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Best Soil Temperatures for Sowing Seeds

March 29, 2020 ♛ By Melissa J. Will

This post contains affiliate links.
Click here for the full disclosure statement.

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Did you know if you sow seeds at certain soil temperatures you will get the best germination rates? Every plant species has its own sweet spot, and the more we accommodate this, the better the results. If you have struggled with seed starting, this may be the tip you need to turn that growing thumb green.

For more beginner tips, also see Seed Starting 101 and clever ways gardeners use warmth to speed up seed germination.

Did you know if you sow seeds at certain soil temperatures you will get the best germination rates? Every plant species has its own sweet spot, and the more we accommodate this, the better the results.

What Seeds Need to Sprout

First, what is a seed?

Seeds contain embryonic tissue and food stores that nourish early growth, enclosed in a protective coat. That coat prevents germination (growth) until conditions change.

Seed Storage

To store seeds and prevent germination, it is recommended to keep them in a cool, dry, dark place with consistent temperatures between 32-41F (0-5C) and not more than 50% humidity levels. Long-term storage requires lower temperatures and humidity levels as they do at the Svalbard Global Seed Vault in Norway.

Seed Germination

To awaken seeds from dormancy and trigger germination, conditions must change. When we sow a seed, we introduce water (moisture), which softens the protective coat, and warmer temperatures which provoke growth. Air and light play a role as well.

When we look at temperatures for seed germination, it is the temperature of the soil or growing medium in direct contact with the seed or roots that is so important. Research shows that every single plant species has its own preferences. Getting the soil temperature right—or within optimum range—can greatly affect germination rates.

Example

  • Ten pepper seeds started at 60°F (15°C) may yield one or two plants, and take weeks to sprout.
  • The same number of seeds started at 85°F (29°C) may sprout within days and yield 8-10 happy plants.
  • But, beyond that temperature, the seeds may not survive.

This is what we refer to as a seed liking ‘warm feet’. Keep in mind that this is just one example and the sweet spot is different for each plant species (and can vary by subspecies as well).

Related: The Global Seed Vault in Norway | Learn how it protects seed species from around the world



Optimum Soil Temperatures

If you made a chart of every seed you sow (which I did before writing this) and noted their optimum soil temperatures, you’ll see that the range is vast, making it would be nearly impossible to accommodate every one of them individually. You’d have to check soil temperatures obsessively and be on high alert with seeds at the ready.

Two Main Seed Sowing Groups

To keep it realistic, and still get decent germination rates, we divide our seeds into two main groups:

WARM CROPS and COOL CROPS

The warm crops like basil, eggplant, pole beans, and tomatoes germinate best with warmer soil temperatures.

The plants in the cool / cold group prefer the lower temperatures of spring and autumn, and includes beets, carrots, salad greens, and turnips (see below).

You will also notice that some crops known to grow nicely in cool conditions actually respond best to extra warmth for seed germination, yet mature best in cooler temperatures. Clearly, nature is not concerned with convenient plant rules.

Related: Is it Okay to Store Seeds in the Fridge at Home?

Also, how long a plant takes to mature is also a key factor for seed sowing, since you want that slow-growing watermelon ripe before autumn frosts set in, so do read your seed packets to get the timing right.

For all seeds, if soil temperatures are too cold, they will not sprout. If temperatures are too warm, they either will not germinate, or, they will grow too rapidly, and fail to mature properly or die off. Ever see a crop bolt? That’s the result of too much heat.

While researching this article, I noticed some discrepancies but the ranges listed will give you a good idea of desirable soil temperatures.

To keep it simple, group your seeds into warm and cool groups and sow when soil temperatures are within the favorable ranges.



How to Measure Soil Temperature

Did you know if you sow seeds at certain soil temperatures you will get the best germination rates? Every plant species has its own sweet spot, and the more we accommodate this, the better the results.

BEST TIP: In spring, pay attention to night-time low temperatures, not day-time highs. When the lows are consistently above 50°F or 10°C, the soil becomes warm enough for some seed sowing.

