You’ve got your seeds, seed starting mix, and pots or trays. What about proper soil temperature? Hmm … if you’re not sure about how warm the soil should be to start your seeds, don’t worry. We’ve got information (and a cool app) that will make it easy.
So, what is the best soil temperature for seed germination? A good soil temperature range for seed germination is 70 to 75 degrees Fahrenheit (21 to 24 degrees Celsius). This range will work for almost all vegetable seeds – and germination will occur quickly (within 1 to 2 weeks for most seeds) with proper soil moisture and planting depth.
The ideal soil temperature range for seed germination depends on the type of plant. Some seeds can germinate in soil close to freezing (32 degrees Fahrenheit or 0 degrees Celsius). Others will do little or nothing in soil cooler than 60 degrees Fahrenheit (16 degrees Celsius).
In this article, we’ll talk about the ideal soil temperature range for various types of seeds (mostly vegetables). We’ll also go over some of the other factors that matter (soil moisture, planting depth, and for a few types of seeds, light!)
Hey – you can get our weekly gardening newsletter here!
Join 1000+ gardeners to get access to news, tips, and information.
Delivered right to your inbox – once per week.
Let’s get started.
Soil Temperature For Seed Germination (Ideal & Marginal)
A soil temperature range of 70 to 75 degrees Fahrenheit (21 to 24 degrees Celsius) is ideal for seed germination for many types of plants.
At this range, most seeds will sprout quickly and also show a good germination rate (the percentage of planted seeds that sprout).
Remember that air temperature is not the same thing as soil temperature. After all, the seeds are in the soil, and they won’t really be exposed to the air until after germination.
Still, air temperature does affect soil temperature – but slowly. For example, if the air temperature rises from 60 Fahrenheit in the morning to 90 Fahrenheit at noon, the soil will not warm up quite as fast.
Use a soil thermometer to keep track of the soil temperature so you know for certain how warm it is, especially outdoors.
Also: if you need ways to keep your seeds warm, you can learn about lots of methods in my article here.
If you want an awesome visual with a thermometer to help you see how various temperature ranges affect time to seed germination, check out my free app here.
Will Seeds Germinate At 40 Degrees? (4 Celsius)
If your soil temperature is 40 degrees Fahrenheit (4 degrees Celsius), some types of seeds will still germinate. However, they may take a little longer to sprout, since the soil is not in the ideal temperature range.
The following types of vegetable seeds will still germinate at a soil temperature of 40 degrees Fahrenheit (4 degrees Celsius):
- Beets (borderline – can take up to 42 days at this temperature)
- Broccoli (borderline)
- Cabbage (borderline)
- Carrots (borderline – can take up to 53 days at this temperature)
- Cauliflower (borderline – can take a few weeks at this temperature)
- Celery (borderline – can take a few weeks at this temperature)
- Chard (borderline – can take a few weeks at this temperature)
- Leeks
- Lettuce (can still take several weeks at this temperature)
- Onions (can still take several weeks at this temperature)
- Parsley (borderline – can take up to 29 days at this temperature)
- Parsnips (can still take several weeks at this temperature)
- Peas (borderline – can take up to 36 days at this temperature)
- Radishes (borderline – can take up to 29 days at this temperature)
- Spinach (can still take several weeks at this temperature)
- Turnips (borderline)
As you can see, several of these plants are borderline at 40 degrees Fahrenheit (they will germinate, but slowly – possibly taking several weeks to start sprouting).
Vegetables that need warmer soil temperatures for germination include asparagus, corn, and tomatoes. They won’t do much, if anything, when planted in soil at a temperature of 40 degrees Fahrenheit.
Will Seeds Germinate At 50 Degrees? (10 Celsius)
If your soil temperature is 50 degrees Fahrenheit (10 degrees Celsius), some types of seeds will still germinate. However, they will take a little longer to sprout, since the soil is not in the ideal temperature range.
The following types of vegetable seeds will still germinate at a soil temperature of 50 degrees Fahrenheit (10 degrees Celsius):
- Asparagus (borderline – can take up to 53 days at this temperature)
- Beets
- Broccoli
- Cabbage
- Carrot
- Cauliflower
- Celery
- Chard (aka Swiss chard)
- Corn (borderline – can take up to 22 days at this temperature)
- Leeks
- Lettuce
- Onions
- Parsley
- Parsnips
- Peas
- Radishes
- Spinach
- Tomato (borderline – can take up to 43 days at this temperature)
- Turnips
As you can see, asparagus, corn, and tomato are seeds that are borderline at 50 degrees Fahrenheit (they will germinate, but slowly – possibly taking several weeks to start sprouting).
Vegetables that need warmer soil temperatures for germination include cucumbers, eggplant, okra, peppers, pumpkins, and squash. They won’t do much, if anything, at a soil temperature of 50 degrees Fahrenheit.
Will Seeds Germinate At 60 Degrees? (16 Celsius)
If your soil temperature is 60 degrees Fahrenheit (16 degrees Celsius), most types of vegetable seeds will germinate. However, some will take a little longer to sprout, since the soil is not in the ideal temperature range.
60 degrees Fahrenheit is a borderline temperature for many types of seeds, including:
- Cucumbers
- Eggplant
- Muskmelons (Cantaloupe)
- Okra
- Peppers
- Pumpkins
- Squash
- Watermelons
Okra and peppers seem to be especially sluggish at 60 degrees Fahrenheit. Their seeds can take almost a month to germinate at this temperature!
