Starting Seeds Indoors: How To Start Seed Indoors

One of the most rewarding of all garden undertakings is starting plants indoors from seed.

Unfortunately, it is frequently also one of the most disappointing, for failure to “bring through” a batch of seedlings spells loss of time and opportunity and leaves the gardener with a deplorably deflated ego.

Growing Seeds IndoorsPin

6 Indoor Seed Planting Requirements

What then can be done to assure success?

There are 6 very definite requirements that should be met.

These are the following:

  1. a loose, porous, fertile soil
  2. sanitation
  3. adequate moisture
  4. optimum heat
  5. light
  6. air free from harmful gases

Neglect of any one of these requirements may lead to failure.

Only by giving careful attention to each one can the gardener feel reasonably sure of success.

Let’s take a look at each requirement to see what can be done to keep operation seed-starting in the groove.

Ideal Soil Conditions

When I refer to the term “soil,” I mean any medium where the seeds are to be started.

It may be plain garden soil or contain several other materials such as the following:

  • vermiculite
  • peat moss
  • sand
  • sphagnum moss

No matter how rich it may be, garden soil seldom has the proper consistency for starting seeds indoors.

If the soil were ideal, it would be both porous enough to let the excess water drain through readily and at the same time absorbent sufficient to retain moisture for an extended period.

Compost, if at just the right stage of decomposition, would meet this requirement. 

However, if you have no compost, you can make up a suitable mixture by combining equal parts of friable garden soil and peat moss.

You may also add to this enough gritty sand so that when the mixture is squeezed in hand, it will fall apart readily, even if quite moist.

Sometimes other materials such as vermiculite or sifted sphagnum moss are used, but they require more expert handling since they lack nutrients.

I find the soil-peat-sand mixture best for the majority of seeds.

Sanitation Deters Disease

By practicing such a sanitary measure as sterilizing the planting mixture, you will protect your seedlings from being destroyed by the well-known fungus disease, damping-off. 

When this disease attacks a seedling, the stem decays at the soil surface, and the seedling falls over.

You can use vermiculite or sphagnum moss to discourage damping-off, both of which are sterile, as the seed-starting medium.

However, as stated above, these materials are difficult to use because nutrients must be added if they are to support growth.

If you decide to use the soil-sand peat mixture, you can render it sterile by one of the following methods:

1. Moisten the mixture (but not so that it’s soggy wet) and heat in a deep pan in the oven for 45 minutes at 200° to 250° degrees Fahrenheit.

Keep the ventilating fan going because the odor is none too pleasant!

2. Fill an ordinary wooden flat (a common size is 13 x 20 x 3 inches) with the material, sprinkle 1 tablespoon of formaldehyde (this can be bought from any druggist) diluted with 5 tablespoons of water over the material, and mix it in thoroughly.

Cover the flat with several layers of moist newspaper and let stand for 24 hours. Then remove the paper and sow the seeds.

Other Practices To Discourage Damping-Off


In addition to sterilizing the medium, a few other practices will also discourage damping-off.

These are the following:

  • sowing seeds thinly
  • providing only enough water—never waterlogging the medium
  • avoiding high temperatures and very high humidity
  • providing adequate ventilation

Proper Watering

Many a flat of seed either fails to come up or is lost after coming up because of careless or improper watering.

If the soil is allowed to dry out between waterings, the growth of seedlings will be checked.

If this happens during the critical period of sprouting and emergence, it may result in total failure.

Proper watering is essential for good germination with excellent seeds, such as petunias and begonias.

Flats are much more apt to dry out in a hot, dry house than in a greenhouse or hotbed. 

However, greenhouse conditions can be simulated by covering the flat with glass or cellophane until the seedlings are well up.

The plastic domes or caps listed in most garden supply catalogs are ideal for this purpose.

Adequate Ventilation

If the air above the flat gets too humid, tilt the dome or glass hack.

Try this “miniature greenhouse” method once for a packet of “difficult” seeds, and you’ll never go back to older methods.

The easiest way to keep the soil flat, evenly moist is to water from below, allowing the water to be absorbed until it seeps to the surface.

Such a watering will keep the soil moist 2 or 3 times as long as a surface drenching, and there will be no danger of washing out small seeds or compacting the surface soil.

Temperature And Germination

The time required for germination varlet varies greatly, depending upon the plant and variety, the temperature, soil, and many other factors.

