Three temperature points: Minimum · Optimum · Maximum.

Germination occurs most rapidly at the optimum (perfect) temperature. Below the minimum, and above the maximum temperature germination will not occur.

Each species of plant has a different minimum, optimum and maximum.

Seeds need moist conditions to germinate, but not too wet, and seedlings are very sensitive to drying out.

Both seeds and seedlings can be killed by waterlogging; if the germinating seed, or the root of the young seedling, becomes completely surrounded by water then they cannot uptake gases for respiration..

Oxygen uptake is essential for germination but becomes impossible for the seeds if the soil is too compacted or become waterlogged.

If a hard crust forms on the soil surface, then oxygen diffusion into the soil is limited, and this reduces germination rate.

Some water plant seeds can only germinate in water and are inhibited by exposure to air.

Many plant species which produce small seeds need light to trigger germination.

For some it is the correct wavelength of the light, and for some it is the correct photoperiod (day length) that triggers germination.

Small seeds do not have big food reserves and so the epicotyl needs to reach the soil surface quickly so that the first leaves can start to photosynthesis to feed the seedling.