Includes tips for starting seedlings successfully indoors
Starting seeds indoors in early to mid-April is a great way to prepare for planting in May in Colorado, especially with our unpredictable spring weather and shorter growing season.
Here are ideal seeds to start now for transplanting outdoors in May:
Vegetables and Herbs to Start Indoors Now
Tips for planting seedlings
Use Quality Seed-Starting Mix
Choose a light, sterile mix specifically made for seedlings—not heavy potting soil. It helps with drainage and root development.
Keep Things Warm
Most seeds germinate best at 65–75°F. Use a heat mat if needed to maintain consistent warmth, especially for heat-loving plants like tomatoes and peppers.
Water Gently and Consistently
Keep the mix evenly moist but not soggy. Mist or water from the bottom to avoid disturbing tiny seeds and seedlings.
Give Them Light
Once seeds sprout, provide 12–16 hours of bright light daily. A grow light set a few inches above the seedlings works best if you don’t have a sunny window.
Lightly Fertilize
Give the seedlings a light (diluted) fertilization once they have germinated, and continue weekly until they're ready for transplanting.
Label Everything
It’s easy to forget what you planted! Use waterproof markers and labels for each tray or cell.
Thin the Seedlings
Once sprouts have a couple of leaves, snip the smaller ones at the soil line so only the strongest plant remains in each cell.
Provide Airflow
A gentle fan or cracked window helps prevent mold and strengthens stems.
Harden Off Before Transplanting
Gradually introduce seedlings to outdoor conditions over 7–10 days before planting them outside to avoid transplant shock.
A note about stratification: Seed stratification is the process of mimicking natural winter conditions—typically through cold storage—to help certain seeds break dormancy and germinate successfully.
Examples of seeds that require stratification to successfully germinate
Lupine
Coneflower
Milkweed
Columbines
Lavender
Penstemon
Asparagus
Rhubarb
Lovage
Parsley
To stratify seeds, place them in a moist medium like damp paper towels or sand, seal them in a plastic bag or container, and store them in the refrigerator (around 34–41°F) for several weeks to simulate winter conditions. Check the recommended time for each seed type, which can range from 2 to 12 weeks. Be sure to label the bag with the seed type and date, and check occasionally for signs of mold or early sprouting.
Here are ideal seeds to start now for transplanting outdoors in May:
Vegetables and Herbs to Start Indoors Now
- Tomatoes – Need 6–8 weeks indoors before transplanting after last frost.
- Peppers (bell, chili, etc.) – Slow growers that benefit from early indoor start. These appreciate a heat mat underneath them for best germination.
- Eggplant – Needs warmth and a long growing season
- Broccoli & Cauliflower – Can be hardened off and planted in early May
- Cabbage – Cold-hardy and perfect for early May planting
- Leeks & Onions (from seed) – Should be started early; transplant when soil is workable
- Spinach
- Lettuce
- Kale
- Basil
- Cilantro
- Parsley
- Zinnias – Fast growers, transplant after frost
- Marigolds – Hardy and pest-resistant
- Petunias – Can take time to bloom, so start early
- Cosmos – Quick to sprout and attract pollinators
- Snapdragons
- Larkspur
- Use a grow light if natural light is limited (seedlings need 16 hours of light per day to germinate and grow big enough to transplant)
- Harden off plants before transplanting by gradually introducing them to outdoor conditions over 7–10 days.
- Watch your local last frost date (typically late May in our area) before moving plants outside.
Tips for planting seedlings
Use Quality Seed-Starting Mix
Choose a light, sterile mix specifically made for seedlings—not heavy potting soil. It helps with drainage and root development.
Keep Things Warm
Most seeds germinate best at 65–75°F. Use a heat mat if needed to maintain consistent warmth, especially for heat-loving plants like tomatoes and peppers.
Water Gently and Consistently
Keep the mix evenly moist but not soggy. Mist or water from the bottom to avoid disturbing tiny seeds and seedlings.
Give Them Light
Once seeds sprout, provide 12–16 hours of bright light daily. A grow light set a few inches above the seedlings works best if you don’t have a sunny window.
Lightly Fertilize
Give the seedlings a light (diluted) fertilization once they have germinated, and continue weekly until they're ready for transplanting.
Label Everything
It’s easy to forget what you planted! Use waterproof markers and labels for each tray or cell.
Thin the Seedlings
Once sprouts have a couple of leaves, snip the smaller ones at the soil line so only the strongest plant remains in each cell.
Provide Airflow
A gentle fan or cracked window helps prevent mold and strengthens stems.
Harden Off Before Transplanting
Gradually introduce seedlings to outdoor conditions over 7–10 days before planting them outside to avoid transplant shock.
A note about stratification: Seed stratification is the process of mimicking natural winter conditions—typically through cold storage—to help certain seeds break dormancy and germinate successfully.
Examples of seeds that require stratification to successfully germinate
Lupine
Coneflower
Milkweed
Columbines
Lavender
Penstemon
Asparagus
Rhubarb
Lovage
Parsley
To stratify seeds, place them in a moist medium like damp paper towels or sand, seal them in a plastic bag or container, and store them in the refrigerator (around 34–41°F) for several weeks to simulate winter conditions. Check the recommended time for each seed type, which can range from 2 to 12 weeks. Be sure to label the bag with the seed type and date, and check occasionally for signs of mold or early sprouting.