Broccoli (Brassica oleracea var. botrytis) is a popular member of the cabbage family of vegetables. This cool-season crop grows well in spring and fall, although some varieties have been bred to produce heads and maintain good eating quality through the heat of summer. Broccoli seeds require warm a temperature for germination, so it can be difficult to get plants off to a good start in early spring when the soil is cold.
Broccoli seeds will germinate over a wide range of temperatures, from 40 degrees to 85 degrees Fahrenheit. However, germination will be slow and unreliable at the lower end of this range, especially when the seeds are planted in ordinary garden soil -- the seeds may rot before they have a chance to sprout.
The theoretical minimum germination temperature for broccoli is approximately 40 degrees F, but the realistic minimum temperature will be determined by your sowing conditions. For example, research on broccoli seed germination indicates that lower temperatures significantly increase the length of time from sowing to seedling emergence. At 50 degrees F, the interval was nine days; at 60 degrees F, the interval was five days; and at 68 degrees F, it was only three days.
If you are sowing broccoli into damp spring soil with high weed pressure, you should probably wait until the soil temperature has reached 60 degrees F. At lower temperatures, germination will be so slow that the seeds might rot, and the weeds will usually germinate and grow more quickly than the broccoli seedlings. If your soil is well drained and your weed situation is manageable, you could try sowing at 50 degrees F.
It is often difficult to obtain good broccoli harvests from direct seeding. Broccoli grows best in the cool days of spring, but unlike other cool-season crops like lettuce and radish, broccoli prefers warm soil for germination. If you wait until the soil is warm to sow the seeds, the weather might turn hot and dry before the broccoli plants form heads. You can avoid this problem by sowing the seeds indoors. For indoor sowing, try to maintain the seed-starting medium near the optimal germination temperature for broccoli, which is about 75 degrees F.
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