Cool vs. Warm Season Vegetables

by Susan Carter, Horticulture Area, CSU Ext. Tri River Area

With the sun shining, birds chirping and moisture in the ground this year, many of us are eager to jump into the garden.  There are several good things to think about before you just go ahead and plant.  Living in the mountains can have its challenges.  Did you know for every 400’ higher in elevation that you lose the number of growing days?  However, other factors can determine your frost-free days.  When I lived in Silverthorne CO, 8730’, we had many cold mornings.  Silverthorne is in the Valley with the Blue River running thru town.  Cold air sinks and follows rivers.  Leadville’s elevation, which is 10,151’ but is a high flat area where cold can drain off to lower elevations.  Leadville has 87 frost-free days and Silverthorne has about 60 growing days.  Is it good to know your average last day of frost: https://www.weather.gov/gjt/avgfrostandfreezedates2 

I now live in Fruita and since it is lower in the Valley, it can be a good 10 degrees colder than Palisade.  This is why most of the fruit and vineyards are in Palisade and why crops like hay and wheat and some vegetables are further down the valley.  Fruita can have a 32-degree frost around Mother’s Day where Palisade can have its last frost date 3 weeks earlier.

CSU Dept. of Atmospheric Science image

For mountain gardens, cool season vegetables are your best bet.  Leafy greens like lettuce, kale and spinach work well.   Broccoli, Brussel sprouts, cauliflowers and many root crops like beets and onions are also great cool season crops.  So why aren’t warm season crops like tomatoes, peppers and squash a good choice?  Well, many of these warm season crops need night temperatures of at least 50 degrees and days up to 90 degrees Fahrenheit.  Moreover, I am not just talking air temperature.  These plants prefer soil temperatures of at least 60 degrees.  Even at lower elevations, these plants are planted too early in the season will suffer from that cold stress and are prone to developing viruses and not thriving.  https://planttalk.colostate.edu/topics/vegetables/1806-growing-cool-season-vegetables/ 

On a smaller scale, you can use microclimates around your house to allow for a longer growing season.  There are methods of season extension that you can use such as frost blankets, walls of water, cold frames, plastic mulches and low or high tunnels formerly called hoop houses.  Being a plant geek, I had to experiment and try plants at high elevation.  My husband would laugh at my attempt every year to grow tomatoes.  I would plant them in dark pots, in mostly sunshine and place them against our homes wall under the overhang to get extra warmth and protection from the frost and cold.  I purchased Siberian tomatoes, which only need 55-60 growing days to develop.  Now growing days does not include seed to maturity, you have to add in time from seedling to germination to seedling plant before you can plant outdoors.   In this example, growing days equaled frost free days not optimal growing days as that is all I had to work with.  For all my effort, I typically would get about 3 small to medium tomatoes, but hey I grew them at high elevation.

CSU does not endorse any seed company. 
This just shows a shorter season tomato variety.

Now I could have used other methods of season extension to grow my tomatoes as mentioned above.  I did however grow many cool season crops like lettuce and spinach.  Did you know years ago there were lettuce farms in Silverthorne?  Sometimes it is much easier to grow what grows best in your area.  Depends on how much time, effort and money you want to put into it.  Happy Growing Season.