top of page

Seed Starts: Indoor Germination - Part Two

Updated: Apr 1, 2023

Welcome to part 2 of 2 covering our seedling chamber.


Purchase your seed chamber shelving with your heat mats in mind as you’ll want each shelf to be large enough to fit the size of the mats. We use 60” x 21” heat maps and 60” x 24” wire shelving. Also make sure the shelving is tall enough to allow ample head space on each row for plant growth. We use 63” tall shelving which allows for three tiers of plants with plenty of headspace above each row. Our unit came with one additional shelf which makes up the top of the chamber.

Heat mats

We use two Jump Start 60” x 21” heat mats on the bottom two tiers of our three tier shelving unit. The top row is reserved for flats that have already germinated and no longer need the additional warmth. Beware: during the day the top row of your chamber may be very warm depending on your light fixture setup and air flow.

The heat mats are daisy chained together and plugged into a temperature controller. A temperature probe runs back to the mat and is placed under a seedling flat, in between the flat and the mat.


Grow lights have improved significantly and pricing has come down dramatically over the years we’ve been starting seeds indoors. The most important factor to remember in regards to light is that plants don’t ‘see’ what we see, but we need to be able to see the plants when we open the seedling chamber.

So here is an overly simplified and brief overview of grow lights. We only care about the wavelengths of light that produce green plant growth. For the market grower, the wavelengths of light for flower and fruiting will be provided by the sun out in the garden beds later on in the season. Instead of bulb watts, lumens, and light color, we look at PAR (photosynthetically active radiation) and PPFD (photosynthetic photon flux density). PAR is the wavelength or color of the light and PPFD is a measure of the intensity of those wavelengths of light. So we want the wavelengths on the blue end of the light spectrum with enough brightness to prevent leggy starts.

We use two GE LED grow light fixtures for seeds and greens for a total of 144 micromoles per second of balanced light on each shelf of our chamber. This setup provides light for plant growth and enough white light for us to see the plants as we care for them.

The grow lights are daisy chained together and plugged into a simple timer which is set to turn on at sunrise and off at sunset. We update the on and off times throughout the season to match the sun.

Note: even the most efficient grow lights produce heat. It is important to make sure this heat is dissipated and allowed to escape the chamber during the day.

Shelving cover

To keep the light and heat on our plants, we wrap our seedling chamber in a highly reflective bubble foil. One brand of this product usually available in home improvement stores is Reflectix.


While we want to maximize the efficiency of our heat mats and grow lights, we don’t want the seedling chamber to overheat, or grow mold and fungus. Constant airflow is a necessity. We use the Airplate cabinet cooling fan system from AC Infinity. These fans are super quiet and run off a controller with a temperature probe zip tied near the top of the inside of the chamber.

We keep the temperature setting of the fans at or near the setting of the heat mats to ensure airflow all day and night. The Airplate fans run at faster speeds as the heat in the chamber increases to quickly dissipate heat produced by the grow lights.

Coming up next...

This concludes our indoor seedling setup series. Check back soon when we'll detail our simple greenhouse and low tunnels.

48 views0 comments

Recent Posts

See All


bottom of page