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Starting Seeds Indoors in Colorado

Starting Seeds Indoors in Colorado

Starting Seeds Indoors in Colorado: A Head Start for a Successful Garden

Colorado’s growing season is short, and very unpredictable. Snow in May? Totally normal. That’s why starting seeds indoors is one of the smartest moves a Colorado gardener can make. It gives plants a strong head start and helps you make the most of our limited frost-free window.

Here’s how to do it right.

Why Start Seeds Indoors in Colorado?

Our high elevation, cool nights, and late spring frosts can stunt or kill young plants if they’re planted outside too early. Starting seeds indoors allows you to:

  • Extend the growing season

  • Grow varieties not available as starts

  • Save money compared to buying transplants

  • Produce healthier, stronger plants

Warm-season crops like tomatoes, peppers, and herbs especially benefit from an indoor start.

When to Start Seeds Indoors

Timing is everything. In most of Colorado, the average last frost date falls between May 10–30, depending on elevation and location.

A good rule of thumb:

  • Start seeds 6–8 weeks before your last expected frost

  • Count backward from your local frost date

Typical indoor start times:

  • Tomatoes & peppers: mid–March

  • Herbs: early to mid–March

  • Brassicas (broccoli, cabbage): late February to early March

  • Flowers: varies by type (check seed packets)

Always read the seed packet—it knows best.

What You’ll Need

Starting seeds doesn’t require fancy equipment. Keep it simple: JAX has everything you need here: https://jaxgoods.com/collections/vendors?q=Jiffy

Colorado homes can be dry in winter, so moisture control matters.

Planting Your Seeds

  1. Fill containers with seed-starting mix and lightly moisten

  2. Plant seeds at the depth listed on the packet

  3. Gently cover and mist with water

  4. Label everything—trust us, you’ll forget

  5. Place containers in a warm spot (65–75°F)

Once seeds sprout, move them under lights immediately.

Light: The Key to Strong Seedlings

Colorado sunshine is great—once plants are outside. Indoors, seedlings often become leggy without enough light.

  • Use grow lights 2–3 inches above seedlings

  • Keep lights on 14–16 hours per day

  • Rotate trays if using window light

Strong light = sturdy stems.

Watering & Care

  • Keep soil moist, not soggy

  • Bottom watering works well and prevents disease

  • Avoid overwatering

Good airflow (a small fan on low) helps strengthen stems and reduce mold.

Hardening Off: Don’t Skip This Step

Before planting outdoors, seedlings need time to adjust to Colorado’s sun, wind, and temperature swings.

About 7–10 days before transplanting:

  • Start with 1–2 hours outside in a sheltered spot

  • Gradually increase exposure each day

  • Bring plants in if temperatures drop or winds pick up

This step prevents shock and sunburn.

When to Plant Outdoors

Even after the last frost date, Colorado weather can surprise you. Watch the forecast and be ready to protect plants with row cover or buckets if needed.

Warm-season crops should go out when:

  • Nighttime temps stay above 50°F

  • Soil has warmed

Patience pays off.

Final Thoughts

Starting seeds indoors gives Colorado gardeners control in an otherwise unpredictable climate. With the right timing, light, and care, you’ll be rewarded with healthier plants and a more productive garden.

And when that first tomato ripens? Totally worth it 🌱