Warm roots are happy roots

In previous postings, I’ve mentioned my greenhouse with its germination chamber and heat mat. This is not as complicated as it sounds, and you don’t actually need a greenhouse to germinate seedlings.

First, please note that my greenhouse is 16 years old and shows it. Most greenhouse operators will cringe at the state of my little hothouse. It is dirty; it is certainly not as sterile as my horticulture teacher from years ago warned me it must be. But for starting vegetable and flower seeds and rooting easy-to-root cuttings, my greenhouse serves me well.

Now on to germination tips. You need to meet two key conditions to ensure rapid seed germination: bottom heat and constant, fairly high humidity. If you provide these conditions, the seeds will sprout.

I use the germination chamber that came with my greenhouse. It’s a big plastic box with a clear plastic top that fits snugly on top of it. Here’s what mine looked like this morning before I removed the top:

Germination Chamber

Note the moisture beading inside the clear plastic top. The pink layer beneath is a piece of insulating foam that Wonder Spouse suggested we use to direct all the heat from the heat mat (the black layer between the insulation and the plastic box) into the germination chamber.

You can buy smaller similar propagation chambers at most garden supply stores or through catalogs. They usually consist of a plastic flat with a clear plastic top that fits over it. However, I’ve often managed simply by stretching clear plastic wrap over pots after I sow seeds in them. I use a pin to prick small holes in the plastic to allow a bit of ventilation.

Here are the Sweet Alyssum seeds I told you about inside the germination chamber after I lifted the top:

Sweet Alyssum seedlings three days after sowing

This is another propagation hack that purists will probably cringe at. I took an old plastic flat with a break in the bottom and cut it cleanly across at the break. I overlapped the two pieces to make a smaller flat that would fit inside my germination chamber.

After filling my smaller flat with potting soil, I created four shallow rows and sprinkled in the Sweet Alyssum seeds. These seeds require light to germinate, so I didn’t cover them with soil. Instead, I used my fingers to lightly press the seeds into the pre-moistened potting mix to ensure that the seeds were in contact with the moist soil.

How did I know the seeds needed light to germinate? The instructions on the seed packet told me so. Always read your seed packets, folks.

The green layer beneath the flat is capillary cloth. I buy mine from a greenhouse supply catalog. It holds water within the cloth and releases it — via capillary action — into pots as the soil dries. It allows me to maintain more even soil moisture levels.

Finally, here’s a closer look at the heat mat beneath the germination chamber:

Heat Mat

A regular heating pad is NOT acceptable. You need a water-proof heat mat designed for seed germination purposes. You can see the electric cord running from the mat. It’s plugged into a handy outlet in the wall of my greenhouse.

This is a relatively large mat, because I needed one big enough to provide heat to my germination chamber. You can buy smaller ones. Some have thermostats for finer temperature control. For the seeds I germinate, I’ve never needed more than what I have.

You’ll also see soil heating cables. You place these cables in the soil beneath your seeds. Although I’m sure these cables have their uses, they seem unnecessarily complicated for my purposes.

If you have a way to control humidity, a sunny window or excellent grow light, and a heat mat, you can start seeds inside your house. No greenhouse required.

Garden supply catalogs often sell these tools, but for more options, try a greenhouse supply company. Here’s one that I often order from.


  1. Sweet Alyssum: Progress Report « Piedmont Gardener

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