A light freeze in Spring can kill tender seedlings that were planted outside too early. Understanding last average frost date is helpful in successfully planting and transplanting seedlings.
In Colorado, the timing for starting seeds indoors depends on the specific plant you're growing and your location, as our climate varies significantly from area to area.
You don't want to start seeds too early, and then have them get leggy or moldy because it is too soon to transplant them into the ground outside.
As a general guideline, we use "last average frost date" as a reference point for when to start seeds indoors. (Last and first frost dates are only 30% probability, according to Farmer's Almanac).
The last frost date in Parker is mid- to late-May according to Almanac. But Plantmaps.com says it is June 1. Many in our area live at altitudes higher than Parker (5900 feet), so we have to take into consideration that our frost dates will be a bit later than in-town Parker, which is what Almanac uses.
That said, late May is a safe date to go with.
Seed packets provide guidance on when to start seeds indoors based on the number of weeks before the last frost date. Always follow the seed packet guidelines!
For example, if the last frost date is around late-May and a particular plant needs six weeks to mature indoors before being transplanted outdoors, you would start the seeds indoors around mid-April.
It's essential to consider the specific requirements of the plants you're growing, as some may need more or less time indoors before being transplanted outdoors.
Factors such as the microclimate of your garden and local weather conditions (wind anyone?) can influence the timing of planting.