Best Temperatures for Seed Germination
What is the ideal temperature for seed germination is a simple question but the answer is actually quite complicated.
Most seeds will germinate over quite a wide range of soil temperatures but the speed of germination will vary. Too cold and they’ll be very slow to sprout and too hot will also reduce the speed of germination. Far too cold or hot and they’ll just fail.
Academic research carried out in the USA has charted these temperature ranges for a number of vegetables. Interestingly the ideal temperature for germination is often far higher than you might expect. In some cases the germination temperature is higher than is ideal for growing.
Temperature for Tomato Germination
A good example of this is the tomato. Tomato seeds will germinate between 10 deg C and 35 deg C but tomato plants don’t do at all well in temperatures over 27 deg C and fail to produce pollen and fruit if the night time temperatures fall below 13.5 deg C. Yet the ideal temperature for germination of tomatoes is 29 deg C
In practice you would most likely germinate your tomatoes with a soil temperature between 20 deg C and 25 deg C which is fairly easy to achieve in a heated propagator.
Curiously carrots’ ideal germination temperature is 27 deg C – I can’t imagine the soil in the UK hits that high often, if at all! The optimum range for carrots is between 7 deg C and 29 deg C with far faster germination rates once the soil hits 10 deg C.
Since carrots are invariably direct sown, knowing your soil temperature will enable you to know when you can sow and expect reasonable germination rates and speed. You can warm up the soil outside by a few degrees if you cover it with horticultural fleece or better still use a cloche which can make a surprising difference to germination rates and times.
A very old method of getting a warm soil to sow in is to use a hot bed cased in fine soil. Not much use for root crops like carrots and parsnips but any crop that can be transplanted will benefit from the bottom heat.
A few points to keep in mind:
- The temperatures quoted are soil or compost temperature, not air temperature. A sunny day in April may be 20ºC but the soil temperature is most likely stuck around 8ºC.
- A soil thermometer can be bought very cheaply, typically less than £10.00, which will give you an accurate figure to work from
- Beware the sunny day when propagating in the greenhouse. Temperatures can soar, basically cooking your seeds or seedlings.
Germination Temperature Charts
The charts have been compiled from a number of academic studies. There are some differences due to differences in methods between studies but I think they’re a fair guide.
Seeds will germinate across a wide range of temperatures but as the temperature falls below the optimum range the length of time it takes for them to germinate increases and the percentage germination rate falls. Long germination times also makes the seeds more vulnerable to disease.
The first chart gives the minimum and maximum temperatures at which seeds will germinate. However, as the chart below – seed germination time against temperature shows, with onions at 1ºC the germination period (about 4 months) would be so long that most of us would have given up!
The best temperatures for seed germination are in the last chart.
|Variety||Min to Max Temp|
|Aubergine||16 to 35|
|Beans Broad||6 to 28|
|Beans French||8 to 35|
|Beans Runner||8 to 35|
|Beet||6 to 35|
|Cabbage||6 to 30|
|Carrot||6 to 35|
|Cauliflower||6 to 30|
|Courgettes||16 to 35|
|Swiss-Chard||6 to 35|
|Sweetcorn||10 to 40|
|Cucumber||16 to 40|
|Lettuce||2 to 35|
|Onion||1 to 35|
|Parsley||6 to 32|
|Parsnip||2 to 29|
|Pea||6 to 29|
|Sweet Peppers & Chillies||16 to 35|
|Radish||6 to 35|
|Spinach||2 to 29|
|Squash & Pumpkins||16 to 38|
|Tomato||10 to 35|
|Turnip & Swede||6 to 40|
Percentage Germination of Seeds and Time to Seedling Emergence at Different Temperatures
Best Temperatures for Seed Germination
As you can see from the charts above for Sweetcorn and Onion Germination, there is a sweet spot where there is high germination in a short time but a temperature a few degrees above or below makes little difference.
There are also variations between batches of seed and varieties, so rather than lay down a single temperature, it’s best to look at a range of temperatures where you can expect reasonable levels of germination.
|Variety||Min to Max Opt. Temp|
|Aubergine||24 to 32|
|Beans Broad||8 to 15|
|Beans French||15 to 30|
|Beans Runner||15 to 25|
|Beet||10 to 25|
|Cabbage||10 to 25|
|Carrot||10 to 35|
|Cauliflower||10 to 25|
|Courgettes||20 to 35|
|Swiss-Chard||10 to 25|
|Sweetcorn||15 to 30|
|Cucumber||15 to 35|
|Lettuce||5 to 25|
|Onion||10 to 25|
|Parsley||10 to 25|
|Parsnip||15 to 20|
|Pea||10 to 25|
|Sweet Peppers & Chillies||20 to 30|
|Radish||15 to 30|
|Spinach||5 to 15|
|Squash & Pumpkins||20 to 30|
|Tomato||15 to 25|
|Turnip & Swede||15 to 35|
Armed with this knowledge you can better judge when to sow outside and what temperature to set a propagator at for good germination.
