Store seeds at 1°C

Question

I have prunus pits that I stratify in a refrigerator during winter.

Normally, the seeds germinate in springtime, which is ideal.

However, often some of the seeds germinate prematurely. When this happens, it's necessary to grow the seeds/plants indoors during winter until they can be planted outdoors (after the last frost date). This is not ideal.

Rather than let the premature pits grow, an alternative would be to slow germination down (or even prevent it) by keeping them at just above freezing. It's my understanding that the colder plants are, the slower they grow (up until freezing, of course--the plants are too young to go dormant for sub-zero temperatures).

So, the question becomes, "Where in my house is the temperature at just above freezing?".

Typically, refrigerators are at a temperature of around 6°C. From experience, I've found that 6°C is too high; the plants grow and eventually die due to lack of light.

How can I store the seeds at 1°C in a household setting?

Answer 1

The temperature control in a fridge is pretty crude. The evaporator is in the freezer and the compressor is controlled with an electric thermostat. The fridge portion however relies on convection from the freezer. The temperature control from the fridge is sometimes nothing more than a slider that establishes how quickly the convected air moves. There are likely better methods employed in more expensive units so you will have to look at your specific fridge. In any case, the temperature can fluctuate quite because the designers were not concerned with a little temperature fluctuation harming food.

Also, if your freezer has an auto defrost cycle this will also increase the temperature for 15 minutes every day (or whatever the timer settings are).

Before we go changing things, it might be helpful to get a wifi temperature logger or something similar so you can see how much the temperature fluctuates and how much modifications improve it.

Assuming this fridge is 100% dedicated to this project; I would try adjusting the fridge temperature to target your 1°C with its current crude control. Fill the rest of the fridge with gallon jugs of 3/4 full of tap water(room for freezing) to help stabilize the temperature. If the freezer is not loaded, also fill it with 3/4 filled gallon jugs of water. This increased thermal mass will help stabilize the temperature. It also helps ensure the temperature does not dip much below 0°C if controls are not ideal.

If that doesn't fully address it, figure out how the auto defrost cycle works on your specific refrigerator/freezer and disable it. This parts site has pretty good schematics of most models. Of course, now you will have to remember to do it manually before and after or your freezer/fridge will run all the time and waste power.

If that doesn't get the temperature stable enough you can replace the passive convection temperature control system with an ac fan and pid controller. The fan turns on when it needs more cold air from the freezer and turns off when it is cold enough. Definitely more work, but it will be able to hold the temperature much closer than with other methods.