Some will be pushing up the soil in 3 or 4 days, and others may take weeks.

Most annuals and vegetables should emerge in 5 to 12 days.

Ideal Temperatures

Temperature is perhaps the most critical factor influencing the length of time required for germination.

While a few species germinate best at comparatively low temperatures, 45° to 50° degrees Fahrenheit, the majority will respond more quickly at 70° or 80° degrees Fahrenheit.

Quick germination is desirable but at too high a temperature.

At the same time, it may hasten germination, increase the rate at which the soil dries out, and encourage weak, spindly growth and damping-off.

The ideal temperature assures prompt germination without the risk of seedling injury.

For the majority of spring-started plants, 60° to 70° degrees Fahrenheit, until the seedlings are well up, with a reduction to 50° to 60° degrees Fahrenheit at night, will be best.

A few plants prefer temperatures about 10° degrees Fahrenheit higher.

These include the following:

  • Celosia
  • Coleus
  • Begonias
  • Impatiens
  • Dahlias
  • Tomatoes
  • Eggplant
  • Peppers

Where it is not convenient to provide a sufficiently high night temperature to secure prompt germination, flats or seed pans may be given bottom heat by placing them on bricks or blocks of wood above a radiator or furnace top.

Never place a fiat directly on a hot surface.

Instead, examine the flat often and when the seedlings appear, remove the flat from the heat at once.

Adequate Light Requirements

The germination process does not require light, but the light is necessary once the seedling emerges above the soil.

Sunlight furnishes the energy needed by the seedling to manufacture food.

The flat should be given all the full, direct sunlight possible from the moment the first cotyledons appear.

If the seedlings don’t receive enough light, they will grow long and spindly.

When seedlings are grown indoors, they should be placed on a table or shelf as close to the south or east window.

Turn the flat every few days to keep the seedling’s growth straight.

If the amount of sunlight is seriously inadequate, fluorescent lights placed 12″ inches above the plants may be used instead. However, this is not as good as sunlight.

Abundant Air

Air, like light, is essential for plant growth.

Few gardeners realize that air is just as crucial to the underground part of the plant as it is to the part above ground.

An abundance of air also discourages the growth of harmful fungi, such as those causing damping-off.

If you have used a glass or plastic covering over the flat or seed pan, remove it as soon as the seedlings are well up.

How to Start The Seeds?

The prepared soil mixture may be placed in any container of convenient shape and size, provided there are drainage holes in the bottom.

When raising a lot of seedlings, an ordinary wooden flat is most commonly used.

For fewer seedlings, flower pots or bulb pans would be more practical, especially when growing the seedlings indoors.

Follow these steps:

  • Place a 1/2-inch-layer of sphagnum moss or coarse compost in the bottom of each container.
  • Then add the soil mixture until it is almost level with the top of the flat or flower pot.
  • Press the mixture down lightly and sow seeds thinly in rows 2” inches apart.
  • The larger seeds such as calendulas, marigolds, and zinnias should be spaced farther apart—about 2” to 3” times their own diameter.
  • The finer seeds may be sown a little closer, but they should not touch.
  • As an extra precaution against damping-off, I treat all seeds with a seed protectant such as Arasan, Semesan, and Spergon.
  • To coat the seeds with the material, take up a bit of the powder on the tip of a knife.
  • Drop the material into the seed packet and shake the seeds in the packet. This method will coat the seeds evenly and thoroughly.

Several handy devices on the market make seed-sowing easier, but the simplest device is a folded piece of smooth, heavy paper.

Fairly large seeds should be pressed into the soil and then covered with a scant 1/8-inch-layer of soil.

Very fine seeds should not be covered but merely pressed into the soil.

Here are the steps to follow:

  • Cover the flat or flower pot as suggested above but often examine for signs of germination.
  • As soon as the seedlings appear, provide sufficient ventilation to prevent excessive condensation on the inside of the cover.
  • If, despite careful sowing, the seedlings crowd each other, thin them out, using tweezers if necessary, and stand free.

A dozen good sturdy seedlings are worth 50 puny, spindly ones.

2 to 4 weeks after the seedlings emerge, they will be large enough to transplant.

The first or second true leaf development is the signal to remove them from the seedbed.

For more details about transplanting seedlings to the garden, see next month’s FLOWER GROWER.