3 Systems for Starting Professional-Quality Seedlings Indoors

Molded plastic seedling trays are known as 1020 trays because their dimensions are approximately 10 inches by 20 inches.

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Cell flats (also known as plug flats) fit inside the 1020 seedling trays, leak-proof trays, or lightweight mesh trays, and are available in multiple sizes, identified by the number of cells in each flat.

The most common cell-flat sizes for veggie transplants are 50's, 72's, 128's and 200's. The cells in these flats are square and very closely spaced, to maximize the number of plants you can fit in your greenhouse.

For larger crops, there are cell flats with 38 or 24 cells per flat. The cells are larger, allowing for larger root balls, and the cells are round and spaced farther apart to allow for greater canopy development of the plants.

Another option is the 20-row seed flat, in which seeds are sown in channels to germinate, then "bumped up" or "potted up" into cell flats when their first true leaves appear. Seed flats are advantageous because they conserve seedling heat mat or germination chamber space.

Many growers consider this to be the most efficient way to start large numbers of seedlings at once.

Advantages. Trays and cell flats are the seed-starting method of choice on most large vegetable farms for several reasons:

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• They are very quick to fill.
• They can be moved around easily.
• They are appropriate for use with mechanical transplanters.
• They are reusable for many seasons — if handled carefully and stored out of direct sunlight.

Disadvantages. Plastic trays and flats do have some drawbacks.

• They need to be stored and cleaned before they can be reused.
• Seedlings can become root-bound in the plastic cells, and there is some transplant shock when plants are removed from the cells.
• They will eventually wear out and need to be replaced. And when they do, they cannot always be recycled. And when they cannot be recycled, they usually end up in landfills, which has an environmental cost.

For sanitizing trays and as a means of disease prevention in your seed-starting program, we recommend a peroxide-based bleach substitute like OxiDate, which is an OMRI-listed disinfectant approved for use in certified-organic agriculture.

Whether or not plastic items such as trays, flats, and domes are recyclable nowadays is essentially dependent upon their composition, which will determine your local recycling center's policies, capabilities, and partnerships. If you use plastic seed-starting and plant containers, we encourage you to check with your local recycling center for more information on their recyclability.

Because of the scale of our operation, we do primarily use 1020 trays for greenhouse propagation and seed-starting. Here are the basic, crop-specific seed-starting practices with cell flats used here at Johnny's:

The 3.5" Black Square Greenhouse Pot is a great size pot for cuttings and annuals, including tomatoes. Our poly-form line of pots provide the economy of a thermoformed container without compromising quality or durability. Form pots feature die-cut side/bottom holes for consistent drainage and are designed to fit securely in carry trays and pro-totes.

Form pots are now manufactured with a new blend of plastic that greatly increases container pliability and reduces the likelihood of cracking or brittleness.

For more information, please visit 144-Cell Heavy Duty Seed Starting Trays.