  • For a good reading, place the thermometer in the soil approximately 3″ deep (or however deep the tip allows) and allow a minute for a definite reading.
  • Measure both in the morning and afternoon, a few days in a row.
  • The process is the same for indoor seed starting: take enough readings that you are confident the soil is consistently within the desired temperature range.
  • Note all the readings and take the average. When the average is in range, it’s sowing time—assuming it’s the right time of year.

Related: How to Know When Watermelon is Ripe for Picking


Thermometers

You can use a kitchen meat thermometer to measure soil temperature or get an official soil thermometer, which is really the same thing. I have a digital thermometer with a metal probe and it works nicely.

Tip for Warming Soil in Spring

For outdoor sowing, one practical way to increase soil temperature is to add a few inches of compost to the soil. This will help keep warmth in and expedite the temperature gain.

Indoors, you can use a seedling heat mat to warm the soil for better and faster seed germination rates if your seed sowing area is unusually chilly (below 60°F | 15°C).

I also have a list here of clever ways gardeners use existing warmth within their homes to help speed up germination.



Optimum Soil Temperatures for Seed Sowing

As mentioned, I could not always find a consensus for optimum temperatures so I’ve listed ranges that should be accurate within a few degrees.

Always read your seed packets for specific sowing tips.Even within one species there are all sorts of exceptions.

If you would like to print out these lists or keep a copy on your device, click here to download it (it’s free).

  Plants Best Soil Temperature for Sowing Seeds Plants per Square Foot Days to Maturity
Asparagus 75°F (24°C) 1 Harvest annually
Arugula 40-53°F (4-12°C) Open spaces 30-50 days
Basil 70°F (21°C) 4 30-45 days
Beans, Broad 50-70°F (10-21°C) 8 75-100 days
Beans, Bush + Pole 70-90°F (21-32°C) 8 50-100 days
Beans, Runner 70-90°F (21-32°C) 8 75 days
Beans, Soya 70-90°F (21-32°C) 9 78-85 days
Beets 50-80°F (10-26°C) 9-16 50-70 days
Broccoli 50-85°F (10-30°C) 1 48-100
Broccoli, sprouting 50-85°F (10-30°C) 1 75-120 days    
Brussels Sprouts 50-85°F (10-30°C) 1 100-120 days
Cabbage 50-85°F (10-30°C) 1 45-120 days
Carrots 45-85°F (7-30°C) 16 55-130 days
Cauliflower 50-85°F (10-30°C) 1 60-270 days
Celery 60-75°F (15-24°C) 4 45-110 days
Celeriac 60-75°F (15-24°C) 4 110 days
Chickpeas 50°F (10°C) 4 90-100 days
Chicory 50-72°F (10-22°C) 1 70-90 days
Chives 70-75°F (21-25°C) 16 ongoing
Cilantro 55-68°F (13-20°C) 1 45 days
Claytonia 50-72°F (10-22°C) Open spaces 55 days
Collards 50-85°F (10-30°C) 1 50-80 days
Corn 60-70°F (15-21°C) 4 70-110 days
Corn Salad 45-65°F (5-18°C) Open spaces 30-50 days
Cress 50-72°F (10-22°C) Open spaces 10-50 days
Cucamelon 60-85°F (15-30°C) 2 67 days
Cucumbers 60-85°F (15-30°C) 2 45-70 days
Eggplant 75-90°F (24-32°C) 1 58-90 days
Endive Radicchio 50-72°F (10-22°C) 4-8 85 days
Garlic Plant in fall 9 ongoing
Kale 50-85°F (10-30°C) 2 40-80 days
Kohlrabi 50-85°F (10-30°C) 9 55-70 days
Leeks 50-75°F (10-25°C) 9 65-135 days
Lettuce 50-72°F (10-22°C) 4 35-80 days
Mesclun Mix 50-72°F (10-22°C) Open spaces 30 days
Mustard 70°F (21°C) 12 35-80 days
Okra 75-90°F (24-32°C) 1 56 days
Onions & Scallions 70-75°F (21-25°C) 9-16 50-180 days
Pac Choi 50-85°F (10-30°C) Open spaces 30-60 days
Parsley 85°F (29°C) 6-8 70-90 days
Parsnips 50-75°F (10-25°C) 4 110-150 days
Peas 50-70°F (10-20°C) 8 55-85 days
Peppers 64-75°F (18-24°C) 1 60-120 days
Potato 65-80°F (15-27°C) 4 110 days
Pumpkin 90°F (32°C) 1 vertically 80-130 days
Purslane 50-72°F (10-22°C) Open spaces 60 days
Quinoa 65-75°F (18-24°C) Check seed packet 100 days
Radish 65-75°F (18-24°C) 16 24-120 days
Rhubarb 60-75°F (16-25°C) 1 12-18 months
Rutabaga 65-70°F (18-21°C) 4 90-100 days
Spinach 45-70°F (5-20°C) 9 30-50 days
Squash 68-95°F (25-35°C) 1-2 vertically 70-115 days
Swiss Chard 50-85°F (10-30°C) 4 50-65 days
Tomatoes 68-95°F (25-35°C) 1 50-93 days
Turnips 65-70°F (18-21°C) 4 35-60 days
Watermelon 75-95°F (24-35°C) 1 vertically 70-120 days  
Zucchini 68-95°F (25-35°C) 1-2 vertically 50-100 days


