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  • Writer's pictureEllen

Building a Mini Germination Chamber

Updated: Sep 19, 2023


Even though we are still in the depths of winter here in Upstate New York, we have spring on the brain. A few weeks ago we started our first flower seeds of the season - an exciting time! When the mercury is below zero and there's a foot of snow on the ground, planting seeds indoors reminds us that warmer weather is around the corner.


This early in the season, we don't have enough plant trays started to warrant firing up the greenhouse. Our heated, insulated greenhouse takes a lot of fuel and effort to run, so it doesn't make sense to start heating it for only a few trays of seedlings. Instead, we start these first few trays in the house under grow lights.


Unfortunately, our wood stove heated house is rather dry and relatively chilly. We always have trouble with germination rate and seedling health in this first round of planting. In the greenhouse we have an insulated germination chamber that keeps seed trays at a constant temperature and humidity to encourage even germination, but that won't be available until the greenhouse is up and running in a few weeks. This year, we decided to build a mini version of our germination chamber at home to try and overcome some of the challenges of starting seeds in the house.


Our large germination chamber in the greenhouse consists of an insulated box with plastic-coated metal racks inside for seedling trays. A water tray with submerged heater and thermostat keep the air at 100% humidity and at a set temperature. You can read more about how we built our large germination chamber in this blog post.




Our mini version is basically the same, but with a few key differences. First, it is much smaller. We decided to use an old two-door filing cabinet as the base for this chamber. Curt removed the drawers, then screwed wooden rails into the interior sides of the cabinet. These rails are perfectly spaced to hold a dozen 10 x 14" plastic lunch trays. The plastic lunch trays are great for starting seeds in small 3/4" soil blocks because they make bottom watering easier.


For the heating element, we purchased this submersible aquarium heater with a built-in temperature sensor on a flexible cable. When used in an aquarium, the sensor stays submerged in order to detect the water temperature. However, we bent the cable so the sensor is up in the air. When we set the thermostat on the heater, it keeps the air temperature at a constant 78°F by heating the metal pan of water. This also keeps the humidity in the chamber at 100%, which is ideal for germination.



Curt insulated the chamber by surrounding it with 1/2" foil faced insulation foam. Although our big germination chamber in the greenhouse has a solid door and walls, we wanted to let some light into this mini version because we start a lot of flowers in the spring that need light to germinate. Curt built a new door for the chamber out of a sheet of corrugated clear plastic roofing.


So far we have started petunias, canna lilies, bacopa, lobelia, lupine, rosemary, and lavender in this chamber. It is doing a great job! One downside is that some of the metal parts of the cabinet have already started to rust - in the future, we may want to use a different shell as the base for the cabinet, or do away with shell completely and just build a foam box around a set of shelves like we did for the full-size version.



If you are a serious gardener and start a lot of your own seeds at home, you may want to give this a try! As with the large germination chamber, make sure you check the water tray regularly and take seedling trays out of the chamber as soon as the first seedlings emerge. After the seeds germinate, we grow them out on seedling heat mats under fluorescent lights.


What seeds are you starting this year? Would you like to build a germination chamber of your own?



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