allows your seedlings to develop sufficiently before they can be safely transplanted outdoors after the risk of frost has passed.

Optimal Timing for Indoor Seed Starting in Denver

The primary factor determining when to sow seeds indoors in Denver is the average last spring frost date. This crucial date signals the approximate time when the danger of freezing temperatures has largely subsided, making it safe for tender plants to be moved outside.

Understanding the Last Spring Frost Date

In Denver, the last spring frost date is generally considered to be May 5. This date serves as a critical benchmark for gardeners to plan their planting schedules, especially for those starting seeds indoors. Planting too early can result in leggy, overgrown seedlings, while planting too late might mean your plants aren't mature enough by the time outdoor conditions become ideal.

Calculating Your Indoor Planting Window

Based on the May 5th last frost date, the optimal period for planting seeds indoors is:

• Earliest Start: 8 weeks before May 5th (approximately March 10)
• Latest Start: 4 weeks before May 5th (approximately April 7)

This 4-8 week window provides flexibility depending on the specific plant's growth rate and your desired transplant size. Faster-growing plants might only need 4-6 weeks indoors, while slower-growing varieties or those needing to reach a larger size before transplanting could benefit from the full 8 weeks.

Key Considerations for Success

• Consult Seed Packets: Always refer to specific seed packet instructions, as they often provide tailored guidance for the particular plant variety.
• Light Requirements: Ensure your indoor seedlings receive adequate light (12-16 hours per day) to prevent them from becoming leggy. Grow lights are highly recommended.
• Temperature Control: Maintain appropriate temperatures for germination and growth, usually between 65-75°F (18-24°C) for most common vegetables and flowers.
• Hardening Off: Before transplanting outdoors, gradually acclimate your seedlings to outdoor conditions (sun, wind, cooler temperatures) over 7-10 days. This process is known as "hardening off" and significantly reduces transplant shock.

Summary Table: Denver Indoor Seed Planting Schedule