Seed-Starting Secrets of a Greenhouse Professional


We began producing certified organic vegetable transplants 23 years ago at The Natural Gardening Company with very little experience under our belt. Since the early days we have produced well over a half million seedlings, learning by trial and error. I have no doubt there is more to learn, but this blog will provide you with a summary of our seed starting technique. Where we learned from mistakes, you can have the benefit of our two+ decades of experience.

Our Soil Mix

We’ve fiddled around with our seed starting mix over the years. In the beginning we used a classic formulation: 1/3 peat moss, 1/3 perlite and 1/3 organic material (usually compost or aged steer manure). These are readily available materials and they work well, with one shortcoming. When seeds using this formula are planted in plug trays and placed in a sunny location, direct sunlight and high heat can cause mold to grow on the surface. This mold eventually becomes a crust. This crusty layer impedes the penetration of water from above and suppresses the upward development of the seed from below. The crust does not completely inhibit the development of seeds in the tray, but it greatly reduces the rate of germination.

By chance, there was a shortage in the perlite supply one season. This forced us to turn to alternative materials. We chose vermiculite, and what a difference it has made. Vermiculite absorbs moisture and keeps the soil materials evenly damp so no crust forms, even under hot, sunny conditions. This moisture retaining property also enhances seed germination, as the most common cause of failure when starting seeds is desiccation – drying out.

Then we simplified our formula further, eliminating the organic material. Seeds carry a layer of starch in the cotyledon and can nourish themselves in the first phase of development. We were already fertilizing our seeds after they germinated with a liquid fertilizer, a combination of fish and kelp, so we stopped using the compost/manure. This left us with a simple, light, easy-to-mix soil formula of equal parts peat moss and vermiculite. This is the mix we use today. If you don’t want to mix it yourself you can find our seed-starting mix on our website.

Our Propagation Trays

seed starting

If you’re seriously interested in success with seeds, and you respect the time and materials you put into this process, there’s only one way to go when you buy your propagation trays: use the trays professionals use when they start seeds. Commercial trays work better, last longer and offer more options than anything you will find at your local garden center. They cost only a fraction more but are much more durable and will last longer.

7/8/2021 9:08:42 AM

As someone else commented disappointed to see this encouragement for single-use plastics - just for an indication of the crisis - if the global yearly consumption of plastic bottles were laid end to end the chain would stretch half way from Earth to the Sun. This of course is for plastic bottles not plastic wrap but its the same family of materials and at least bottles can recycled - plastic wrap is not generally recycled and ends up in landfill.

3/12/2020 11:06:40 AM

I use a heat rope in my tiny greenhouse. It stays at 71 and has a built it thermostat. It lies under four inches of soil in a raised corner bed. I have a roll down sheet of vapor barrier plastic that keeps out the cold.(the rest of the greenhouse is not heated.) I have also used a small electric heater to give germination a boost .Within this area I have at least a 90% germination rate. It is too cold to start my peppers so I start these indoors.

2/22/2020 6:02:31 AM

As others have said, use coconut coir instead of non renewable peat and buy trays with reusable plastic domes if you really want to cover. Also I just discovered cow pots, made from cow manure they will totally degrade when planted and then nurture your plants. Win win.

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