Quick Answer
The optimal soil temperature for starting cold hardy seeds is between 40°F (4°C) and 70°F (21°C), with some varieties tolerating temperatures as low as 32°F (0°C) and as high as 80°F (27°C). Soil temperatures above 70°F can inhibit germination, while temperatures below 40°F can slow it down.
Choosing the Right Seeds
When selecting cold hardy seeds, look for varieties that are specifically bred for cooler soil temperatures. Examples include broccoli, kale, spinach, and carrots. These seeds can tolerate light frosts and will germinate when soil temperatures are between 40°F and 60°F (4°C to 15°C). Some popular cold hardy varieties include ‘Deep Purple’ broccoli, ‘Lacinato’ kale, and ‘Tyee’ spinach.
Preparing the Soil
To create optimal conditions for cold hardy seeds, prepare the soil 2-3 weeks before planting by loosening it to a depth of 8-10 inches (20-25 cm). Add a 2-inch (5 cm) layer of compost or well-rotted manure to improve soil structure and fertility. Cover the soil with a thin layer of mulch or straw to retain moisture and regulate soil temperature.
Planting Techniques
Sow cold hardy seeds directly into the soil when it reaches the optimal temperature for germination. Plant seeds 1/4 to 1/2 inch (6-13 mm) deep, depending on the variety, and 1-2 inches (2.5-5 cm) apart. Water gently but thoroughly after planting, and keep the soil consistently moist during the first few weeks after sowing.
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