There is an optimum temperature, the most ideal temperature, for vegetable seed to germinate, and it varies from one type of veg to another.
The optimum temperature is printed on the back of some seed packets.
Seeds will germinate over a range of temperatures, but at the optimum temperature :
- more seeds germinate – the percentage of seeds to germinate is highest
- germination is quicker – the number of days to emergence is less
The benefits of increasing germination rate by hitting the optimum temperature are:
- seed cost per veg plant produced is less
- return on effort of seeding is higher
The benefits of decreasing days to emergence by hitting the optimum temperature are:
- less time for seed to rot off
- less time for seed to be eaten by predators
- shorter growing period & earlier harvest
Seeds will germinate at temperatures lower & higher than the optimum temperature, but fewer seeds will germinate & they will take longer to come through.
For example, the optimum germination temperature for asparagus is 77F, when 95% of seeds are likely to germinate, and the asparagus seedlings take 10 days to emerge. Either side of the optimum temperature, the number of seeds germinating reduces as shown in the table below; and the number of days for the seedlings to emerge increases.
| Asparagus seed | ||
| Temperature F | %germination | Days to emerge |
| 32 | 0 | |
| 41 | 0 | |
| 50 | 61 | 53 |
| 59 | 80 | 24 |
| 68 | 88 | 15 |
| 77 | 95 | 10 |
| 86 | 79 | 12 |
| 95 | 37 | 19 |
| 104 | 0 | |
Starting seeds off indoors, or outdoors in pots or under glass is worth considering because:
- germination temperatures for vegetable seeds often differ from the ideal temperatures for plant growth.
- soil temperature takes a long time to heat up & waiting for the optimum soil temperature will prolong harvest dates, if indeed, there will be enough days left for a full growing season.
Vegetable gardeners will find that some types of veg, and also different varieties of particular veg, do better in their particular locality. Some varieties will germinate at lower temperatures than other varieties of the same type of vegetable.
Here is a chart of some ideal temperatures:
| Germination temperature | Growing temperature | |||
| Asparagus | 77 | 60-70 | ||
| Beans – french | 75-85 | 60-65 | ||
| Beans – runner | 75-85 | 65-75 | ||
| Broccoli | 80 | 60 – 65 | ||
| Brussels Sprouts | 75 – 80 | 60 – 65 | ||
| Cabbage | 75 – 85 | 60 – 65 | ||
| Carrots | 75 | 60 – 70 | Sow Directly into soil | |
| Does not transplant well. | ||||
| Cauliflower | 80 | 60 – 70 | ||
| Celery | 70 | 60 – 70 | ||
| Leeks | 75 | 75 | ||
| Lettuce | 40 – 60 | 40 – 60 | ||
| Onions | 65 – 85 | 65 – 85 | ||
| Parsnips | 65 – 75 | 65 – 75 | ||
| Peas | 40 – 75 | 40 – 75 | Sow Directly into soil | |
| or sow in gutter | ||||
| Potatoes | 45 | |||
| Spinach | 50 – 75 | 60 – 65 | ||
| Swiss Chard | 50 – 85 | 60 – 65 | ||
| Turnip | 50 – 95 | 50 – 95 |
We put seeds in a propagator because they require a higher temperature to germinate. But once they have germinated, a lower temperature will encourage the seedlings to grow slowly and to develop a strong stem.
seedlings in pot
Once the seed has germinated, it should be removed from the propagator and placed in a warm, light place. It will need to be protected from cold temperatures for a few weeks.
elongated seedling
with weak stem
If the seedling is kept in the propagator for a day or more, it will become leggy, with a long, weak stem.
Remove the plant from the propagator at the first sight of the seed leaves.
The germination temperature for vegetable seeds is different from the ideal temperature for plant growth