Seeds germinate from the soil's warmth. Here's why cold ground rots warm-season seed, why cool-season crops want the opposite, and how to tell the ground is ready.
Photo: Rainer Zenz (uploader), via Wikimedia Commons · CC BY-SA 3.0, via Wikimedia Commons.
The date is a proxy; the soil is the thing
Every seed packet and every planner talks in dates: “sow after the last frost”, “start in April”. A date is a convenient stand-in, but what a seed responds to is the temperature of the ground it’s sitting in. Germination is run by enzymes, and enzymes work within a temperature range; each kind of seed has its own.
A seed comes up from the warmth of the soil, not from a date on the calendar. Each kind of seed has a soil-temperature range it germinates in; sown into ground colder than that range it comes up slowly and patchily or rots before it can emerge, and ground hotter than the range stalls it too.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Germination is driven by temperature-dependent enzymes that mobilise the seed's stored reserves, so in soil below the species' range those enzymes work too slowly for the seedling to emerge before soil fungi rot the water-swollen seed, and in soil above the range that same enzyme machinery is suppressed. That is why the soil's own temperature is the reliable signal, and the day of the year is not.
Evidence: checked against 2 sources in the research-based literature.
In our rapidly changing climate, a calendar date is not the best predictor for an extra early onset of spring weather in March or an extremely late, cold and wet April
Remedy
Read the soil to know when to sow. A soil thermometer pushed to sowing depth for a minute gives the number. Sow each crop once the soil sits in its germination range (warm-season crops wait for warm ground even after the frost has passed, and cool-season crops go into cool soil on purpose), or start seed in a warm flat indoors and set the young plants out. Each plant's own range is on its page.
Crops fail in the gap between the proxy and the real thing. The frost-free date can arrive weeks before the soil is really warm, and a warm-season seed sown on that date sits in cold, damp ground, the worst place for it.
Why cold soil rots a warm-season seed
The classic case is the bean, and it’s the mistake a calendar walks you into most reliably:
A warm-season bean germinates from warm soil, whatever the calendar says. Sown into cold ground it rots or emerges weak, even after the frost has passed.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Warm-season legumes take up water fast at sowing (imbibition), and in cold soil that first rush of cold water injures the seed tissue before the seedling can establish (imbibitional chilling), so the seed rots or comes up damaged. Soil below about 15 °C (60 °F) at sowing depth is where this bites, and the frost-free date can arrive weeks before the soil is that warm.
Evidence: checked against 2 sources in the research-based literature.
Bean seed planted in cold soil may rot rather than germinate, and plant growth will be very slow in cooler weather
Remedy
Wait for the soil to warm to about 15 °C (60 °F) at sowing depth, which a soil thermometer reads in a minute, or start the beans in warm cells and set them out. Peas are the opposite and are out of scope here. Half-hardy, they germinate in cold soil and are sown early on purpose.
A warm-season seed takes up water fast the moment it’s sown. In cold ground that first rush of cold water injures the seed tissue before the seedling can establish, and the soil fungi that rot seed are most at home in exactly those cold, wet conditions. So the seed doesn’t just germinate slowly. It rots. Waiting for the soil to reach the crop’s range decides whether you get a stand or a gap.
Cool-season crops want the mirror image
That doesn’t mean you should always wait for warmth. Cool-season crops (peas, lettuce, spinach, radishes) germinate in cool soil and are sown early on purpose. Their range sits low, and holding them back for warm ground wastes weeks of the season they’re built for. So the rule is “sow in the seed’s range”, because the range is what differs from crop to crop.
How to read the ground
A soil thermometer pushed to sowing depth for a minute gives you the number, and it costs a few dollars. Check it in the morning, when the soil is at its coldest, and give a cold spell a few days to pass before you trust a warm reading. Each plant’s page carries its germination range, so you can hold the number you measured against the number the crop needs, and sow the day the two line up, whatever the calendar says.
