March in Denver may still deliver snowstorms on a whim, but indoors? It’s officially growing season. Starting seeds inside gives gardeners a jump on Colorado’s short growing window… especially for heat-loving plants that need extra time to mature.
What you need to get growing: The basics are simple. CSU Denver Extension Horticulture recommends a soil-less seed-starting mix (light and fluffy is key), containers or trays with drainage, labels, consistent moisture, and light (lots of it!). A sunny window helps, but an overhead grow light on a timer produces sturdier, less leggy seedlings. Add a gentle fan to encourage strong stems and reduce disease, and a heat mat if you want faster germination.
What to start and when: In Denver, aim to start seeds indoors 4–8 weeks before the average last frost (typically early May), which makes March prime time for long-season veggies like tomatoes and peppers. The Clemson Cooperative Extension recommends skipping the indoor phase altogether for fast growers and root crops, such as carrots, beans, and corn, and planting them directly outdoors later.
Small pot or big pot? Both work! You can sow directly into individual cells or small compostable pots (the “one-step” method) or start seeds in a flat and transplant once true leaves appear. Just handle seedlings by the leaves, not the stem, and use a sterile seed-starting mix, not garden soil.
Top tip before planting outside: Hardening off is key. Gradually introduce seedlings to the outdoors over a week or two so they don’t go into shock. CSU Denver Extension Horticulture suggests taking them outside and bringing them back inside, adding more and more time outside each day, for a total of five to seven days.
Starting seeds indoors may feel like a small project, but it pays off in a big way. With a little planning, the right light, and some patience, you’ll head into planting season with healthy transplants that are ready to take off the moment they hit the garden. In a climate where spring weather keeps us guessing, seed-starting puts you back in control and sets the stage for a more productive, more rewarding summer garden.