Will Seeds Germinate At 70 Degrees? (21 Celsius)
If your soil temperature is 70 degrees Fahrenheit (21 degrees Celsius), just about every type of vegetable seed will germinate just fine. In fact, most seeds will germinate about as fast as they can at this temperature or close to it (70 to 75 Fahrenheit or 21 to 24 Celsius).
Will Seeds Germinate At 80 Degrees? (27 Celsius)
If your soil temperature is 80 degrees Fahrenheit (28 degrees Celsius), just about every type of vegetable seed will germinate just fine. It is a little above the ideal range for some plants (such as lettuce, peas, and spinach), but they will still germinate in a fairly short time.
Will Seeds Germinate At 90 Degrees? (32 Celsius)
If your soil temperature is 90 degrees Fahrenheit (32 degrees Celsius), many seed will still germinate just fine. However, some seeds will have trouble or fail to germinate at such a high soil temperature.
For example, celery, lettuce, lima beans, parsnips, peas, and spinach may show little or no germination at a soil temperature of 90 degrees Fahrenheit. If a few seeds do sprout, the germination rate will probably be very low.
Will Seeds Germinate At 100 Degrees? (38 Celsius)
If your soil temperature is 100 degrees Fahrenheit (38 degrees Celsius), some seeds will still germinate. However, lots of seeds will have trouble or fail to germinate at such a high soil temperature.
For example: as mentioned above, celery, lettuce, lima beans, parsnips, peas, and spinach, will have trouble germinating at high soil temperatures.
We will also have trouble germinating asparagus, carrot, eggplant, green bean, pepper, and tomato seeds at a soil temperature of 100 degrees Fahrenheit. If a few seeds do sprout, the germination rate will probably be very low.
Hey – want to take my FREE 1-week seed starting email mini-course?
Learn about starting plants from seed – in just a few minutes per day.
Sign up for the course and get started today!
Other Factors That Affect Seed Germination
Remember that the above information assumes that other germination factors are correct, including proper soil moisture, planting depth, and light.
Soil Moisture For Seed Germination
Seeds need water to germinate – no matter what the type of seed is. Giving seeds enough water (but not too much) is a careful balancing act.
Lack of water will stop germination. If you start watering and then the soil dries out, it can hurt or kill seeds.
Too much water will drown seeds before they sprout. Seeds “breathe”, so they must be able to absorb oxygen and exhale carbon dioxide. Too much water prevents seeds from breathing.
How Much Water Do Seeds Need To Germinate?
Seeds need consistent moisture to germinate. This means the soil should be damp, but not soaked. According to the Penn State University Extension, soil that is 50% to 75% saturated with water is ideal.
When there is too much water in the soil, it gets soggy for a long time after watering. You can always add water if the soil gets a little dry, but it is hard to take water out of the soil after you add too much
In places with dry air, high temperatures, and strong wind, you will probably need to water a little bit every day to keep the soil wet enough for seed germination.
How To Keep Soil Moist For Seeds
There are two methods you can use to keep soil moist enough for seed germination:
- Spray bottle – this lets you to water precisely, and it makes things easier if you want to avoid over watering. A spray bottle also lets you to water only part of a seed tray if necessary.
- Humidity dome – this gives seeds an environment (such as a plastic tray and cover with a vent) that holds moisture in the air and soil. You won’t need to water as much if you have a humidity dome.
As a bonus, a humidity dome helps to keep soil warmer, which also helps seeds to germinate. If you like, you can use both a spray bottle (for precision watering) and a humidity dome (to retain moisture).
You don’t need a humidity dome for seed germination. Seeds will still sprout without it. However, it may increase germination rates while also saving you time.
Just be sure to remove a humidity dome right after the first leaves appear on seedlings. Otherwise, your seedlings are more likely to suffer from mold problems.
You can learn more about humidity domes (and how to make your own!) in my article here.
Seed Planting Depth
A good rule is to plant a seed at a depth of 2 times its diameter. For example:
- A seed that is 1 inch in diameter should be planted at a depth of 2 inches.
- A seed that is ¼ inch in diameter should be planted at a depth of ½ inch.
It is possible to plant seeds too shallow or too deep.
If a seed is too shallow, there is more chance that it will dry out, due to accidental air contact. The sprouted seed may also work itself out of the soil.
If a seed is too deep, it may fail to germinate, stay below soil after germination, emerge late, or grow poorly after emergence.
You can find more information (and recommended seed planting depths for various plants) in this article.
Light For Seed Germination
All seedlings need light after the seed germinates. But did you know that some seeds need light to germinate?
It’s true! Some seeds only want a thin layer of soil on top of them, which allows some light to get through to them.
The following types of seeds need a little light to germinate:
- Ageratum
- Alyssum
- Browalia
- Celery
- Coleus
- Dill
- Dusty Miller
- Impatiens
- Lettuce
- Lobelia
- Nicotiana
- Petunia
- Portulaca
- Salvia
- Scarlet sage
- Snapdragon
- Torenia
- Wax begonia
Conclusion
Now you have a good idea of what types of seeds germinate at various soil temperatures. You also have an idea of how long it can take, and what other factors come into play when sprouting seeds.
If you want to learn more about starting seeds (either indoors or outdoors), sign up for my free 7-day seed starting mini course here.
You can learn how to save seeds from your garden here.
I hope you found this article helpful. If so, please share it with someone who can use the information.
To find books, courses, seeds, gardening supplies, and more, check out The Shop at Greenupside!
Hey – you can get our weekly gardening newsletter here!
Join 1000+ gardeners to get access to news, tips, and information.
Delivered right to your inbox – once per week.
If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here. Enjoy!
~Jonathon