It’s January and spring is right around the corner! Time to get your seed orders in and start thinking about how you’re going to grow the best transplants this year.

This is our first year as market farmers, and we intend to transplant more than we direct seed, so we better learn how to do it well!

Much of the information in this article is from J.M. Fortier’s the market gardener, which I highly recommend. Let’s learn from others who have been there before.

If intensifying production on a small plot of land is your goal, learning how to start seeds indoors should be a priority.

Failed germination, slow growth, diseases, and any other problems at the seeding stage can have disastrous consequences on the production calendar you rely on for timely harvests.

Indoor seeding requires competence and attention to detail. 

Advantages of Transplanting

There are several advantages of using transplants early in the season, such as:

  • Seeding begins before the start of the frost-free period, thereby extending the growing season considerably.
  • Germination and growing conditions are controlled in the early stages when plants are most vulnerable.
  • The chances of crop success are improved because seeding density is perfect and crops have a head start over weeds.
  • It is possible to do succession plantings by starting crops in the greenhouse before garden space is available.

seedlings for transplants

Starting Seeds in Cell Flats

There are many techniques for starting seeds indoors. Most amateur gardeners use open polystyrene flats or individual pots made of coconut fibre.

Eliot Coleman uses soil-blocking, a technique in which seeds are germinated in blocks of soil created by a press. We plan on using soil blocking for starting our seeds, but for the sake of this article, we are going to follow the practices of J.M. Fortier, which is growing transplants in cells.

Cell flats are plastic containers separated into many compartments in which the roots of the seedlings begin to grow.

Flats are placed in trays that add support for moving them around.

Most flats are 11” wide and 21” long, an industry standard that allows for uniformity in tables and other nursery equipment, such as harvest carts for transporting transplants.

Flats have anywhere from 24 to 200 cells, which determines how big the cell size is. You choose the correct cell size in accordance with the soil volume required by the roots and with the number of days each crop will spend in the cell.

Each plant’s root system will develop within its own cell, making it easy to transplant them individually to specific sites or containers (their roots do not become tangled).

Advantages of Cell Flats

There are many advantages to working with cell flats, such as:

  • Easy to handle and fill
  • Drain well after watering
  • Create clumps of soil that hold together well
  • Durable
  • Reusable

One drawback of using cell flats is that they are not totally indestructible, some won’t last the season and will have to be thrown out.

You also have to make sure they don’t become hosts of different plant diseases. However, in all of the years that J.M. has used them, they have never had any problems.

J.M. always empty’s them completely and spreads them out to dry in the sun for a few hours at the end of each transplanting session.

soil for transplants

Soil Mix for Transplants

Working with the correct soil mix is a very important aspect of growing transplants in cell flats.

Since all basic plant needs (air, water, minerals, etc.) must be met with only a small amount of growing medium, the ingredients of the mix must have specific characteristics – drainage, water retention, aeration, fertilization, salinity, pH, etc.

A soil mix is something that cannot be improvised, so buying a commercial mix may be a better option than making it yourself. Especially for noobs like myself.

When choosing a commercial soil mix, make sure it is high quality and not treated with synthetic wetting agents. Most certified organic mixes are suitable.

How To Make Your Own Soil Mix

For those adventurous types, here’s how you prepare your own soil mix.

This is the secret recipe (just kidding, it’s published in the market gardener).

  • 3 buckets of peat moss
  • 2 buckets of perlite
  • 2 buckets of compost
  • 1 bucket garden soil
  • 1 cup blood meal
  • ½ cup agricultural limestone

There are a few important details when preparing your own soil mix:

  • Peat moss is the main component of the mix and should be top-quality.
  • Perlite serves as an aggregate and plays a key role in drainage and aeration.
  • Compost must not be leached of its nutrients and must be fully mature in order to avoid germination problems.
  • Garden soil is used to cut the compost and lower the salinity. Use a light soil, J.M. uses soil from his garden to do this.
  • Blood meal supplies the extra nitrogen required by heavy feeders.
  • Agricultural limestone is used to raise the pH of the mix, which has a tendency to be low because of the natural acidity of peat moss.

The mixing of all the ingredients can be done directly in a wheelbarrow. 

For best results mix the limestone into the peat moss first. The rest of the ingredients are incorporated one by one with a shovel. 

Working with dry ingredients makes for a more homogeneous mix, but it’s important that in the end, the mix be well moistened throughout. You can add water throughout the mixing process if you prefer.

The mix must be sifted to remove rocks and other large debris to ensure a consistent product. 

You can accomplish this by using a wooden frame with a metal mesh stretched across it. 

The holes of the mesh are about half an inch square.

Filling Cell Flats

Cell flats must be filled with the proper technique in order to conserve as much air retention capacity as possible in the soil mix. This makes a big difference with regard to the proper root development of the seedling.


Make sure the mix is thoroughly wet – it should be sticky.

  1. Fill the cell flats to the brim with the sticky mix, any excess is cleared away with a straight piece of wood or a brush.
  2. Cells should be filled uniformly.
  3. Pack the mix tightly by lifting the flats about 2” into the air and letting them drop onto a flat surface.
  4. Once cells have been seeded, cover the seeds with a fine layer of dry soil mix to prevent them from drying out too quickly.

Flats should be ⅚ full to ensure enough space for water retention.

Arrange the seeded flats in the nursery.

Group flats with the same number of cells together for uniform watering.

The Seedling Room

Crops that you want to produce early- tomatoes, leeks, and onions, must be started as soon as spring arrives to be ready to harvest early in the season.

Starting the transplants inside your house is a good idea.

You will need an area where flats can be spread out and watered, and where soil can be mixed and handled.

This space is called a seedling room.

Our seedling room is the back corner of our basement. It doesn’t have to be fancy!

Criteria of A Good Seedling Room

The main objective of a seedling room is to be able to control growing conditions perfectly.

The ideal average temperature of plant growth is 18 to 23 degrees Celsius (64 to 73 degrees Fahrenheit) in the daytime and 18 degrees Celsius (64 degrees Fahrenheit) at night.

The relative humidity in the room should be 60-90% which is easily maintained with a timer-controlled vaporizer.

The seedling room should be lined with polyethylene to trap heat and humidity.

A small fan is also helpful to prevent fungal disease resulting from a buildup of stagnant air.


Extra lighting is required for optimal plant growth as the day length in February and March is too short. The seedlings need to receive 14 to 16 hours of light per day.

The simplest and cheapest option is to position fluorescent tubes above the flats.

You will need both cool white and warm white fluorescent bulbs in order to provide the plants with the full spectrum of light.

To prevent plants from withering, the lights should be adjustable in height and position about 4” above the tips of the plants as they grow.


Most vegetables require a higher temperature to germinate than they do to grow. 

In order to ensure optimal growth, the seedling room can be equipped with heat mats.

These mats help ensure an optimal germination temperature is kept at 25 degrees Celsius (77 degrees Fahrenheit) day and night.

Next Steps

A seedling room is ideal for starting a small amount of indoor production, but once a certain number of seedlings are started, it becomes necessary to move into a bigger space. 

In my next article, I will be covering the plant nursery options and transplanting them into the garden.

How do you start your transplants? Let us know in the comments below.

Stay Local,

Kathy & Jon

your friendly neighbourhood growers


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