Warm Crops

To simplify seed sowing, you can organize your crops into warm and cool crops. This is a realistic way to get your crops germinating as fast as possible while allowing enough time for the plants to mature during the growing season.

Mainly Tender Summer Crops

warmth-loving garden crops

Approximate Seed Sowing Soil Temperature Range: 68-79°F  | 20 -26°C 

Basil
Bean
Cabbage*
Cilantro/ coriander
Corn
Cucumber
Dill
Eggplant
Melons
Mustard
Nasturtium
Okra
Onion
Peppers
Pole bean
Pumpkin
Sage
Squash – Summer + Winter
Sweet potato
Tomato
Watermelon
Zucchini

Cool & Cold Crops

Spring and Autumn Crops

These crops do best during spring and fall when outdoor temperatures are cooler although some of them like a warmer start for the seeds (see the chart). You can grow them in summer—and we do—but there’s always the risk of bolting (skipping right to the seed production stage) in excessive heat.

Many of these crops can be over-wintered in cold frames.

cool temperature garden crops

Approximate Seed Sowing Soil Temperature Range: 50-68°F  | 10-20°C  

Asparagus
Beet
Broad bean
Broccoli
Broccoli raab
Brussels sprouts
Cabbage*
Carrot
Cauliflower
Celery
Celeriac
Chervil
Chives
Claytonia
Collards
Endive
Garlic
Kale
Kohlrabi
Leafy greens
Leek
Lettuce
Mache
Mibuna
Mizuna
Mustard
Onion
Pak choi
Parsley
Parsnip
Pea
Potato
Radish*
Rosemary
Rutabaga
Sage
Salad greens
Scallion
Spinach
Swiss Chard
Tatsoi
Thyme
Turnip*

*Cabbage, radish, and turnip are considered cool crops but do like extra warmth for seed germination.

Related: How to Winter Sow Seeds Outdoors in Milk Jugs

Free Printable

If you would like download the soil temperature chart, click on Download Now (below).

New to downloading files? There are step-by-step instructions here.


If you are eager to sow seeds in spring or fall, check the cool crop list for candidates, and grab your thermometer. Start checking your soil temperature and sow when conditions are right.

Warm crops can often be slow-growers and do best with indoor seed starting to allow enough time for the plant to mature and fruit.

~Melissa the Empress of Dirt ♛

Next

What to Sow in Fall Before It Snows
10 Best Tips for Growing Sweet Peas
Low-cost seed starting supplies including reused pots.
Best Low-Cost Indoor Seed Starting Supplies
See More >>
Best Soil Temperatures for Sowing Seeds
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Melissa J. Will - Empress of DirtWelcome!
I’m Melissa J. Will
a.k.a. the Empress of Dirt
(Ontario, Canada).
Join me as I share creative + frugal home & garden ideas with a dash of humor.
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