However, keep in mind that not all plants will thrive after germination at the higher temperatures of the optimum range for germination.
Onions are best started at 15ºC or even 10ºC and even heat loving crops like tomatoes will do better if the temperature is dropped on or just prior to seedling emergence to 20ºC
Find the best temperatures for seed germination for a range of vegetable seeds. What temperature will seeds germinate at. How are the optimum germination temperatures calculated.
Do You Know the Optimum Temperatures for Your Seeds?
by Dr. Ed Brotak
Give seeds and transplants their best shot at growing success by understanding optimum temperatures, both low and high. Home gardeners and farmers alike all know how important soil conditions are for a successful crop, be it flowers or corn. But we don’t typically think about soil temperature. Why is soil temperature important? It affects plant growth. If it’s too cold or too hot, plants won’t grow well, if at all. Soil nutrients and useful soil organisms have optimal soil temperatures. Soil moisture and aeration also relate to temperature. But soil temperature is at its most important in the critical early stages of plant life: the germination of the seed and the development of the seedling before it breaks the surface. If the soil is too cold, the seed may not germinate; even if it does, growth may be permanently affected.
Optimum Temperatures: How Temperature Changes
We often just assume soil temperatures mirror the air temperature. They don’t. There are different processes involved in how heat is moved. Let’s look at a sunny day. Most of the sun’s rays pass through the atmosphere and hit the earth’s surface, thus heating it. The air is heated from below and with hot air rising (convection), the heat is moved upward quickly. Obviously, that can’t happen below the ground. The heat is only taken slowly downward by the process of conduction. To illustrate how heat works its way down: For a typical soil, the peak temperature at a depth of 2 inches will occur about an hour after the peak surface temperature and it will be 30 percent less. The maximum temperature at 4 inches down occurs 2 or 3 hours later and is 66 percent less. The opposite happens at night. The surface radiates heat out into space and cools rapidly. The air next to the ground is now cooled from below and air temperatures fall. But below the surface, heat travels only slowly upward. Therefore, subsurface soil temperatures remain the same. In winter, the slow loss of heat from the ground helps prevent freezing temperatures from penetrating too deep into the soil. Thus, the root systems of perennials are protected. Interestingly, a snow cover on the surface is actually protective. The layer of snow insulates the ground below from the often extreme air temperatures, which can get below 0°F in northern areas.
Soil moisture, the amount of water in the soil, also affects soil temperature. Water has a high heat capacity—it can absorb a lot of heat without changing temperature. Also, when water evaporates, it uses energy or heat. In the spring, wet soils will take longer to warm up. Soil moisture content is strongly related to soil type. Sandy soils drain well. Their surface heats up a lot during the day and cools off rapidly at night. Clay soils hold water; their temperatures change slowly. Especially in the spring, clay soils tend to be too cold. The best soils (for temperature and moisture) are loams, which are mixtures of the other soil types.
What Optimum Temperatures Do for Your Seeds
Spring planting time is when soil temperatures are most critical. When do you put those seeds into the ground? That depends on what you’re planting. Different plants require various soil temperatures. And there’s more to consider once you’ve determined what you’re planting. For every type of plant, there is a minimum temperature for seeds to germinate and an optimum temperature for seed germination. For example, a cool-season vegetable like lettuce can germinate with soil temperatures just above freezing, but the best temperature for germination is 75oF. If you plant lettuce seeds with soil temperatures in the 30s, they’ll develop slowly, with emergence taking over a month. Wait until the soil warms up some—at least into the 40s—and everything speeds up. But don’t wait until the soil temperature hits 70°F because even though that’s great for seed germination, the warm summer temperatures would hinder the subsequent plant growth.
Fortunately, all the information you need to know about when to plant various flowers and vegetables is readily available. Check on the back of seed packs or in seed catalogs, or online from a variety of sources including local agricultural agencies and extension services. Do keep in mind that even though soil temperatures may be warm enough for germination and growth, you may have to still worry about air temperatures suddenly getting below freezing. This holds especially true in an abnormally warm late winter or early spring.
Measuring soil temperature isn’t that difficult or expensive. Simple soil thermometers are available at many garden or farm supply shops and online. You can get one for under 10 dollars. Stick the thermometer down into the soil as deep as you would plant the seeds. There are also various websites you can check to see local soil temperatures. Check with your state agricultural service or your State Climatologist Office.
Dr. Ed Brotak is a retired meteorologist who lives and gardens in North Carolina.
Home gardeners and farmers alike all know how important soil conditions are for a successful crop, be it flowers or corn. But we don’t typically think about soil temperature. Why is soil temperature important? It affects plant growth. If it’s too cold or too hot, plants won’t grow well, if at all.