I live in Sacramento, CA (zone 9b). Winters are mild. Dec/Jan average temps range from 55F - 40F during day/night. However, depending on year, we can get some frost from mid Dec - mid Jan. I would like to start seedlings (marigolds, zinnias, petunias, violas, salvias, vincas, pansies, snapdragons) around early/mid Jan for transplanting by March.

Right now, I am using two seed trays and a Philzon grow light. Results are very good. However, space is limited and I foresee using about 6 seed trays per year to cover all the flowers for the year for front/back garden.

I was thinking of building a small cold frame in my backyard, and putting the seed trays in there in early/mid Jan for sprouting / growing. Is it a good idea and will it work?

Something like this: https://www.pinterest.com/pin/525091637802640544/?nic=1

2 Answers 2


Your link points to images which show mostly the upright type of cold frame; I have no experience with these but do have some experience with the flat horizontal type, so take my comments with this in mind. I guess the upright type reflects the fact that gardens are a lot smaller than they used to be.

Frames are great protection against wind, can improve natural light exposure and can level out some of the overnight ambient temperature changes. The flat type can even be loaded up with partly decomposed material to "heat" the inside of the frame, extending the growing season into early spring and late fall. And of course they look very cute and romantic.

Now for some practicalities:

  • They require a lot of attention since the sun rises and sets and cloudy can turn to very sunny in short order; in the days of frames managed by a team of 30 gardeners 24/7 this was not so much an issue.
  • The cold frame is small meaning little air circulation unlike a larger greenhouse.
  • While freeze is less likely, fry is a distinct possibility.
  • Humidity can rise very fast in a closed frame, leading to increased risk of fungal disease such as damping off in seedlings; automation of ventilation can help but these things can fail and it only takes one fail to ruin your investment.
  • Windy conditions can break ventilation windows if they are not secured.
  • High humidity leads to rot of wooden parts.

Bottom line is if you have the time to watch over this wild animal of a tool in fair and foul weather then they can be fun toy to play with.


Cold frames work very well with one giant caution; They will get to very high temperature very fast when the sun hits them ( real greenhouse effect). I built one in zone 5 with a home-made operator to open it depending on the temperature. My design was not reliable over time so I just opened it every morning which was not very good. Today I expect there are temperature controlled openers or fans that would make cold frames very useful.

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