Primary Science


Popular Primary Science Question: Plant Reproduction (Effect of Overcrowding)

Primary Science: Plant Reproduction – Overcrowding of seeds and its effects on plant growth

What is overcrowding?

What happens when you plant too many seeds in a pot? This is a common P5 plant reproduction topic- specifically on the effects of overcrowding.

Understanding the concept of overcrowding helps us see how living things interact and adapt in different environments.

Seeds need to be dispersed to avoid overcrowding


Firstly, we need to know the four types of seed dispersal and why do they even need to be dispersed?

There are four types of seed dispersal methods:

  • Wind – e.g. Dandelions/ Shorea seeds
  • Water – e.g. Coconuts/ Pong Pong seeds
  • Animals – e.g. Mangoes/ Burdock seed
  • Explosive action/ splitting – e.g. Himalayan balsam/ Pea pods


Seeds must be dispersed far from the parent plant and other seeds.


Seed dispersal prevents overcrowding, which reduces competition for water, light, space and mineral salts/nutrients so that the seedlings can grow healthier.

Remember: We Love Singapore McNuggets! (water, light, space, mineral salts/Nutrients)

Let’s look at the following question on Plant Reproduction (Primary Science):

Some green bean seeds were planted in Pot A and Pot B.

The pots were placed next to a window, exposed to the same amount of sunlight.

The same amount of water and soil were given to each pot.



The average height of the plants were recorded daily. The plants in which pot will be taller?

The correct answer is Pot B!

It may seem counterintuitive, but the overcrowded seedlings (Pot B) will actually grow taller than their non- overcrowded counterparts (Pot A)!

This is because the overcrowded seedlings will compete for sunlight, and the seedlings will have to grow taller to not be blocked and continue to photosynthesise.

overcrowding-effect-on-plant-reproduction-average height of plants-1

The question mentioned that seeds were planted. So the height of the seedlings at the start are similar, as the seedlings have seed leaves which store food and there was no need to compete for sunlight.

Once the plants needed to make their own food, plants in pot B grew taller and thinner due to competition for sunlight. They will grow very fast and have thin stems, using up their remaining stored food in the seed leaves very fast to try to reach for more sunlight.

Afterwards, without sufficient light and water, photosynthesis slows down and respiration slows down, which releases less energy that is needed for life processes and the seedlings slowly die.


Read Also: Transport in Plants and Humans (Upper Block Science)

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