Propagation by Seed

What is propagation by seed?

Propagation by seed is a form of sexual reproduction. Unlike vegetative propagation, such as cuttings or division, seed is produced following the reproductive process involving genetic material from parent plants.

This means plants grown from seed are not necessarily genetically identical to the parent plant.

Seed propagation is one of the most important methods of producing plants and is used for:

• vegetables
• annual flowers
• herbaceous plants
• trees
• shrubs
• wildflowers
• crops

For a horticulturist, successful seed sowing depends on understanding both the seed and the conditions needed for germination.

What is a Seed?

A seed contains a developing plant, together with stored food reserves and a protective covering.

The basic structures include:

• seed coat – protects the seed
• embryo – the young developing plant
• food reserves – provide energy during early development

When suitable conditions are provided, the seed absorbs water and begins to germinate.

The process can be simplified as:

Water + oxygen + suitable temperature → germination → root → shoot → seedling

Light is also important for many species, particularly after germination, although not every seed requires light to germinate.

Seed Size Matters

Seed size has a direct effect on how it should be handled and sown.

For practical horticulture, seeds can be broadly divided into:

Large seed

Examples:

• sweet pea
• broad bean
• runner bean
• sunflower
• acorn

Medium seed

Examples:

• pea
• French bean
• nasturtium
• calendula
• many vegetable and flower seeds

Small seed

Examples:

• lettuce
• carrot
• poppy
• foxglove
• many herbs
• many wildflower species

These categories are not strict botanical classifications. They are useful practical sowing categories.

Large Seed

Large seeds are easier to handle individually.

They can usually be:

• picked up by hand
• spaced accurately
• pushed into the soil
• covered relatively deeply

Large seeds generally contain larger food reserves.

This allows the emerging seedling to push through a greater depth of growing medium.

Typical technique

A hole or drill is made to the appropriate depth.

The seed is placed into the hole.

The soil is drawn over the seed and gently firmed.

For many large seeds, sowing depth is often around 2–3 times the seed’s diameter, although the correct depth depends on the species.

Medium Seed

Medium-sized seeds can usually be handled individually or sown using controlled methods.

Examples include many:

• vegetables
• lettuce
• carrots
• annual flowers
• herbs

They generally require a shallower sowing depth than large seeds.

The seed should have sufficient contact with the growing medium without being buried excessively.

A useful principle is:

Small seed → shallow sowing.
Large seed → deeper sowing.

However, always check the requirements of the individual species.

Small / Fine Seed

Small seed presents a different challenge.

It may be difficult to pick up and space individually.

Examples include:

• Papaver
• Digitalis
• Lobelia
• some herbs

Small seed is generally sown very shallowly – sometimes just on the surface.

Some seed may only require a light covering of compost or fine soil or none at all.

Very small seeds may even require light to germinate and should be sown on the surface or covered only very lightly, depending on the species.

Comparing Seed Sizes

Seed size	Handling	Typical sowing depth	Common technique
Large	Easy to handle individually	Relatively deep	Individual holes or deep drill
Medium	Usually manageable individually	Moderate	Shallow drill or individual sowing
Small	Difficult to handle individually	Very shallow	Fine drill, surface sowing or careful broadcasting

Never assume all seed should be sown at the same depth.

The size of the seed gives the gardener an important clue.

What is a Drill?

A drill is a shallow channel made in the soil into which seed is sown.

Drills make sowing more controlled and organised.

They allow the gardener to:

• control sowing depth
• create straight rows
• space plants consistently
• identify the crop
• distinguish seedlings from weeds
• cultivate between rows
• water accurately
• monitor germination

Drills can have different shapes.

Flat-Bottom Drill

A flat-bottom drill has a relatively flat base.

It can be useful where the gardener wants to create a consistent shallow sowing zone.

The seed sits along the bottom of the drill.

A simple cross-section looks like:

Soil surface

________________________

      _________

     |         |

     |  SEED   |

_____|_________|________

Advantages

• provides a consistent sowing depth
• suitable for certain medium-sized seed
• gives good contact between seed and soil
• relatively simple to create

The drill should not be excessively deep.

