Grow It in ColoradoStarting Seeds Indoors
Around Colorado Springs, Fountain and El Paso County, the average last spring frost usually comes in mid-May. A practical, conservative time to set frost-sensitive transplants outdoors is after about May 15, once the forecast looks clear. Count back from that date to decide when to start each crop indoors.
The windows below are general starting points for this area. The 2027 season dates count back from about May 15, 2027. For regional planting windows, see the Planting Calendar. If frost threatens, see Protecting Your Garden for how to cover plants.
Indoor start dates (count back from mid-May)
| Crop | Weeks before last frost | Indoor start window | 2027 season dates | Move-outside timing |
|---|---|---|---|---|
| Onions, leeks | 10–12 weeks | Late February to early March | Feb 20 – Mar 6 | Transplant after hardening off. Onions and leeks tolerate cool weather. |
| Peppers, eggplant | 8–10 weeks | Early to mid-March | Mar 6 – 20 | After about May 15, once frost danger has passed |
| Tomatoes | 6–8 weeks | Late March to early April | Mar 20 – Apr 3 | After about May 15, once frost danger has passed |
| Broccoli, cabbage, cauliflower, kale | 6–8 weeks | Early to mid-March | Mar 1 – 15 | Around mid-April; these crops tolerate cool weather |
| Lettuce, basil, parsley | 4–6 weeks | Early to mid-April | Apr 3 – 17 | Lettuce and parsley can go out in cool spring weather; basil after about May 15, once frost danger has passed |
| Squash, cucumbers, melons, pumpkins | 3–4 weeks | Mid- to late April | Apr 17 – 24 (or direct sow late May) | After about May 15, once frost danger has passed. Many gardeners sow these directly in the garden in late May instead. |
For the 2027 season, begin hardening off about May 5–8 so seedlings are ready around mid-May. If late frost is common in your area, shift these start and transplant dates a week later.
Harden off seedlings
Before planting outdoors, harden off seedlings for 7–10 days. Colorado's strong sun, wind and cold nights can shock plants that have been raised indoors.
Set trays outside in a sheltered, shaded spot for a short time the first day, then gradually increase the time and the exposure to sun and wind. Bring plants in overnight if temperatures will drop sharply. After a week to 10 days, they should be ready to transplant.
Basic indoor setup
- Use a sterile seed-starting mix, not garden soil, in clean trays or pots with drainage.
- Place grow lights about 2–3 inches above the seedlings and run them 14–16 hours a day, raising the lights as the plants grow.
- Peppers and tomatoes often germinate more reliably with extra warmth, such as a heat mat under the trays.
- Water from the bottom when you can, by setting trays in a shallow pan of water and letting the mix soak up moisture, then pouring off the excess. This helps keep the soil surface drier.
If aphids show up on seedlings, see Aphids in Your Indoor Garden for how to isolate plants, clean the grow area and keep them from coming back.
If you garden in the mountains or at higher elevations, add 2–4 weeks to these dates and check your local frost date. Conditions vary widely with elevation and exposure. See Colorado State University Extension for research-based guidance and local frost information.