Thursday, February 26, 2009

When to plant vegetables

Warm La Nina effect weather along the Front Range this year has many people anxious to plant. In this post I want to discuss the common wisdom of planting by holidays and the science behind it. As always, individual decisions on planting date are based on risk tolerance. In some years early planting may prove a good bet and in others, an impatient action.

The holiday rule of thumb is to plant peas and lettuce on St. Patrick’s Day and peppers and tomatoes on Memorial Day along the Front Range.

The science behind this is that cool season vegetables are planted when soil temperatures are sufficiently warm for seed germination. These vegetables are able to withstand cold air temperatures. Warm season vegetables require warmer soil temperatures for seed germination and root growth, and warm, stable air temperatures for plant tops that are generally intolerant of freezing air temperatures. Many but not all warm season vegetables are planted as transplants and not direct seeded.

Soil temperatures for vegetable seed germination*

Cool season vegetables –

35 degrees - lettuce and onions

40 degrees – peas, radishes, spinach, cabbage

Warm season vegetables –

50 degrees – tomato, peppers, corn

55 degrees – beans

60 degrees – cucumbers, squash, eggplant

* soil temperatures measured with a soil thermometer at 4 inches at 8:00 a.m.

How does the planting by holidays prescription stack up with the science? It turns out to be a relatively safe guide when used in conjunction with observable weather trends.

Dates for soil warming for years 2005 to 2008*

40 degrees - Mar 28, Mar 28 and Mar 4, Mar 24

50 degrees – May 5, May 6, May 8, Apr 28

60 degrees – Jun 11, May 13, Jun 10, Jun 14

* recorded at the Lory Student Center weather station on the Colorado State University campus in Fort Collins (monitored by the Department of Atmospheric Sciences)

For more information on vegetables and their temperature tolerances, see CSU Extension Garden Notes Planting Guide #720.


  1. Carl - Co you have a recommendation as to a resource for someone interested in food gardening for the first time? It would be useful to have a listing of recommended foods to grow, what size of space is needed for the garden and how to prepare the soil, how many seeds of each variety to plant, and how to care for the garden through the year ~ essentially what I would love is a simplistic A-Z approach / guide for such an undertaking. If you could point me in the right direction, I would be very appreciative :o)

    Thank you!

  2. There are any number of books on vegetable gardening. Perhaps readers can name their favorites. The only trick is ignoring the information that doesn't apply here (adding lime and epsom salts to soils for example). For some local expertise, try the vegetable page of the Denver Master Gardener website or the Colorado Master Gardener site

  3. Carl- I live in Estes Park. How many weeks later would you guess I could start my cool-weather vegetables?

  4. Perhaps some readers in the mountains might comment or you might try contacting a local nursery or the Larimer County CSU Extension at 970-498-6000. The best way of course is measuring soil temperature.

  5. Carl... What if i want to plant my vegetables in pots but have them out doors? Can i just move them in on cold days but keep them out in the sun on nice days to start growing sooner?