V-Drill

A V-shaped drill has sloping sides that meet towards the bottom.

Soil surface

________________________

       \       /

        \     /

         \   /

          \_/

           ●

          SEED

The V-shape can help position seed towards the base of the drill.

It is particularly useful for accurate sowing of small quantities of seed.

The depth should still be appropriate for the species.

A V-shaped drill is not automatically better than a flat-bottom drill.

The choice depends on:

• seed size
• soil condition
• crop
• sowing method
• desired spacing

Why Sow in Lines?

Sowing in lines is not simply about making the garden look tidy.

Straight rows provide several practical advantages.

1. Easier identification

Seedlings appear in a predictable line.

This helps distinguish them from weeds.

2. Easier weeding

The gardener can hoe or cultivate between rows without disturbing the crop.

3. Easier maintenance

Watering, thinning and monitoring become easier.

4. Consistent spacing

Plants have more predictable access to:

• light
• water
• nutrients
• space

5. Easier harvesting

Rows make crops easier to locate and harvest.

6. Better use of space

The gardener can plan plant spacing rather than allowing plants to compete randomly.

Why Sow in Staggered Rows?

Staggered sowing means that plants in adjacent rows are offset rather than directly opposite one another.

For example:

Row 1:   ●     ●     ●     ●

Row 2:      ●     ●     ●     ●

Row 3:   ●     ●     ●     ●

This can improve the use of available space.

When plants are staggered:

• there may be less direct competition between neighbouring plants
• the available growing area can be used more efficiently
• leaves may have improved access to light
• root systems may have more evenly distributed growing space

Staggered spacing is particularly useful when plants are grown relatively close together.

Lines vs Broadcasting

There are two broad approaches to sowing.

Row sowing

Seed is placed in organised rows.

Advantages:

• easy to identify
• easy to weed
• easy to thin
• controlled spacing
• easier crop management

Broadcasting

Seed is scattered across an area.

This is commonly used for:

• wildflower areas
• lawns
• some green manures
• certain ornamental displays

Broadcasting can produce a more natural appearance, but accurate spacing and subsequent weeding become more difficult.

Seedbed Preparation

Successful seed propagation begins before the seed is sown.

The seedbed should generally be:

• free from large weeds
• reasonably fine and friable
• adequately moist
• free from large stones and debris
• suitable for the crop

A fine seedbed is particularly important for small seed.

Large clods can create problems because small seeds can fall into deep gaps and become buried too deeply.

Why Soil Structure Matters

Seed requires close contact with the growing medium.

Too many large air gaps can prevent consistent moisture around the seed.

However, excessive compaction can reduce oxygen availability and restrict emerging roots.

The ideal seedbed provides:

Moisture + oxygen + contact + suitable temperature

This is why soil preparation matters so much.

Sowing the Seed

Step 1 — Prepare the area

Remove weeds and prepare a suitable seedbed.

Step 2 — Mark the row

Use a line or marker to create a straight row.

Step 3 — Create the drill

Make either the appropriate flat-bottom or V-shaped drill.

Step 4 — Sow the seed

Place the seed at the correct spacing.

Step 5 — Cover

Cover with an appropriate depth of fine soil or compost.

Step 6 — Firm

Gently firm the surface to ensure good contact.

Step 7 — Water

Water carefully without washing the seed away.

Step 8 — Label

Record:

• plant name
• variety
• date
• sowing method

Thinning

Seeds are often sown more closely than the final plants require.

Once seedlings have emerged, they may need to be thinned.

Thinning involves removing some seedlings so that the remaining plants have sufficient space.

This is particularly important where direct-sown crops have been sown in rows.

The gardener should avoid pulling neighbouring seedlings out of the soil accidentally.

A useful technique is to remove unwanted seedlings carefully while the soil is moist enough for easy removal.

Germination Requirements

Different seeds have different requirements, but the main factors are:

Water

The seed must absorb water before germination can begin.

Oxygen

Respiration is required to provide energy for growth.

