Best Soil Temperatures for Sowing Vegetable Seeds

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. Start measuring your soil temperature to know the best time to sow seeds.

For more beginner tips, also see Seed Starting 101 and clever ways gardeners use warmth to speed up seed germination. This lists best soil temperatures for flowers and herbs.

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Using Soil Temperatures For Better Germination

This is one of the best kept secrets in gardening. Each type of seed has a sweet spot where germination is most successful. If conditions including temperature are right, you get maximum germination rates and faster growth.

When growing vegetables, the closer we are to best soil temperatures for germination, the more plants we will have.

Let’s have a look at what seeds need to sprout, the two main vegetable sowing groups, how to measure your soil temperature, and what our crops like.

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Contents

• What Seeds Need to Sprout
• Two Main Sowing Groups
• How to Measure Soil Temperature
  • Thermometers
  • Tips For Warming Soil in Spring
• Vegetable Soil Temperature Sowing Chart
• Warm Crops
• Cool or Cold Crops
• Frequently Asked Questions
• Resources
  • Free Printable List

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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.

• Best Ways to Store Seeds at Home (Fridge & Other Options)
• How Seeds Are Kept in Long-Term Storage (Global Seed Vault)

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: Seed Starting for Beginners

Two Seed Sowing Groups

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 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? Two known causes are excessive heat and longer days. This explains more about bolting and how to prevent it.

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 while ensuring there are enough days available for the plant to mature.

How to Measure Soil Temperature

• For a good temperature 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: Best Temperatures for Sprouting Flower & Herb Seeds

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.

Vegetable Soil Temperature Sowing Chart

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.

Most of this information comes from studies and growers with much more controlled environments than we have in our gardens. But the ranges can give us a general idea of what plants seem to like.

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 save it (it’s free).

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

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

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.

This explains when it’s safe to plant or sow outdoors in spring before last frost and this has more tips on direct sowing before last frost.

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

*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

Frequently Asked Questions

Resources

Free Printable

If you would like save the soil temperature chart, click on button (below).

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

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 ♛