Best Soil Temperatures for Sowing Seeds | Free Printable

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 60F, may yield one or two plants, and take weeks to sprout.
The same number of seeds started at 85F may sprout within days and yield 8-10 happy plants.

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).

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.

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 preparing this article, I noticed some variations in the research so I used specific numbers where there seems to be a consensus. For starting a variety of seeds at home, I have provided the two main sowing groups (below), so you can come up with a realistic sowing plan and work the soil temperatures to your advantage.