When to plant vegetable gardens is determined by the soil temperature a seed needs to germinate as well as its ability to withstand frost. Most common vegetables we eat do not fare well against frost.
For Colorado, a safe time to plant is at the end of the second week of May, around Mother’s day. Although snows can come as late as June, the average late frost has shifted by 10 ten days with our change in climate. So I base my seed starting around a May 15 – 20 plant time.
If you intend to plant early spring seeds like radishes, beets, and lettuce, you can find the average soil temperature based on month at Colorado Climate Center
There are many things that can affect soil temperature including the soil humidity, and the evaporation rate off of soil. Moisture cools soils as does evaporation. Soil contents such as sand can increase soil temperatures due to the conductivity of the rock material. Even the angle at which the sun hits the soil can have an affect. Soils on a slope tend to run hotter than those on the level. So with that the said each person’s soil temperatures could differ.
Raised beds also impact the soil temperatures as the tend to be drier and evaporate faster. Although I could not find any definitive study showing the difference in temperatures for equivalent soil types, it is believed beds can increase soil temps by as much as 8 degrees.
I’ve created the following chart for knowing when to start indoor and outdoor seeds.
|DATE||Soil Temp*||Task||Plant||Plant Temp|
|3/1||< 40||12 week sow indoors|
|3/15||< 45||8 week sow Indoors||tomatoes, jalepenos, brussel|
|Prep beds for early spring plants|
|3/30||45||6 week sow indoors|
|4/7||4 week sow indoors|
|4/15||50||Sow early spring plants||beets||55 – 70|
|5/1||55||Harden Off Indoor starters|
|Plant Outdoors||corn||60 – 80|
|Sow early spring plants||radishes, lettuce||65 – 70|
|5/7||60||Sow 2nd batch||radishes, lettuce||65 – 70|
|5/17||65||Plant Outdoors||brussel sprouts||68|
|Sow last batch||radishes, lettuce|
|5/30||70||Plant Outdoors||tomatoes, pole beans||70|
|Harvest first batch||radishes, lettuce|
|6/7||80||Harvest 2nd batch||radishes, lettuce|
|6/15||80||Harvest||2nd batch radishes, lettuce|
|*Note soil temperatures are higher earlier in raised
beds and containers.