Temperature

Each species has a suitable temperature range for germination.

Light

Some seeds germinate better in darkness, some require light, and many are relatively unaffected within a suitable range.

Therefore:

Always check the requirements of the individual seed.

Seed Dormancy

Some seeds do not germinate immediately even when conditions appear suitable.

This may be because they are dormant.

Dormancy can be broken using treatments such as:

• stratification
• scarification
• soaking
• alternating temperatures
• exposure to light or darkness

These techniques are species-specific.

Direct Sowing vs Sowing Under Protection

Direct sowing

Seed is sown directly into the final growing position.

Advantages

• less transplanting
• less root disturbance
• simple
• useful for many vegetables and hardy annuals

Disadvantages

• seedlings exposed to pests and weather
• weeds compete directly with seedlings
• germination can be less reliable in poor conditions

Sowing under protection

Seed is sown in:

• trays
• modules
• pots
• propagators
• greenhouses

Advantages

• greater environmental control
• protection from adverse weather
• easier monitoring

Disadvantages

• additional equipment
• seedlings eventually need transplanting
• risk of damping off if conditions are poorly managed

PPE and Health & Safety

Seed sowing is generally a low-risk task, but normal horticultural precautions still apply.

Activity	Hazard	Control / PPE
Preparing soil	Dust	Avoid creating dust; suitable gloves
Using hand tools	Cuts	Use tools correctly; keep them maintained
Handling compost	Dust and microorganisms	Gloves; wash hands after use
Raking soil	Flying particles	Eye protection where appropriate
Repetitive sowing	Hand or wrist strain	Good working position; appropriate tools
Watering	Slips	Clean spills and maintain good housekeeping

Common Mistakes

Sowing too deeply

This is particularly common with small seed.

Sowing too densely

Overcrowded seedlings compete for resources.

Poor soil preparation

Large clods make consistent sowing difficult.

Washing seed away

Heavy watering can move seed from its intended position.

Failing to label

Different seedlings can look remarkably similar.

Confusing seedlings with weeds

This is one reason straight rows are so useful.

Allowing the seedbed to dry out

Newly sown seed requires appropriate moisture.

Waterlogging

Too much water can reduce oxygen and encourage disease.

Practical Seed-Sowing Activity

Prepare and sow a seedbed

Plant: ______________________________

Seed size: Large / Medium / Small

Date: ______________________________

Sowing method: Direct / Protected

Drill type: Flat-bottom / V-shaped / Other

Row spacing: _________________________

Seed spacing: ________________________

Sowing depth: ________________________

PPE used: ____________________________

Student Observation

Why was this sowing depth selected?

Why was this drill shape selected?

Why were the seeds sown in lines?

Would staggered rows be useful for this crop?

What could happen if the seed were sown too deeply?

Seed Size Challenge

Match the seed to the likely sowing technique.

A — Large seed

Sweet pea

Likely technique:

B — Medium seed

Calendula

Likely technique:

C — Small seed

Poppy

Likely technique:

Now explain:

Why should the three seeds not necessarily be sown at the same depth?

Remember

Good seed sowing is not simply:

“Put seed in soil.”

It is about giving every seed the best possible opportunity to germinate.

Think about:

Seed size → sowing depth → spacing → soil condition → moisture → temperature → light

The shape of the drill matters because it helps control where the seed sits.

The arrangement of the rows matters because it affects how the crop will be managed.

And the size of the seed matters because a tiny seed does not have the same resources as a large seedling embryo.

A professional horticulturist therefore does not sow every seed in the same way.

Look at the seed. Understand the plant. Then choose the technique.

Key references

• Royal Horticultural Society (RHS), guidance on sowing seed and raising plants from seed.
• Royal Horticultural Society (RHS), Raising Plants from Seed.
• Hartmann, H.T. et al., Plant Propagation: Principles and Practices.
• Bewley, J.D. et al., Seeds: Physiology of Development, Germination and Dormancy.
• Taiz, L. et al., Plant Physiology and Development.
• Health and Safety Executive (HSE), guidance on safe use of hand tools and horticultural materials.